Illinois state income tax: engine rules, 2026

How the tax is computed

Every step the engine takes for Illinois, in the order it takes them. Steps marked with a filing status or household type apply only there. Each step links the section below that carries Illinois's figures and the official sources for it; the calculation order page describes every step for all jurisdictions.

What each filing status gets, read back from the engine for a household with $1 of income (so below every phase-out and income test):

1. Per-person mode joint returns with the spouse's age given On a joint return with the spouse's age given, every age test the state applies per taxpayer is evaluated for each spouse at their own age. Without it one household age applies to both.
   engine step (compute_state_tax_detail). Engine convention, no external source: The engine cannot know the second spouse's age unless told.
2. Blindness and disability, per person only when the caller says a person is blind or disabled The filer is person one; the spouse counts only on a joint return and only when the caller says so (there is no household convention as for age). Where a state lets a disabled person (and, where it says so, a blind one, or one whose spouse is disabled) qualify for its general retirement exclusion before the age, that person is treated as having reached the age for that exclusion only. Of the 42 taxing jurisdictions, these have a blind amount: Arizona, Arkansas, California, Colorado, Delaware, the District of Columbia, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, New Jersey, New Mexico, North Dakota, Oklahoma, Oregon, Utah, Vermont and Virginia; these have none: Alabama, Connecticut, Georgia, Louisiana, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, West Virginia and Wisconsin. A disability route is calculated in Arkansas, the District of Columbia, Georgia, Hawaii, Idaho, Iowa, Maryland, Michigan, New Jersey, Oregon and West Virginia.
   engine step (compute_state_tax_detail). Engine convention, no external source: Who is blind or disabled is the caller's statement; the engine has no household convention for it.
3. Rate schedule and standard deduction by filing status Single uses the single schedule and deduction; married filing jointly the joint ones. Head of household and married filing separately follow the two steps below.
   Illinois's values · engine step (select_brackets). Sources (Rate schedule): 2025 Form IL-1040 Instructions (checked 2026-11-02); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-11-02)
4. Married filing separately Where the state publishes its own separate schedule, that one (with half the joint deduction unless a separate deduction is set). Where one schedule applies to every status, that schedule unscaled with the single deduction. Otherwise the joint schedule with every threshold and the deduction halved (computed as the joint tax on twice the income, halved).
   Illinois's values · engine step (select_brackets). Sources (Rate schedule): 2025 Form IL-1040 Instructions (checked 2026-11-02); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-11-02)
5. Head of household The state's head-of-household schedule where it has one, else the single schedule; the head-of-household deduction where set, else the single deduction.
   Illinois's values · engine step (select_brackets). Sources (Rate schedule): 2025 Form IL-1040 Instructions (checked 2026-11-02); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-11-02)
6. The income total every income test uses Other ordinary income + retirement distributions (before any exclusion) + conversion + federally taxable Social Security + long-term gains + Railroad Retirement Tier 2. It stands in for federal adjusted gross income in every threshold, cliff and phase-out below; state subtractions do not reduce it. Each income line a threshold reads (this total and the states' own AGI lines) is rounded to cents first, as the form's line is, so a sum of cent amounts that a binary double holds a hair under a threshold tests as the amount it is.
   engine step (compute_state_tax_detail). Engine convention, no external source: The inputs are the federal AGI components, so their sum is federal AGI for these income types.
7. Interest on state, local and U.S. obligations The state's own treatment of interest: the federally tax-exempt interest on other states' obligations (outOfStateMunicipalInterest) is added where the state taxes it, and its own (inStateMunicipalInterest) where it taxes that too; the interest on U.S. obligations (usObligationInterest, a part of ordinaryIncome) is taken out (31 U.S.C. 3124). Both change the state's income, its own income tests and its base, never the federal income total above. Each state's rule is on its page (interest). Illinois adds all of the federal 2a interest (IL-1040 line 2), so the part of taxExemptInterest not split into the two municipal parts is added too.
   Illinois's values · engine step (compute_state_tax_detail). Sources (Interest on state, local and U.S. obligations): 2025 Form IL-1040 Instructions (checked 2026-11-02); Illinois: Schedule M instructions, tax year 2025 (checked 2026-11-02)
8. Attributing income to each spouse joint returns with each spouse's own amounts given With a caller's split (the API's spouse object), each spouse's own distributions (employer-plan money included), pension, public pension, conversion, wages and military pay; spouse 2's shares are clamped into the household totals and spouse 1 gets the rest. Without a split, in per-person mode, every item is divided 50/50. Social Security is divided by each spouse's own gross benefits where the caller gives spouse.socialSecurityGross, else 50/50; other ordinary income and gains are always divided 50/50 where a rule needs a per-person amount. The split is used only where a state's rule is per taxpayer; household totals stay authoritative for the base and the income tests.
   engine step (compute_state_tax_detail). Engine convention, no external source: Equal attribution is the neutral split when the owner of a dollar is not known.
9. Disability pay taxed as ordinary income only when the caller gives the person facts (a civil service annuity, disability pay, Social Security disability, survivor benefits, an IRA basis or an exception) Where only government disability plans are subtracted, disability pay (read as a non-government plan's; a government disability pension is entered as a public pension) leaves the retirement dollars and is taxed as ordinary income, each spouse's share by the split, else half.
   Illinois's values · engine step (compute_state_tax_detail). Sources (Blindness and disability): 2025 IL-1040 instructions (checked 2026-11-02)
10. Keeping the per-spouse shares inside the household joint returns with each spouse's own amounts given After the public-pension and employer-pension carve-outs, if the two spouses' remaining shares add up to more than the household's exclusion-eligible distributions (an inconsistent split), spouse 1's share is reduced so they do not.
    engine step (compute_state_tax_detail). Engine convention, no external source: Input normalisation.
11. Military retirement pay The state's military rule runs first, on military pay alone, per taxpayer: each spouse's own pay at their own age when attributed, otherwise half each at the household age on a joint return. Exempt dollars leave the base; the rest become pension dollars for the general retirement rules. Where the state lets a disabled retiree qualify at any age, a disabled person is treated as having reached the rule's age.
    Illinois's values · engine step (compute_state_tax_detail). Sources (Military retirement pay): Retirement income (checked 2026-11-02)
12. One taxpayer's military exemption Exempt: all of it (below the age split, up to the younger cap). Capped: up to the cap for the age tier, plus the earned-income bonus below the age split when the taxpayer's own wages exceed the threshold, less the taxpayer's own Social Security where the rule says so. Percentage: the percentage (below the age split, up to the younger cap). Then the income limit: above it the exemption is lost, or phased out over the range where one is set.
    Illinois's values · engine step (military_exempt_one). Sources (Military retirement pay): Retirement income (checked 2026-11-02)
13. Capital-loss deduction off the base The deduction comes off the other ordinary income (negative income, as Form 1040 line 7a is), so the base and every state-AGI figure summed from it carry it; WI no more than Wisconsin ordinary income (Schedule WD line 28(c)). States whose base is federal taxable income (with the caller's figure) already have it there and take it only once. Engine 1.13.1: the allowed rental loss comes off the same way, on its own line (rental_loss_deduction).
    Illinois's values · engine step (compute_state_tax_detail). Sources (Capital gains): 2025 Form IL-1040 Instructions (checked 2026-11-02); IL-1040 Instructions (checked 2026-11-02); 2025 Schedule D (Form 1040), Capital Gains and Losses (checked 2026-11-02)
14. Social Security exempt None of the federally taxable benefit enters the base.
    Illinois's values · engine step (ss_amount_in_state_base). Sources (Social Security): 2025 Form IL-1040 Instructions (checked 2026-11-02); Publication 120, Retirement Income (R-12/25) (checked 2026-11-02)
15. Retirement exclusion per spouse (both ages known) joint returns with the spouse's age given Full-exclusion states exclude each qualifying spouse's own dollars. Capped states give each spouse the single cap for their own age against their own eligible dollars (pension dollars only where the exclusion is pension-only; employer dollars only below the IRA age), less their military exclusion and their own Social Security offset (by the spouse's stated gross benefits, else half). Per-return states (and the pooled-if-both-qualify rule when both qualify) apply the joint cap to the couple's pooled eligible dollars when either spouse qualifies. The income tests then apply once to the summed deduction.
    Illinois's values · engine step (compute_state_tax_detail). Sources (IRA, 401(k) and private pensions): Publication 120, Retirement Income (R-12/25) (checked 2026-11-02); 2025 Form IL-1040 Instructions (checked 2026-11-02); Publication 120, Retirement Income (checked 2026-11-02)
16. Retirement exclusion (household) The distributions eligible for the exclusion go through the exclusion below. Taxable public-pension dollars left by a cap are not eligible; below the IRA age only employer-plan dollars are.
    Illinois's values · engine step (compute_state_tax_detail). Sources (IRA, 401(k) and private pensions): Publication 120, Retirement Income (R-12/25) (checked 2026-11-02); 2025 Form IL-1040 Instructions (checked 2026-11-02); Publication 120, Retirement Income (checked 2026-11-02)
17. Conversion taxed or exempt In full-exclusion states a conversion is exempt at the qualifying age (at any age where the state says so). Elsewhere, unless pooled above, it is ordinary income taxed in full.
    Illinois's values · engine step (compute_state_tax_detail). Sources (Roth conversions): Publication 120, Retirement Income (R-12/25) (checked 2026-11-02)
18. Full exclusion All distributions are excluded at or above the exclusion age; below it they are taxed.
    Illinois's values · engine step (retirement_distributions_in_state_base). Sources (IRA, 401(k) and private pensions): Publication 120, Retirement Income (R-12/25) (checked 2026-11-02); 2025 Form IL-1040 Instructions (checked 2026-11-02); Publication 120, Retirement Income (checked 2026-11-02)
19. Long-term gains in the base Where gains are taxed as ordinary income: net long-term gain less the flat exclusion (never below zero), times the inclusion share. Elsewhere none enters.
    Illinois's values · engine step (compute_state_tax_detail). Sources (Capital gains): 2025 Form IL-1040 Instructions (checked 2026-11-02); IL-1040 Instructions (checked 2026-11-02); 2025 Schedule D (Form 1040), Capital Gains and Losses (checked 2026-11-02)
20. Exemption for blindness only when the caller says a person is blind or disabled An extra exemption for each blind person (the filer, and the spouse on a joint return). Where the state's exemption is for a person who is blind or disabled, a disabled person below the state's age limit (if any) qualifies too, one exemption per person either way.
    Illinois's values · engine step (compute_state_tax_detail). Sources (Blindness and disability): 2025 IL-1040 instructions (checked 2026-11-02)
21. Dependent exemptions and deductions only when the caller lists dependents An amount per dependent subtracted like the personal exemption: the state's flat amount, or the stepped per-exemption amount the filer gets, or the state's own income steps; nothing above an income limit where there is one; a percentage phase-out per income step where there is one; one more for a dependent in an age band or a student up to an age, for a dependent born in the year, and for a disabled dependent; a deduction per dependent beyond the first on joint and head-of-household returns; and a child deduction by an income table for dependents under an age. A head of household's status exemption that already holds one dependent's amount counts one fewer. Where the state says so, the phase-out is measured on the state AGI (the base before deductions).
    Illinois's values · engine step (compute_state_tax_detail). Sources (Deductions, exemptions and credits): Informational Bulletin FY 2026-15, What's New for Illinois Income Taxes (checked 2026-11-02); 2025 Form IL-1040 Instructions (checked 2026-11-02); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-11-02); 2025 Schedule IL-E/EITC Illinois Exemption and Earned Income Tax Credit (checked 2026-11-02); 2025 Schedule ICR Illinois Credits (checked 2026-11-02)
22. Age subtraction At or above the age: the single amount, or the joint amount on a joint return (both spouses at the household age). In per-person mode, half the joint amount (or the single amount) per qualifying spouse. Plus the low-income amount below the income test; less the military exclusion where the statute says so; limited to the spouse's own income where the statute says so. Where the statute counts income from any source, the taxable remainder of the spouse's own public pension is part of that own income. Where the state says so, the Social Security that offsets a person's amount is that person's own.
    Illinois's values · engine step (compute_state_tax_detail). Sources (Age 65+ subtractions and credits): 2025 Form IL-1040 Instructions (checked 2026-11-02)
23. The federal below-the-line deductions The amounts the federal return deducts below AGI, which a state that allows them takes as they are (26 U.S.C. 224, 225, 163(h)(4) and 170(p), the federal caps and phase-outs as the statutes print them): the Schedule 1-A qualified tips deduction (the employee tips up to the per-return cap, less the statute's step for each whole thousand dollars of modified AGI over its single or joint start; none on a separate return), the qualified overtime deduction (its cap, doubled on a joint return, the same reduction; none on a separate return), the qualified passenger vehicle loan interest deduction (its cap, less the statute's step for each thousand dollars, or part, over its start) and the non-itemizer charitable deduction (cash gifts to public charities other than donor-advised funds, up to its cap, doubled on a joint return, and the 60 percent of AGI limit). Modified AGI is the engine's federal AGI. With computeFederal the federal return's own tips, overtime and 170(p) amounts are used.
    Illinois's values · engine step (state_tax_federal_bl_deductions). Sources (Deductions, exemptions and credits): Informational Bulletin FY 2026-15, What's New for Illinois Income Taxes (checked 2026-11-02); 2025 Form IL-1040 Instructions (checked 2026-11-02); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-11-02); 2025 Schedule IL-E/EITC Illinois Exemption and Earned Income Tax Credit (checked 2026-11-02); 2025 Schedule ICR Illinois Credits (checked 2026-11-02)
24. No exemptions above an income limit Above the federal AGI limit (the joint limit on a joint return) there is no exemption allowance at all: the personal amount (Illinois carries it as its standard deduction), the 65+ and the blind exemptions are all zero.
    Illinois's values · engine step (compute_state_tax_detail). Sources (Deductions, exemptions and credits): Informational Bulletin FY 2026-15, What's New for Illinois Income Taxes (checked 2026-11-02); 2025 Form IL-1040 Instructions (checked 2026-11-02); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-11-02); 2025 Schedule IL-E/EITC Illinois Exemption and Earned Income Tax Credit (checked 2026-11-02); 2025 Schedule ICR Illinois Credits (checked 2026-11-02)
25. State taxable income Other ordinary income + Social Security, retirement income, conversion and gains left in the base − standard deduction − personal exemption − age subtractions.
    engine step (compute_state_tax_detail). Engine convention, no external source: The order of subtraction follows the state forms; each amount is sourced at its own step.
26. 2025 tax line rounded to the whole dollar (VA, MO, WV, DE, CA, OR, GA, ME, VT, MT, PA, MI, IL, OH, AZ, ID, UT, LA, NE, IN, MS, NC, CT, AR, MN, CO, KS, NY, NM, WI, HI, MA, AL, OK, SC, KY, DC) Engine 1.20.0, a 2025 return in a state whose 2025 instructions say the tax line is a whole-dollar line (VA Form 760 line 16, MO-1040 lines 30Y/30S for each spouse, WV IT-140 line 8, DE PIT-RES line 24, CA Form 540 line 31, OR-40 line 20, GA Form 500 line 16; engine 1.21.0 adds ME Form 1040ME line 20 and VT IN-111 line 8 where the rate schedule gives them, MT Form 2 page 2 lines 8, 10 and 12, PA-40 line 12, MI-1040 line 17 and IL-1040 line 12, each rounded; engine 1.24.0 adds AZ Form 140 line 46, ID Form 40 line 20, UT TC-40 line 10, LA IT-540 line 11, NE Form 1040N line 15, IN IT-40 line 8 and MS Form 80-105 line 15, and OH IT 1040 line 8a with its schedule base amounts, ty25-oh-whole-dollar-tax): the rate-schedule tax is rounded to the nearest dollar, half a dollar up: since engine 1.24.0 from the exact amount (ty25-round-exact), except California and West Virginia, whose rate-schedule worksheets write the product to the cent first. Table rows are whole dollars already. The instructions' own worked examples are quoted at the marker and pinned in the printed-answer fixtures. Golden round 10 adds NC (D-400 line 15), CT (Tax Calculation Schedule line 4), AR (AR1000F line 29, and each column of filing status 4), MN (M1 line 10), CO (DR 0104 line 13 and its worksheet above the table), KS (K-40 line 8; above the table the Tax Computation Worksheet with its printed whole-dollar subtraction amounts, not the exact bracket sums), NY (IT-201 line 39 and each line of the tax computation worksheets), NM (PIT-1 line 18 and its schedule above the table) and WI (Form 1 line 12): these nine round half a dollar up from the exact amount, with no cents step (st25_dollar6), so a product whose cents are 49.73 is rounded down (CT Table B, golden round 10 CT-09). Golden round 10, group C adds HI (N-11), MA (Form 1 line 22, line 24 apart), AL (Form 40 line 17), OK (Form 511 / 511-NR), SC (SC1040 line 6, the printed schedule above the table), KY (Form 740 line 12, each status-2 column) and DC (D-40 line 20), in both the tax-table and the rate-schedule mode (ty25-whole-dollar-c).
    engine step (st25_whole_dollar_tax). Engine convention, no external source: The 2025 instructions' whole-dollar rule and worked examples, quoted at CALC-STEP ty25-whole-dollar-tax (ME, VT and MT at st25_whole_lines: ME 'Use whole dollar amounts.' (https://www.maine.gov/revenue/sites/maine.gov.revenue/files/inline-files/25_1040me_gen_instr_w_cover_pg.pdf); VT 'Use Whole Dollars. Round entries to the nearest whole dollar.' (https://tax.vermont.gov/sites/tax/files/documents/IN-111-Instr-2025.pdf); MT: every money line of the 2025 Form 2 prints the cents as '00'). PA: 'On the PA-40 form and schedules, show money amounts in whole-dollars.' (https://www.pa.gov/content/dam/copapwp-pagov/en/revenue/documents/formsandpublications/formsforindividuals/pit/documents/2025/2025_pa-40in.pdf); MI: 'Report all amounts in whole dollars.' (https://www.michigan.gov/taxes/-/media/Project/Websites/taxes/Forms/IIT/TY2025/MI-1040-Book.pdf); IL: 'You must round cents to whole dollars on Form IL-1040 and most schedules, as directed.' (https://tax.illinois.gov/content/dam/soi/en/web/tax/forms/incometax/documents/currentyear/individual/il-1040-instr.pdf). NJ's rounding is optional and stays in cents. Engine 1.24.0 (golden round 10): AZ 'You must round dollar amounts to the nearest whole dollar. If 50 cents or more, round up to the next dollar. If less than 50 cents, round down. Do not enter cents.' (https://azdor.gov/sites/default/files/document/FORMS_INDIVIDUAL_2025_140i.pdf); ID 'Round the amounts on your return to the nearest whole dollar. Round down if under 50 cents, round up if 50 cents or more.' (2025 Idaho individual income tax instructions); UT 'Round off cents to the nearest whole dollar. Round down if under 50 cents; round up if 50 cents and above. Do not enter cents on the return.' (https://files.tax.utah.gov/tax/forms/current/tc-40inst.pdf); LA 'All numbers should be rounded to the nearest dollar.' and 'Line 11 - Multiply Line 10 by .03. Round to the nearest dollar.' (2025 IT-540 instructions); NE 'Enter All Amounts as Whole Dollars. Do not include cents on the return or schedules.' (2025 Form 1040N booklet); IN 'You must round your amounts to the nearest whole dollar.' (2025 IT-40 booklet p.5); MS 'All dollar amounts should be rounded to the nearest whole dollar (no pennies).' (2025 Form 80-100 instructions); OH 'Round all figures to the nearest dollar.' (https://dam.assets.ohio.gov/image/upload/tax.ohio.gov/forms/ohio_individual/individual/2025/it1040-booklet.pdf). Golden round 10: NC 'Round off to the nearest whole dollar. Drop amounts under 50 cents and increase amounts from 50 cents to 99 cents to the next whole dollar.' (2025 D-401 instructions); CT-1040 'Whole Dollars Only'; AR 'ROUND ALL AMOUNTS TO WHOLE DOLLARS' (2025 AR1000F); MN 'Round the dollar amounts to the nearest dollar. For example: 129.49 becomes 129, and 129.50 becomes 130.' (2025 M1 instructions); CO 'Round all dollar amounts to the nearest whole dollar.' (2025 DR 0104 booklet); KS 'ENTER AMOUNTS IN WHOLE DOLLARS ONLY' (2025 K-40) and the 2025 Tax Computation Worksheet (IP25 p.34); NY 'enter whole dollar amounts only. This includes amounts on any credit forms, schedules, or other forms you submit with your New York State return.' (2025 IT-201-I); NM 'Round all numbers and enter only whole dollar amounts.' (2025 PIT-1 instructions); WI 'All amounts filled in on the form should be rounded to the nearest dollar.' (2025 Form 1 and Form 1NPR instructions).
27. 2025 whole-dollar returns, line by line (ME, VT, MT, VA, OR, MO, DE, GA, WV, CA, PA, MI, IL, OH, AZ, ID, UT, LA, NE, IN, MS, NC, CT, AR, MN, CO, KS, NY, NM, WI, HI, MA, AL, OK, SC, KY, DC); RI, MD, ND and IA lines to the cent Engine 1.21.0, a 2025 return in a state whose 2025 instructions say every amount on the return is a whole-dollar amount (ME "Use whole dollar amounts", VT "Round entries to the nearest whole dollar", MT Form 2 "00" cents, VA, OR, MO, DE, GA, WV, CA, PA, MI, IL): a worksheet's or a credit's product is rounded where the form writes it (half a dollar up from the exact amount since engine 1.24.0, ty25-round-exact; California and West Virginia to the cent first): ME standard/itemized deduction and exemption phase-out line 7, pension deduction, property tax fairness credit lines; VT Social Security / military / non-covered pension partial exemptions, the IN-153 cap on federal taxable income, the minimum tax, the IN-119 early-distribution tax, the charitable credit; MT Schedule II, 2EC and the itemized add-back; VA Form 763 lines 17-18 and the itemized total; OR-40-N line 42 (the deduction plus the federal tax subtraction prorated as one product); MO-1040 line 33 per spouse and the federal tax deduction; GA's part-year retirement exclusion and Schedule 3 line 13 of a separate return; WV Schedule M line 34; CA FTB 3805P line 4 and Form 540NR lines 35/37/39; PA Schedule SP line 16; MI-1040CR lines 12, 34 and 38/41, Worksheet 5 and Form 4884 Worksheet 3.3 line 4; IL Schedule ICR line 4g and the child tax credit on the rounded IL EITC. Engine 1.24.0 adds OH (Schedule of Credits line 12, the joint filing credit; IT NRC line 20; line 13, the share of the federal EIC), UT (TC-40 lines 16 and 19, the AH, AJ, code 18 and AT credit worksheets: ty25-ut-whole-dollar-credits), AZ (the reduced dependent tax credit), ID (Form 43 line 63, the part-year food tax credit), IN (CT-40 line 3, the county tax: ty25-whole-dollar-local) and the prorated deduction and exemption lines of AZ, ID, MS and IN part-year and nonresident returns (ty25-py-whole-lines); LA, NE and MS lines the engine computes are the tax line and the ratio products. The eight states of engine 1.24.0 round from the exact amount, with no cent step (st25_dollar6). Golden round 10 adds NC, CT, AR, MN, CO, KS, NY, NM and WI, rounded half a dollar up from the exact amount (st25_dollar6): NC D-400 line 14 (Schedule PN: line 12b x the line 24 decimal) and line 15; CT Tax Calculation Schedule line 9 (line 7 x the Table E decimal) and CT-1040NR/PY line 10; AR1000NR line 38D and AR line 32 (the early-distribution tax); MN M1NR line 32 and Worksheet A for line 4 steps 3, 5 and 8; CO DR 0104PN line 36; KS K-40 line 10; NY IT-203 line 46, the NYC and Yonkers lines and the household and school credits; NM PIT-B line 14 and the low- and middle-income exemption worksheet line 5; WI Form 1NPR line 39, the married couple credit and the line 25 early-distribution tax. RI-1040 lines keep the cents ("must include cents"). The sources are quoted at st25_whole_lines and st25_cents_lines; the printed-answer fixtures pin the forms' examples. Golden round 10, group C adds HI, MA, AL, OK, SC, KY and DC to the whole-dollar returns and MD to the cents returns ("State calculations are rounded to the nearest cent"); their own lines are at ty25-whole-dollar-c.
    engine step (st25_in). Engine convention, no external source: The 2025 instructions' whole-dollar (and RI cents) rules, quoted at st25_whole_lines / st25_cents_lines and CALC-STEP ty25-whole-dollar-lines.
28. 2025 whole-dollar lines: one rounding, from the exact amount (CA and WV to the cent first) Engine 1.24.0 (golden round 10): every whole-dollar line of a 2025 return (ty25-whole-dollar-tax, ty25-whole-dollar-lines, ty25-whole-dollar-c and the state lines that cite them) is rounded by one function (st25_dollar6) on the one list of 2025 whole-dollar returns: half a dollar up from the exact amount, with no step to the cent first, as each state's form says (Virginia: "Amounts less than 50 cents should be rounded down while all amounts of 50 cents - 99 cents should be rounded up"). An amount whose cents are at least 49.5 but under 50 is rounded down: a Connecticut Table B product with 49.73 cents is entered a dollar lower than engines 1.20.0-1.23.0 entered it (they rounded it to the cent first, golden round 10 CT-09). California and West Virginia keep the step to the cent: their rate-schedule worksheets write the product to the cent before the total is rounded (California's Step 4, "round the amount from Step 3 to two decimals"; West Virginia's Rate Schedule example). 2026 answers are unchanged.
    engine step (st25_dollar6). Engine convention, no external source: VA: 'All amounts entered on your return must be rounded to the nearest dollar. Amounts less than 50 cents should be rounded down while all amounts of 50 cents - 99 cents should be rounded up.' (2025 Form 760 instructions, https://www.tax.virginia.gov/sites/default/files/vatax-pdf/2025-760-instructions.pdf); MO: 'For 1 cent through 49 cents, round down to the previous whole dollar amount. For 50 cents through 99 cents, round up to the next whole dollar amount.' (2025 MO-1040 instructions); DE: 'amounts from 50 to 99 cents are increased to the next dollar' (2025 PIT-RES instructions); OR: '$99.49 becomes $99, and $99.50 becomes $100' (2025 OR-40 instructions); MI: 'Round down amounts of 49 cents or less. Round up amounts of 50 cents or more.' (2025 MI-1040 book); ME: 'Round down to the next lower dollar any amount less than 50 cents.' (2025 Form 1040ME instructions); IL: 'drop amounts under 50 cents and increase amounts of 50 to 99 cents to the next dollar' (2025 IL-1040 instructions). CA: 'They round the amount from Step 3 to two decimals (if necessary) and add it to the tax amount for their income range. After rounding the result, they will enter $4,768' (2025 Form 540 booklet, How to Figure Tax Using the 2025 California Tax Rate Schedules); WV: '$ 2,778.01 Tax on excess of $57,635 ... $ 4,831.51 Total Tax on $117,635 (Round to nearest whole dollar)' (2025 IT-140 booklet, Rate Schedule I).
29. 2025 whole-dollar returns: the amounts you enter are whole dollars too (37 jurisdictions); ND and IA lines to the cent Golden round 11: on a 2025 return in one of the 37 whole-dollar jurisdictions every amount the form makes you enter is a whole dollar, not only the lines it computes: each income amount of the request (one entry of the form: a person's wages, the household's interest, a part-year return's state-source amounts), each federal-return line the state return copies (with computeFederal: the taxable Social Security, the federal tax lines, Form 1040 lines 12e and 13b one by one, so Montana's line 3 is the entered federal AGI less each entered deduction line) and a credit's facts (the property tax paid) are entered rounded, half a dollar up, and a figure the form reads from its own charts by income (a standard deduction or child deduction table, an exemption tier, a tax-table row) is read on the entered amount. The request is validated on the amounts as sent (a negative cent is still refused); input shows the amounts as entered and yearNotes says so. North Dakota's and Iowa's 2025 lines are written to the cent, as Rhode Island's and Maryland's (Iowa's whole-dollar rounding is optional). 2026 answers are unchanged.
    engine step (state_tax_ty25_whole_entries). Engine convention, no external source: PA: 'On the PA-40 form and schedules, show money amounts in whole-dollars. Eliminate any amount less than $0.50 and increase any amount that is $0.50 or more to the next highest dollar.' (2025 PA-40 instructions p.4, https://www.pa.gov/content/dam/copapwp-pagov/en/revenue/documents/formsandpublications/formsforindividuals/pit/documents/2025/2025_pa-40in.pdf); VA: 'All amounts entered on your return must be rounded to the nearest dollar.' (2025 Form 760 instructions); IA: 'You can round off cents to whole dollars on your return and schedules. If you do round to whole dollars, you must round all amounts.' (2025 IA 1040 expanded instructions, What's New).
30. 2025 whole-dollar returns: the request validated as sent, evaluated as entered Golden round 11: a 2025 request for one of the 37 whole-dollar jurisdictions is validated on the caller's own amounts, then evaluated on each amount rounded to the dollar as the form enters it (ty25-whole-entries); a part of two amounts (early distributions of the IRA and employer-plan money, the out-of-state part of the tax-exempt interest) stays within their entered sum. Should the entered amounts break a rule the caller's own met, the caller's amounts are evaluated as given.
    engine step (stj_ty25_enter). Engine convention, no external source: The forms' whole-dollar rules quoted at ty25-whole-entries and ty25-whole-dollar-lines.
31. 2025 refundable credit lines in whole dollars (ME, VT, MT, VA, OR, MO, DE, GA, WV, CA, PA, MI, IL, OH, AZ, ID, UT, LA, NE, IN, MS, NC, CT, AR, MN, CO, KS, NY, NM, WI) Engine 1.21.0, a 2025 return of a whole-dollar state (ty25-whole-dollar-lines): each refundable credit line is entered in whole dollars (ME earned income credit worksheet line 3, VT IN-112 Part II, OR Kids Credit, VA Form 760 line 23's refundable share of the federal EIC, ...); a line that rounds to nothing drops out of refundableCredits. Golden round 10 adds NC, CT, AR, MN, CO, KS, NY, NM and WI (CT EITC, CO, MN, NM, NY and NYC credit lines).
    engine step (state_tax_refundable_credits). Engine convention, no external source: The 2025 instructions' whole-dollar rules, quoted at st25_whole_lines and CALC-STEP ty25-whole-dollar-credits.
32. No tax at or below zero State taxable income at or below zero returns a tax of 0 (credits are not refundable), unless the state has a tax computed apart from taxable income that still applies (next step).
    engine step (compute_state_tax_detail). Engine convention, no external source: Every credit the engine carries is nonrefundable, so nothing below can make a zero tax negative; each credit is sourced at its own step.
33. Separate-return scaling married filing separately For the halved-joint rule, taxable income is doubled before the schedule and the result halved.
    Illinois's values · engine step (compute_state_tax_detail). Sources (Rate schedule): 2025 Form IL-1040 Instructions (checked 2026-11-02); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-11-02)
34. The rate schedule Each bracket's rate on the part of taxable income inside it, summed. No rounding: the result is carried in full precision.
    Illinois's values · engine step (apply_state_brackets). Sources (Rate schedule): 2025 Form IL-1040 Instructions (checked 2026-11-02); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-11-02)
35. Credits cannot make the tax negative The credits are subtracted and the result floored at zero.
    engine step (compute_state_tax_detail). Engine convention, no external source: Every credit the engine carries is nonrefundable; each credit is sourced at its own step.
36. Qualifying surviving spouse qualifying surviving spouse returns A qualifying surviving spouse (filingStatus qss) is one person on the return, taxed from the single figures with the state's own treatment of the status laid over them: where the state says so, the joint rate schedule, the joint standard deduction, the joint personal exemption, the joint income limits and phase-outs (only those the state sets for joint returns), the married 65+ and blindness amounts, or the head-of-household return; where the state gives the status nothing of its own, the single return. The federal figures the engine derives for the status are the federal qualifying-surviving-spouse ones (the joint standard deduction, the married 65+ amount).
    Illinois's values · engine step (state_tax_effective_config). Sources (Rate schedule): 2025 Form IL-1040 Instructions (checked 2026-11-02); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-11-02)
37. Railroad Retirement benefits only when the caller gives Railroad Retirement benefits Railroad Retirement benefits are never taxed by a state (45 U.S.C. §231m). Tier 2 (railroadRetirementTier2) is in the income total the tests use but never in the base. Tier 1 (railroadRetirementTier1) is a part of the gross benefits (socialSecurityGross); its share of the taxable benefits is taken out of the Social Security every state rule sees, so a state that taxes Social Security never reaches it. Where a state's worksheet says so, the benefits reduce a retirement cap or subtraction (ID, IN, MD, ME, MN: both tiers; MI: the taxable railroad benefits); Minnesota's and Utah's Social Security rules test an income without them, Minnesota's, New Jersey's and California's retirement income tests an income without Tier 2; Ohio and Delaware test their own AGI everywhere, which never holds the benefits.
    Illinois's values · engine step (compute_state_tax_detail). Sources (Social Security): 2025 Form IL-1040 Instructions (checked 2026-11-02); Publication 120, Retirement Income (R-12/25) (checked 2026-11-02)
38. The capital-loss deduction the state allows With a net capital loss (capitalLoss): the smaller of the loss and the state's limit (the federal $3,000 / $1,500 where the state follows the federal return; MA $2,000, and no more than the interest and dividends); AL the loss less the part carried in, unlimited; PA and NJ none. It comes off the income total the income tests use (and the Social Security worksheets' income); the gross-income filing test (LA) reads the total before it. Engine 1.13.1: with a rental loss (rentalNetIncome below 0, the federal Schedule E line 26 amount) the state allows, the whole loss where the state starts from federal AGI or follows the federal schedule, none in PA and NJ (rental_loss_no_offset): it comes off the same income the same way; a rental profit is other ordinary income.
    Illinois's values · engine step (compute_state_tax_detail). Sources (Capital gains): 2025 Form IL-1040 Instructions (checked 2026-11-02); IL-1040 Instructions (checked 2026-11-02); 2025 Schedule D (Form 1040), Capital Gains and Losses (checked 2026-11-02)
39. Each spouse's own Social Security joint returns On a joint return, each spouse's own Social Security is the spouse's share of the benefits by spouse.socialSecurityGross when the caller gives it (Tier 1 railroad benefits left out), else half each. It is what the per-spouse rules read: Colorado's per-spouse subtraction, Rhode Island's per-spouse modification, and the Maine, Maryland and other retirement caps reduced by the benefits each spouse received (with each spouse's own railroad benefits where the state counts them).
    Illinois's values · engine step (compute_state_tax_detail). Sources (Social Security): 2025 Form IL-1040 Instructions (checked 2026-11-02); Publication 120, Retirement Income (R-12/25) (checked 2026-11-02)
40. Property tax credit (IL) Schedule ICR: a share of the property tax on the principal residence, nonrefundable, at AGI at most $250,000 ($500,000 joint).
    Illinois's values · engine step (st_compute). Sources (Deductions, exemptions and credits): Informational Bulletin FY 2026-15, What's New for Illinois Income Taxes (checked 2026-11-02); 2025 Form IL-1040 Instructions (checked 2026-11-02); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-11-02); 2025 Schedule IL-E/EITC Illinois Exemption and Earned Income Tax Credit (checked 2026-11-02); 2025 Schedule ICR Illinois Credits (checked 2026-11-02)
41. Refundable state credits The credits paid beyond the tax (reported in refundableCredits; netTax = totalTax less them): earned income credits as a share of the federal EIC with each state's own age band (OK on the 2020 federal credit, DC's own childless schedule, the CalEITC table, MN's Working Family Credit; PA's Working Pennsylvanians Tax Credit, a tenth of the federal EIC, from 2025), New York's Empire State child credit (IT-213; state engine 1.16), the refundable child and dependent credits figured from the dependents' ages (MA Form 1 line 46, ME DETC, NJ line 65, NM PIT-RC line 25, OR Kids Credit, VT IN-112, CO DR 0104CN with 2025's Family Affordability Tax Credit, IL Schedule IL-E/EITC Step 5, MD Worksheet 21C, CA's young child tax credit, and MN's child tax credit with the Working Family Credit on Schedule M1CWFC), the NYC EIC and school tax credits, and the property-tax, renter, grocery and senior credits of MA, MI, NJ, ID, HI, NM, OK, AZ, NY, RI, WV, ME, DC, MN, MO, WI, IN and MT, each on its own income measure. Engine 1.13.1: a part-year NYC resident's school tax credits (the Table 2 proration chart and the rate-reduction amount on IT-360.1 line 47; IT-203 lines 60 / 60a for a part-year state resident). Golden round 3: Hawaii's food/excise bands are "under" their upper amounts, read on the N-11's whole-dollar AGI. Engine 1.19.0 (tax year 2025, the printed charts): the MO-PTS Property Tax Credit Chart, Oklahoma's 511-EIC 2020 EIC Table and Colorado's TABOR State Sales Tax Refund (DR 0104 line 38).
    Illinois's values · engine step (state_tax_refundable_credits). Sources (Deductions, exemptions and credits): Informational Bulletin FY 2026-15, What's New for Illinois Income Taxes (checked 2026-11-02); 2025 Form IL-1040 Instructions (checked 2026-11-02); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-11-02); 2025 Schedule IL-E/EITC Illinois Exemption and Earned Income Tax Credit (checked 2026-11-02); 2025 Schedule ICR Illinois Credits (checked 2026-11-02)
42. 2025 part-year / nonresident prorated exemption allowance in whole dollars (MI, IL) only when the caller gives a part-year or nonresident residency Engine 1.21.1, a 2025 part-year or nonresident return in Michigan or Illinois: the prorated exemption allowance (MI Schedule NR line 19, line 15 x the line 18 percentage, or Worksheet 6 when one spouse is a full-year resident; IL Schedule NR line 50, line 49 x the line 48 decimal) is rounded to the cent and then to the nearest dollar, half a dollar up, like every other line of the two whole-dollar returns (ty25-whole-dollar-tax). The engine's own prorated total (per-spouse factors included) is rounded and the difference carried on the deduction slot the filing status reads, where both states carry the personal amount. 2026 answers are unchanged.
    Illinois's values · engine step (py_ty25_whole_exemption). Engine convention, no external source: MI: '19. If both spouses are part-year or nonresidents, multiply line 15 by the percentage on line 18 and enter here and on MI-1040, line 15.' (2025 Schedule NR, the line prints '00'; https://web.archive.org/web/20260321195952id_/https://www.michigan.gov/taxes/-/media/Project/Websites/taxes/Forms/IIT/TY2025/Schedule-NR.pdf) and 'Report all amounts in whole dollars. Round down amounts of 49 cents or less. Round up amounts of 50 cents or more.' (2025 MI-1040 book p.5; https://www.michigan.gov/taxes/-/media/Project/Websites/taxes/Forms/IIT/TY2025/MI-1040-Book.pdf). IL: '50 Multiply Line 49 by the decimal on Line 48. This is your Illinois exemption allowance.' (2025 Schedule NR, the line prints '.00'; https://tax.illinois.gov/content/dam/soi/en/web/tax/forms/incometax/documents/currentyear/individual/il-1040-schedule-nr.pdf) and 'You must round cents to whole dollars on Form IL-1040 and most schedules, as directed.' (2025 IL-1040 instructions; https://tax.illinois.gov/content/dam/soi/en/web/tax/forms/incometax/documents/currentyear/individual/il-1040-instr.pdf).

Worked examples

Computed by the engine for the fixed households below (the same ones for every state, so states compare directly). Federal tax is not supplied, so a federal-tax deduction is not taken; the income tests use the sum of the components as income.

Line by line

The intermediate lines the engine records while it computes each example (its detail record, printed as it comes out; lines that are zero are left out). The engine's documentation build checks that the lines add up to the taxable income and the tax, for every state, filing status and a grid of households. Each line links the engine step that produces it.

After 2026: how the amounts move

A request with projection (API) estimates a later tax year from these rules: every dollar amount and age test below moves the way Illinois's law moves it. Rates and the rules themselves stay the 2026 ones (how projection works).

Enacted for later years, not applied by the projection

None found: the research found no change enacted for years after 2026 beyond the schedules above.

What the engine does not calculate, and known approximations

• The form is in whole dollars; the engine keeps cents, so results can differ by under $1.
• disabilityIncome is read as pay from a disability plan that is not a government plan, which Illinois taxes (engine 1.8.0). Enter a government or military disability pension as pensionPublic: it is subtracted like other government retirement income.
• Dependents: the Illinois child tax credit (Schedule IL-E/EITC Step 5; IL-1040 line 30), a share of the Illinois EITC with a dependent child under 12, is figured in refundableCredits; a dependent under 12 is read as the filer's child (relationship and residence are not inputs). Sources: Informational Bulletin FY 2026-15, What's New for Illinois Income Taxes (checked 2026-11-02); 2025 Form IL-1040 Instructions (checked 2026-11-02); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-11-02); 2025 Schedule IL-E/EITC Illinois Exemption and Earned Income Tax Credit (checked 2026-11-02); 2025 Schedule ICR Illinois Credits (checked 2026-11-02).
• Municipal bond interest: interest on the Illinois bonds a statute exempts (Schedule M line 34, held directly, net of premium amortization), on Puerto Rico, Guam and Virgin Islands bonds (line 35) and the expenses of federally tax-exempt income (line 17) are subtracted; none is calculated: inStateMunicipalInterest is added in full. Sources: 2025 Form IL-1040 Instructions (checked 2026-11-02); Illinois: Schedule M instructions, tax year 2025 (checked 2026-11-02).

What the engine does not calculate for any state (credits beyond those each page lists, credits for tax paid to other states, state alternative minimum taxes, and more) is listed on the calculation order page.

Verification

Worksheet-tested The engine's test suite replays Form IL-1040 (2025, with the 2026 $2,925 exemption) line by line: line 5 Social Security and retirement subtraction (IRA, pensions, military, conversions), the line 10a and 10b exemption allowance per filer and per 65+ box, and line 12 at 4.95%.

• Golden scenarios (hand-computed expected tax, asserted to within $1): S15, XIL1.
• State-specific unit tests: test_il_flat_rate, test_ss_exempt_states_dont_tax_ss, test_full_exclusion_states_exempt_pension_and_ira, test_full_exclusion_states_also_exempt_roth_conversion, test_v3_folds_v2, test_replay_il_rp_election, test_paths, test_s2_il_disability, test_il_senior_per_spouse, test_older_spouse_unlocks, test_c_states, test_2025_round_exact.
• Structural tests covering every state: 12.
• State forms and worksheets followed in the test derivations: Form IL-1040 (2025) lines 1-12, with the 2026 exemption amount; IL-1040 instructions (2025) line 5 and line 10 exemption chart; Publication 120, Retirement Income.