Montana state income tax: engine rules, 2026

How the tax is computed

Every step the engine takes for Montana, 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 Montana'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.
   Montana's values · engine step (select_brackets). Sources (Rate schedule): MCA 15-30-2103 Rate of tax -- net long-term capital gains -- definitions (Montana Code Annotated 2025; Wayback Machine capture of mca.legmt.gov, 2026-05-12) (checked 2026-09-28); HB 337: 2026-2027 Montana Individual Income Tax Changes (Wayback Machine capture of revenue.mt.gov, 2026-09-09) (checked 2026-10-12); Montana tax simplification resource hub (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)
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).
   Montana's values · engine step (select_brackets). Sources (Rate schedule): MCA 15-30-2103 Rate of tax -- net long-term capital gains -- definitions (Montana Code Annotated 2025; Wayback Machine capture of mca.legmt.gov, 2026-05-12) (checked 2026-09-28); HB 337: 2026-2027 Montana Individual Income Tax Changes (Wayback Machine capture of revenue.mt.gov, 2026-09-09) (checked 2026-10-12); Montana tax simplification resource hub (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)
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.
   Montana's values · engine step (select_brackets). Sources (Rate schedule): MCA 15-30-2103 Rate of tax -- net long-term capital gains -- definitions (Montana Code Annotated 2025; Wayback Machine capture of mca.legmt.gov, 2026-05-12) (checked 2026-09-28); HB 337: 2026-2027 Montana Individual Income Tax Changes (Wayback Machine capture of revenue.mt.gov, 2026-09-09) (checked 2026-10-12); Montana tax simplification resource hub (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)
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.
   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. 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.
8. 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.
9. 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.
   Montana's values · engine step (compute_state_tax_detail). Sources (Military retirement pay): Wmre (checked 2026-09-28); Working military retirement exemption (checked 2026-09-27)
10. 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).
    Montana's values · engine step (compute_state_tax_detail). Sources (Capital gains): MCA 15-30-2103 Rate of tax -- net long-term capital gains -- definitions (Wayback Machine capture, 2026-05-12) (checked 2026-09-28); HB 337: 2026-2027 Montana Individual Income Tax Changes (Wayback Machine capture, 2026-09-09) (checked 2026-10-12); 2025 Montana Individual Income Tax Return (Form 2) (archived) (checked 2026-10-12); 2025 Schedule D (Form 1040), Capital Gains and Losses (checked 2026-10-12)
11. Which Social Security threshold applies Joint returns use the joint threshold; head of household uses it too where the state says so, otherwise the single one; married filing separately uses its own threshold where set, else the single one. Benefits are tested against the income total above.
    Montana's values · engine step (ss_amount_in_state_base). Sources (Social Security): Montana Tax Simplification Resource Hub (Wayback Machine capture of revenue.mt.gov, 2026-05-14) (checked 2026-10-12)
12. Social Security: the federal amount The federally taxable amount enters the base as it is.
    Montana's values · engine step (ss_amount_in_state_base). Sources (Social Security): Montana Tax Simplification Resource Hub (Wayback Machine capture of revenue.mt.gov, 2026-05-14) (checked 2026-10-12)
13. 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.
    Montana's values · engine step (compute_state_tax_detail). Sources (IRA, 401(k) and private pensions): Montana Tax Simplification Resource Hub (Wayback Machine capture of revenue.mt.gov, 2026-05-14) (checked 2026-10-12); 2025 Montana Form 2 Individual Income Tax Return (Wayback Machine capture of revenuefiles.mt.gov, 2026-01-05) (checked 2026-10-12)
14. 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.
    Montana's values · engine step (compute_state_tax_detail). Sources (Roth conversions): MCA 15-30-2120 Adjustments to federal taxable income to determine Montana taxable income (Wayback Machine capture, 2026-06-11) (checked 2026-09-28)
15. 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.
    Montana's values · engine step (compute_state_tax_detail). Sources (Capital gains): MCA 15-30-2103 Rate of tax -- net long-term capital gains -- definitions (Wayback Machine capture, 2026-05-12) (checked 2026-09-28); HB 337: 2026-2027 Montana Individual Income Tax Changes (Wayback Machine capture, 2026-09-09) (checked 2026-10-12); 2025 Montana Individual Income Tax Return (Form 2) (archived) (checked 2026-10-12); 2025 Schedule D (Form 1040), Capital Gains and Losses (checked 2026-10-12)
16. Additional standard deduction at 65 The unmarried amount for a single or head-of-household filer 65 or older, the married amount for each spouse 65 or older, added to the standard deduction before any phase-out.
    Montana's values · engine step (compute_state_tax_detail). Sources (Deductions, exemptions and credits): Montana Tax Simplification Resource Hub (Wayback Machine capture of revenue.mt.gov, 2026-05-14) (checked 2026-10-12); 0150 0300 0210 0030 (checked 2026-09-28); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-10-12); 2025 Montana Individual Income Tax Return Form 2 (checked 2026-10-12); 2024 Montana Form 2 Instructions (latest instructions retrievable; 2025 instructions not archived) (checked 2026-10-12); 2025 Montana Form 2 Instructions (Schedule 2EC pp. 38-41) (checked 2026-10-12); MCA 15-30-2318 (Internet Archive copy) (checked 2026-10-12); 2025 Montana Form 2 Instructions (Internet Archive copy) (checked 2026-10-01)
17. Additional standard deduction for blindness only when the caller says a person is blind or disabled The unmarried amount for a blind single or head-of-household filer, the married amount for each blind spouse, added to the standard deduction before any phase-out. Age and blindness are separate boxes, so a person 65 or older and blind gets both.
    Montana's values · engine step (compute_state_tax_detail). Sources (Blindness and disability): Rev. Proc. 2025-32 (2026 inflation adjustments) (checked 2026-10-12)
18. 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.
    Montana's values · engine step (compute_state_tax_detail). Sources (Age 65+ subtractions and credits): 2025 Montana Form 2 Individual Income Tax Return (Wayback Machine capture, 2026-01-05) (checked 2026-10-12); MCA 15-30-2120 (Wayback Machine capture, 2026-06-11) (checked 2026-09-28); Rev. Proc. 2025-32 (2026 inflation adjustments) (checked 2026-10-12); Schedule 1-A (Form 1040), Additional Deductions (checked 2026-10-12); 2025 Montana Form 2 (archived copy) (checked 2026-10-12)
19. Federal senior deduction taken against the state base For each taxpayer 65 or older: the amount, reduced by the rate times the income total above the status's start (joint start on a joint return); none for married filing separately. In the states whose base is federal taxable income it is taken only when the caller does not supply that figure (it is already in it).
    Montana's values · engine step (compute_state_tax_detail). Sources (Age 65+ subtractions and credits): 2025 Montana Form 2 Individual Income Tax Return (Wayback Machine capture, 2026-01-05) (checked 2026-10-12); MCA 15-30-2120 (Wayback Machine capture, 2026-06-11) (checked 2026-09-28); Rev. Proc. 2025-32 (2026 inflation adjustments) (checked 2026-10-12); Schedule 1-A (Form 1040), Additional Deductions (checked 2026-10-12); 2025 Montana Form 2 (archived copy) (checked 2026-10-12)
20. 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.
21. Starting from federal taxable income only when the caller supplies federal taxable income (the planner; not the API) When the caller supplies federal taxable income (the planner does; the API does not), the base starts from it, adds the gains in it (less the state's gain subtraction), and subtracts the state's own exclusions and subtractions; the state's deduction is not subtracted again.
    Montana's values · engine step (compute_state_tax_detail). Sources (Interaction with the federal return): MCA 15-30-2120 Adjustments to federal taxable income to determine Montana taxable income (Montana Code Annotated 2025; Wayback Machine capture of mca.legmt.gov, 2026-06-11) (checked 2026-09-28); 2025 Montana Form 2 Individual Income Tax Return (Wayback Machine capture of revenuefiles.mt.gov, 2026-01-05) (checked 2026-10-12)
22. 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.
23. Separate-return scaling married filing separately For the halved-joint rule, taxable income is doubled before the schedule and the result halved.
    Montana's values · engine step (compute_state_tax_detail). Sources (Rate schedule): MCA 15-30-2103 Rate of tax -- net long-term capital gains -- definitions (Montana Code Annotated 2025; Wayback Machine capture of mca.legmt.gov, 2026-05-12) (checked 2026-09-28); HB 337: 2026-2027 Montana Individual Income Tax Changes (Wayback Machine capture of revenue.mt.gov, 2026-09-09) (checked 2026-10-12); Montana tax simplification resource hub (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)
24. 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.
    Montana's values · engine step (apply_state_brackets). Sources (Rate schedule): MCA 15-30-2103 Rate of tax -- net long-term capital gains -- definitions (Montana Code Annotated 2025; Wayback Machine capture of mca.legmt.gov, 2026-05-12) (checked 2026-09-28); HB 337: 2026-2027 Montana Individual Income Tax Changes (Wayback Machine capture of revenue.mt.gov, 2026-09-09) (checked 2026-10-12); Montana tax simplification resource hub (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)
25. Own schedule for long-term gains The gain in the base (at most the taxable income) is taken out of the schedule and taxed at the lower gains rate up to the first ordinary bracket's top, stacked on the ordinary income, and at the upper rate above; this replaces the schedule part of the tax, and amounts added on top of the schedule before it (a fixed amount, recapture) are kept.
    Montana's values · engine step (compute_state_tax_detail). Sources (Capital gains): MCA 15-30-2103 Rate of tax -- net long-term capital gains -- definitions (Wayback Machine capture, 2026-05-12) (checked 2026-09-28); HB 337: 2026-2027 Montana Individual Income Tax Changes (Wayback Machine capture, 2026-09-09) (checked 2026-10-12); 2025 Montana Individual Income Tax Return (Form 2) (archived) (checked 2026-10-12); 2025 Schedule D (Form 1040), Capital Gains and Losses (checked 2026-10-12)
26. 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.
27. 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).
    Montana's values · engine step (state_tax_effective_config). Sources (Rate schedule): MCA 15-30-2103 Rate of tax -- net long-term capital gains -- definitions (Montana Code Annotated 2025; Wayback Machine capture of mca.legmt.gov, 2026-05-12) (checked 2026-09-28); HB 337: 2026-2027 Montana Individual Income Tax Changes (Wayback Machine capture of revenue.mt.gov, 2026-09-09) (checked 2026-10-12); Montana tax simplification resource hub (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)
28. 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.
    Montana's values · engine step (compute_state_tax_detail). Sources (Social Security): Montana Tax Simplification Resource Hub (Wayback Machine capture of revenue.mt.gov, 2026-05-14) (checked 2026-10-12)
29. 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.
    Montana's values · engine step (compute_state_tax_detail). Sources (Capital gains): MCA 15-30-2103 Rate of tax -- net long-term capital gains -- definitions (Wayback Machine capture, 2026-05-12) (checked 2026-09-28); HB 337: 2026-2027 Montana Individual Income Tax Changes (Wayback Machine capture, 2026-09-09) (checked 2026-10-12); 2025 Montana Individual Income Tax Return (Form 2) (archived) (checked 2026-10-12); 2025 Schedule D (Form 1040), Capital Gains and Losses (checked 2026-10-12)
30. 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).
    Montana's values · engine step (compute_state_tax_detail). Sources (Social Security): Montana Tax Simplification Resource Hub (Wayback Machine capture of revenue.mt.gov, 2026-05-14) (checked 2026-10-12)
31. 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).
    Montana's values · engine step (state_tax_refundable_credits). Sources (Deductions, exemptions and credits): Montana Tax Simplification Resource Hub (Wayback Machine capture of revenue.mt.gov, 2026-05-14) (checked 2026-10-12); 0150 0300 0210 0030 (checked 2026-09-28); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-10-12); 2025 Montana Individual Income Tax Return Form 2 (checked 2026-10-12); 2024 Montana Form 2 Instructions (latest instructions retrievable; 2025 instructions not archived) (checked 2026-10-12); 2025 Montana Form 2 Instructions (Schedule 2EC pp. 38-41) (checked 2026-10-12); MCA 15-30-2318 (Internet Archive copy) (checked 2026-10-12); 2025 Montana Form 2 Instructions (Internet Archive copy) (checked 2026-10-01)

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 Montana'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 senior subtraction is the 2025 indexed amount ($5,660), used for 2026 until the 2026 figure is published.
• The working military retirement exemption (the lesser of half the military retirement pay or qualifying Montana income, for up to five consecutive years) is not calculated, nor its counterpart for military survivor benefits (up to half of a Survivor Benefit Plan annuity): both depend on residency and work history the inputs do not carry. Sources: Wmre (checked 2026-09-28); Working military retirement exemption (checked 2026-09-27).
• Montana has no dependent exemption and no nonrefundable child credit: nothing to calculate. Sources: Montana Tax Simplification Resource Hub (Wayback Machine capture of revenue.mt.gov, 2026-05-14) (checked 2026-10-12); 0150 0300 0210 0030 (checked 2026-09-28); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-10-12); 2025 Montana Individual Income Tax Return Form 2 (checked 2026-10-12); 2024 Montana Form 2 Instructions (latest instructions retrievable; 2025 instructions not archived) (checked 2026-10-12); 2025 Montana Form 2 Instructions (Schedule 2EC pp. 38-41) (checked 2026-10-12); MCA 15-30-2318 (Internet Archive copy) (checked 2026-10-12); 2025 Montana Form 2 Instructions (Internet Archive copy) (checked 2026-10-01).
• Schedule I adjustments for income the inputs do not separate are not calculated: interest from other states' municipal bonds (added), and state income tax refunds, interest from federal obligations, and Montana medical savings, 529 and ABLE account deposits (subtracted). Sources: Montana Tax Simplification Resource Hub (Wayback Machine capture of revenue.mt.gov, 2026-05-14) (checked 2026-10-12); 0150 0300 0210 0030 (checked 2026-09-28); Irs releases tax inflation adjustments for tax year 2026 including amendments from the one big beautiful bill (checked 2026-10-12); 2025 Montana Individual Income Tax Return Form 2 (checked 2026-10-12); 2024 Montana Form 2 Instructions (latest instructions retrievable; 2025 instructions not archived) (checked 2026-10-12); 2025 Montana Form 2 Instructions (Schedule 2EC pp. 38-41) (checked 2026-10-12); MCA 15-30-2318 (Internet Archive copy) (checked 2026-10-12); 2025 Montana Form 2 Instructions (Internet Archive copy) (checked 2026-10-01).
• Federal deductions the inputs do not carry are not subtracted: Montana starts from federal taxable income, so the federal Schedule 1-A deductions for tips, overtime and car-loan interest (and itemized deductions) would lower it; only the standard deduction, the age and blindness amounts and the senior deduction are taken. Sources: MCA 15-30-2120 Adjustments to federal taxable income to determine Montana taxable income (Montana Code Annotated 2025; Wayback Machine capture of mca.legmt.gov, 2026-06-11) (checked 2026-09-28); 2025 Montana Form 2 Individual Income Tax Return (Wayback Machine capture of revenuefiles.mt.gov, 2026-01-05) (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 A golden row follows Form 2 line 3 (federal taxable income less federal deductions).

• Golden scenarios (hand-computed expected tax, asserted to within $1): M10.
• State-specific unit tests: test_mt_reduced_ltcg_rates, test_compute_federal_ss_parts, test_mt, test_hoh_federal_deduction_states, test_conformity_component_path, test_gains_schedules_keep_addons.
• Structural tests covering every state: 10.
• State forms and worksheets followed in the test derivations: Form 2 line 3.