State tax docs / States / Illinois

Illinois state income tax: engine rules, 2026

How the QuantCalc engine computes Illinois tax for tax year 2026, rule by rule, with the official source for each rule. Verification status: Worksheet-tested (what this means).

On this page: How the tax is computed · Rate schedule · Deductions, exemptions and credits · Age 65+ subtractions and credits · Social Security · IRA, 401(k) and private pensions · Public pensions · Military retirement pay · Roth conversions · Capital gains · Blindness and disability · Part-year and nonresident returns · Worked examples · After 2026 · Not calculated · Verification. Machine-readable: this state's entry in the engine config and the provenance file (key IL).

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):

	Single	Married filing jointly	Married filing separately	Head of household
Rate schedule used	single schedule	joint schedule	joint schedule, thresholds halved	single schedule
Top rate	4.95% flat	4.95% flat	4.95% flat	4.95% flat
Standard deduction, under 65	$2,925	$5,850	$2,925	$2,925
Standard deduction at 70 (joint: both 70)	$2,925	$5,850	$2,925	$2,925
Personal exemption	none	none	none	none
Age subtractions at 70 (joint: both 70)	$1,000	$2,000	$1,000	$1,000

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.
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.
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 2026-10-12); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-10-12)
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 2026-10-12); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-10-12)
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 2026-10-12); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-10-12)
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.
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.
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.
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 2026-10-12)
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.
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 2026-10-12)
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 2026-10-12)
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 2026-10-12); IL-1040 Instructions (checked 2026-10-12); 2025 Schedule D (Form 1040), Capital Gains and Losses (checked 2026-10-12)
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 2026-10-12); Publication 120, Retirement Income (R-12/25) (checked 2026-10-12)
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 2026-10-12); 2025 Form IL-1040 Instructions (checked 2026-10-12); Publication 120, Retirement Income (checked 2026-10-12)
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 2026-10-12); 2025 Form IL-1040 Instructions (checked 2026-10-12); Publication 120, Retirement Income (checked 2026-10-12)
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 2026-10-12)
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 2026-10-12); 2025 Form IL-1040 Instructions (checked 2026-10-12); Publication 120, Retirement Income (checked 2026-10-12)
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 2026-10-12); IL-1040 Instructions (checked 2026-10-12); 2025 Schedule D (Form 1040), Capital Gains and Losses (checked 2026-10-12)
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 2026-10-12)
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 2026-10-12); 2025 Form IL-1040 Instructions (checked 2026-10-12); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-10-12); 2025 Schedule IL-E/EITC Illinois Exemption and Earned Income Tax Credit (checked 2026-10-12); 2025 Schedule ICR Illinois Credits (checked 2026-10-12)
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 2026-10-12)
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 2026-10-12); 2025 Form IL-1040 Instructions (checked 2026-10-12); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-10-12); 2025 Schedule IL-E/EITC Illinois Exemption and Earned Income Tax Credit (checked 2026-10-12); 2025 Schedule ICR Illinois Credits (checked 2026-10-12)
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.
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.
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 2026-10-12); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-10-12)
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 2026-10-12); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-10-12)
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.
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 2026-10-12); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-10-12)
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 2026-10-12); Publication 120, Retirement Income (R-12/25) (checked 2026-10-12)
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 2026-10-12); IL-1040 Instructions (checked 2026-10-12); 2025 Schedule D (Form 1040), Capital Gains and Losses (checked 2026-10-12)
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 2026-10-12); Publication 120, Retirement Income (R-12/25) (checked 2026-10-12)
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 2026-10-12); 2025 Form IL-1040 Instructions (checked 2026-10-12); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-10-12); 2025 Schedule IL-E/EITC Illinois Exemption and Earned Income Tax Credit (checked 2026-10-12); 2025 Schedule ICR Illinois Credits (checked 2026-10-12)
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), 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).
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 2026-10-12); 2025 Form IL-1040 Instructions (checked 2026-10-12); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-10-12); 2025 Schedule IL-E/EITC Illinois Exemption and Earned Income Tax Credit (checked 2026-10-12); 2025 Schedule ICR Illinois Credits (checked 2026-10-12)

Rate schedule

A flat 4.95% on state taxable income.

Source review notes (what the sources do not state outright, or what the engine does not carry):

No 2026-dated official page stating the 4.95% rate was fetched; 35 ILCS 5/201 on ilga.gov timed out.

2025 Form IL-1040 Instructions, Illinois Department of Revenue. Illinois taxes net income at a single flat rate of 4.95 percent. Checked 2026-10-12.
Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill, irs.gov. irs.gov: "for heads of households, the standard deduction will be $24,150" Checked 2026-10-12.

Deductions, exemptions and credits

Standard deduction (or the exemption the engine carries in its place): $2,925 single, $5,850 married filing jointly, and the single amount for head of household.

Informational Bulletin FY 2026-15, What's New for Illinois Income Taxes, Illinois Department of Revenue. The Illinois personal exemption allowance for tax year 2026 is $2,925 per exemption ($5,850 for a joint return with two exemptions). Engine 1.7.0: IL-1040 line 10d: the exemption amount ($2,925 for 2026) per dependent; no exemption allowance above $250,000 of AGI ($500,000 joint). Checked 2026-10-12.
2025 Form IL-1040 Instructions, Illinois Department of Revenue. The exemption allowance is disallowed entirely when federal AGI exceeds $250,000 (single and other non-joint statuses) or $500,000 (married filing jointly). The engine removes the whole allowance (10a, 10b and 10c) above those limits (engine 1.6.0). Engine 1.7.0: IL-1040 line 10d: the exemption amount ($2,925 for 2026) per dependent; no exemption allowance above $250,000 of AGI ($500,000 joint). Earned income credit (engine 1.12.0, refundableCredits): 20% of the federal EIC, a childless filer from 18. Property tax credit (engine 1.12.0): Schedule ICR, 5% of the property tax on the principal residence, nonrefundable, AGI at most $250,000 / $500,000 joint. Checked 2026-10-12.
Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill, irs.gov. irs.gov: "for heads of households, the standard deduction will be $24,150" Checked 2026-10-12.
2025 Schedule IL-E/EITC Illinois Exemption and Earned Income Tax Credit, Illinois Department of Revenue. Earned income credit (engine 1.12.0, refundableCredits): 20% of the federal EIC, a childless filer from 18. Checked 2026-10-12.
2025 Schedule ICR Illinois Credits, Illinois Department of Revenue. Property tax credit (engine 1.12.0): Schedule ICR, 5% of the property tax on the principal residence, nonrefundable, AGI at most $250,000 / $500,000 joint. Checked 2026-10-12.

Age 65+ subtractions and credits

At age 65 or older: $1,000 is subtracted from the base ($2,000 for a married couple filing jointly when both qualify), against any income. With the spouse's age given, each spouse qualifies on their own age.

2025 Form IL-1040 Instructions, Illinois Department of Revenue. Each taxpayer (and spouse on a joint return) age 65 or older adds $1,000 to the Illinois exemption allowance. Checked 2026-10-12.

Social Security

Social Security benefits are fully exempt: none of the federally taxable amount enters the state base.

API value: socialSecurityRule: "exempt". The engine's input is the federally taxable amount; income tests use the sum of the income components as the adjusted-gross-income proxy.

2025 Form IL-1040 Instructions, Illinois Department of Revenue. Federally taxed Social Security benefits included in federal AGI are subtracted on Form IL-1040 Line 5, so Illinois does not tax Social Security. Checked 2026-10-12.
Publication 120, Retirement Income (R-12/25), Illinois Department of Revenue. Social Security benefits reported on the federal return are subtractable retirement income. Checked 2026-10-12.

IRA, 401(k) and private pensions

Pension, annuity and IRA/401(k) distributions are fully excluded at any age. No dollar cap and no income limit.

Publication 120, Retirement Income (R-12/25), Illinois Department of Revenue. Federally taxed distributions from qualified employee benefit plans (including 401(k)), IRAs and SEP plans are fully subtracted, with no age or income limit. Checked 2026-10-12.
2025 Form IL-1040 Instructions, Illinois Department of Revenue. Line 5 subtracts federally taxed retirement income from qualified employee benefit plans (including 401(k) plans) and IRAs. Checked 2026-10-12.
Publication 120, Retirement Income, Illinois Department of Revenue. Only government retirement and government disability plans (military included) are subtracted; disability pay from any other plan stays taxable. Engine 1.8.0 taxes disabilityIncome in Illinois. Checked 2026-10-12.

Public pensions

No separate treatment: public pensions follow the rules for other retirement income above.

Publication 120, Retirement Income (R-12/25), Illinois Department of Revenue. Government retirement and disability plans, including military plans, and state or local governmental 457 deferred compensation are fully subtractable. Checked 2026-10-12.

Military retirement pay

Military retirement pay is fully exempt, at any age.
Rules are applied per taxpayer at that person's own age when the spouse's age is given. API value: rules.militaryRetirement.treatment: "exempt".

Retirement income, Illinois Department of Revenue. Government retirement income, including military plans, is subtracted from Illinois income. Checked 2026-10-12.

Roth conversions

A traditional-to-Roth conversion is not taxed at any age.

Publication 120, Retirement Income (R-12/25), Illinois Department of Revenue. A traditional-to-Roth IRA conversion is federally taxed IRA income that Illinois allows as a Line 5 subtraction, so it is not taxed by Illinois. Checked 2026-10-12.

Capital gains

Net long-term gain is taxed as ordinary income.
Short-term gains are ordinary income.

Source review notes (what the sources do not state outright, or what the engine does not carry):

Illinois has no capital-gains preference; the instructions do not state 'capital gains are taxed as ordinary income' in words, the treatment follows from the federal-AGI starting point and flat rate.

2025 Form IL-1040 Instructions, Illinois Department of Revenue. Illinois starts from federal adjusted gross income and applies the single 4.95% rate, so long-term capital gains are taxed as ordinary income. Checked 2026-10-12.
IL-1040 Instructions, Illinois Department of Revenue. The state return starts from federal adjusted gross income (federal taxable income in CO, IA, ND), so the federal capital-loss deduction (up to $3,000, $1,500 married filing separately) and its carryover are in the state base (engine 1.9.0: cap_loss_limit). Checked 2026-10-12.
2025 Schedule D (Form 1040), Capital Gains and Losses, Internal Revenue Service. The federal return deducts a net capital loss up to $3,000 ($1,500 married filing separately) on Form 1040 line 7a; a state that starts from federal adjusted gross income or federal taxable income carries that deduction. Checked 2026-10-12.

Blindness and disability

An extra exemption of $1,000 for each person who is blind (the filer, and the spouse on a joint return).
The API applies these through its blind, disabled, spouse.blind and spouse.disabled fields; absent means false.

2025 IL-1040 instructions, Illinois Department of Revenue. A $1,000 exemption for each legally blind taxpayer. Checked 2026-10-12.

Part-year and nonresident returns

Part-year resident (Schedule NR): the tax on the state's own income (what was received while a resident, plus the state-source income while a nonresident), with the deductions and exemptions the form prorates scaled by its ratio.
Nonresident (Schedule NR): the tax on the state's own income (what was received while a resident, plus the state-source income while a nonresident), with the deductions and exemptions the form prorates scaled by its ratio.
The ratio: the state's own AGI (after its subtractions) of the state's income over the state AGI of the whole year, rounded to 3 places, at most 1.
Prorated on a part-year return: the standard deduction, the personal exemptions, the age-65 subtraction or exemption, dependent exemptions.
Prorated on a nonresident return: the standard deduction, the personal exemptions, the age-65 subtraction or exemption, dependent exemptions.
A nonresident's retirement income is not taxable by the state (4 U.S.C. §114); the API refuses it in residency.stateIncome. The credit for tax paid to another state and the earned income credits on these returns are not computed.

Married couples whose residency differs (engine 1.13.0)

With residency.spouse on a joint request, each allowed treatment is calculated and reported in residency.couple; tax is the required one, else the state's default, else the couple's residency.election, else the joint return the request's mfj names. Each rule is quoted from the state's instructions (engine 1.14.0: and its statutes, regulations, bulletins and other official guidance) in the sources below.

Spouses	Illinois's rule
One spouse a full-year resident, the other a part-year resident	separate returns, unless the couple elects a joint resident return (both taxed as full-year residents).
One spouse a full-year resident, the other a nonresident	separate returns, unless the couple elects a joint resident return (both taxed as full-year residents).
One spouse a part-year resident, the other a nonresident	a joint part-year / nonresident return is required.
Both part-year residents, different resident periods	a joint part-year / nonresident return is required.
Both nonresidents, only one with income from the state	a joint part-year / nonresident return is required.

Schedule NR IL-1040 Instructions 2025, Illinois Department of Revenue. Part-year / nonresident return (engine 1.12.0, residency): Checked 2026-10-12.
IL-1040 Schedule NR 2025 (R-12/25), Illinois Department of Revenue. Part-year / nonresident return (engine 1.12.0, residency): Checked 2026-10-12.
IL-1040 Instructions 2025, Illinois Department of Revenue. Part-year / nonresident return (engine 1.12.0, residency): Checked 2026-10-12.
2025 Form IL-1040 Instructions, Illinois Department of Revenue. Married couples whose residency differs (engine 1.13.0): Checked 2026-10-12.
2025 Schedule NR Instructions (Nonresident and Part-Year Resident Computation of Illinois Tax), Illinois Department of Revenue. Married couples whose residency differs (engine 1.13.0): Checked 2026-10-12.

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.

Household (engine inputs)	State tax
Single, 67: $30,000 private pension, $20,000 IRA, $10,000 interest, $24,000 Social Security ($17,000 federally taxable), $5,000 long-term gain	$548.21
Married filing jointly, 70: $40,000 public pension, $40,000 IRA, $20,000 interest, $40,000 Social Security ($34,000 federally taxable), $10,000 long-term gain	$1,096.43
Single, 60: $60,000 other ordinary income and a $50,000 Roth conversion	$2,825.21
Married filing jointly, 66: $30,000 IRA, a $40,000 Roth conversion, $30,000 Social Security ($25,500 federally taxable)	$0
Single, 58: $36,000 military retirement pay and $30,000 wages	$1,340.21
Married filing jointly, ages 67 and 61: $50,000 IRA, $10,000 interest, $30,000 Social Security ($25,500 federally taxable), each spouse's age given	$155.93

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.

Single, 67: $30,000 private pension, $20,000 IRA, $10,000 interest, $24,000 Social Security ($17,000 federally taxable), $5,000 long-term gain: $548.21

Line	Amount
Income total used by the income tests	$82,000
Retirement income and conversions excluded (not in the base)	$50,000
+ Other ordinary income in the base	$10,000
+ Long-term gain in the base	$5,000
− Standard deduction	$2,925
− Age subtractions and deductions	$1,000
= State taxable income	$11,075
Tax from the rate schedule	$548.21
= Tax before credits	$548.21
= Tax	$548.21

Married filing jointly, 70: $40,000 public pension, $40,000 IRA, $20,000 interest, $40,000 Social Security ($34,000 federally taxable), $10,000 long-term gain: $1,096.43

Line	Amount
Income total used by the income tests	$144,000
Retirement income and conversions excluded (not in the base)	$80,000
+ Other ordinary income in the base	$20,000
+ Long-term gain in the base	$10,000
− Standard deduction	$5,850
− Age subtractions and deductions	$2,000
= State taxable income	$22,150
Tax from the rate schedule	$1,096.43
= Tax before credits	$1,096.43
= Tax	$1,096.43

Single, 60: $60,000 other ordinary income and a $50,000 Roth conversion: $2,825.21

Line	Amount
Income total used by the income tests	$110,000
Retirement income and conversions excluded (not in the base)	$50,000
+ Other ordinary income in the base	$60,000
− Standard deduction	$2,925
= State taxable income	$57,075
Tax from the rate schedule	$2,825.21
= Tax before credits	$2,825.21
= Tax	$2,825.21

Married filing jointly, 66: $30,000 IRA, a $40,000 Roth conversion, $30,000 Social Security ($25,500 federally taxable): $0

Line	Amount
Income total used by the income tests	$95,500
Retirement income and conversions excluded (not in the base)	$70,000
− Standard deduction	$5,850
− Age subtractions and deductions	$2,000
= State taxable income	$0
Tax from the rate schedule	$0
= Tax	$0

Single, 58: $36,000 military retirement pay and $30,000 wages: $1,340.21

Line	Amount
Income total used by the income tests	$66,000
Military pay excluded (not in the base)	$36,000
+ Other ordinary income in the base	$30,000
− Standard deduction	$2,925
= State taxable income	$27,075
Tax from the rate schedule	$1,340.21
= Tax before credits	$1,340.21
= Tax	$1,340.21

Married filing jointly, ages 67 and 61: $50,000 IRA, $10,000 interest, $30,000 Social Security ($25,500 federally taxable), each spouse's age given: $155.93

Line	Amount
Income total used by the income tests	$85,500
Retirement income and conversions excluded (not in the base)	$50,000
+ Other ordinary income in the base	$10,000
− Standard deduction	$5,850
− Age subtractions and deductions	$1,000
= State taxable income	$3,150
Tax from the rate schedule	$155.93
= Tax before credits	$155.93
= Tax	$155.93

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).

Amounts (engine fields)	After 2026	What the law says, and the source
`std_deduction_single`, `std_deduction_mfj`, `dep_exemption`	Indexed: CPI-U, average of the 12 months ending August 31 of the preceding calendar year over calendar 2011 (35 ILCS 5/204(d-5)); enacted only through taxable years ending 12/31/2028; the increase rounded down to a multiple of $25.	35 ILCS 5/204(b)(7),(d-5): for taxable years ending on or before December 31, 2028 the basic exemption (per taxpayer, spouse and dependent) is $2,050 plus a cost-of-living adjustment: $2,050 times the rise in CPI-U (average of the 12 months ending August 31 of the preceding year over 2011), the adjustment rounded down to $25 ($2,925 for 2026). The statute sets NO amount for taxable years ending after December 31, 2028 (item (7) expires then), so the value after 2028 depends on future legislation. 35 ILCS 5/204 (Standard exemption), Wayback copy 2025-09-05 (Illinois General Assembly, statute): Indexing clause, $25 rounding and the 2028 end date. Quoted: "2024 and on or before December 31, 2028, $2,050 plus the cost-of-living adjustment under subsection (d-5)."; "If any cost-of-living adjustment is not a multiple of $25, that adjustment shall be rounded to the next lowest multiple of $25."; "The term "Consumer Price Index" means the last Consumer Price Index for All Urban Consumers". Checked 2026-09-30. Informational Bulletin FY 2026-15 (Illinois Department of Revenue, agency_page): 2026 exemption $2,925. Quoted: "The personal exemption amount for tax year 2026 will increase to $2,925.". Checked 2026-10-12.
`senior_subtraction_single`, `senior_subtraction_mfj`, `blind_exemption`	Fixed in law: held at the rules-year amount in every later year.	35 ILCS 5/204(d): an additional $1,000 per taxpayer (and qualifying spouse) 65 or older and $1,000 per blind taxpayer; fixed amounts. 35 ILCS 5/204 (Standard exemption), Wayback copy 2025-09-05 (Illinois General Assembly, statute): Fixed $1,000. Quoted: "$1,000 for the taxpayer if he or she has attained the age of 65 before the end of the taxable year."; "$1,000 for the taxpayer if he or she is blind at the end of the taxable year.". Checked 2026-09-30.
`senior_subtraction_min_age`	Attained age: the same age in every year.	Attained age 65 before the end of the taxable year: a fixed attained age. 35 ILCS 5/204 (Standard exemption), Wayback copy 2025-09-05 (Illinois General Assembly, statute): Age 65. Quoted: "has attained the age of 65 before the end of the taxable year". Checked 2026-09-30.
`exemption_agi_cliff_single`, `exemption_agi_cliff_mfj`, `dep_exemption_agi_limit_single`, `dep_exemption_agi_limit_mfj`	Fixed in law: held at the rules-year amount in every later year.	35 ILCS 5/204(g): no exemption when AGI exceeds $500,000 (joint) or $250,000 (others); fixed. 35 ILCS 5/204 (Standard exemption), Wayback copy 2025-09-05 (Illinois General Assembly, statute): Fixed AGI limits. Quoted: "no taxpayer may claim an exemption under this Section if the taxpayer's adjusted gross income for the taxable year exceeds (i) $500,000". Checked 2026-09-30.
`cap_loss_limit`, `cap_loss_limit_mfs`	Fixed in law: held at the rules-year amount in every later year.	The state follows the federal net capital loss limit of IRC 1211(b): $3,000 ($1,500 MFS), a fixed amount with no inflation adjustment. 26 U.S.C. 1211 (limitation on capital losses) (Cornell LII (U.S. Code), federal): The capital-loss deduction against ordinary income is a fixed $3,000 ($1,500 MFS); section 1211 contains no inflation adjustment. Quoted: "$3,000 ($1,500 in the case of a married individual filing a separate return), or". Checked 2026-10-12.
`pt_credit_agi_limit`, `pt_credit_agi_limit_mfj`	Fixed in law: held at the rules-year amount in every later year.	35 ILCS 5/208: no property tax credit above $250,000 of AGI ($500,000 joint); fixed amounts. IL-1040 Instructions (Illinois Department of Revenue, instructions): The AGI limits. Quoted: "is greater than $500,000, you are not entitled to a property tax credit"; "$250,000, you are not entitled to a property tax credit or a K-12". Checked 2026-10-12.

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.
The child tax credit is refundable and is not calculated. Sources: Informational Bulletin FY 2026-15, What's New for Illinois Income Taxes (checked 2026-10-12); 2025 Form IL-1040 Instructions (checked 2026-10-12); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-10-12); 2025 Schedule IL-E/EITC Illinois Exemption and Earned Income Tax Credit (checked 2026-10-12); 2025 Schedule ICR Illinois Credits (checked 2026-10-12).

What the engine does not calculate for any state (itemized deductions, credits beyond those listed, part-year residents, and more) is listed on the calculation order page.

Verification

Worksheet-tested backend/tests/test_state_worksheets_1.c 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.
Structural tests covering every state: 10.
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.