Idaho state income tax: engine rules, 2026

How the tax is computed

Every step the engine takes for Idaho, 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 Idaho'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. 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.
   Idaho's values · engine step (select_brackets). Sources (Rate schedule): Individual Income Tax Rate Schedule (checked 2026-09-27); Idaho Code 63-3024 (checked 2026-09-27); SECT63 3024 (checked 2026-09-27)
3. 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).
   Idaho's values · engine step (select_brackets). Sources (Rate schedule): Individual Income Tax Rate Schedule (checked 2026-09-27); Idaho Code 63-3024 (checked 2026-09-27); SECT63 3024 (checked 2026-09-27)
4. 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.
   Idaho's values · engine step (select_brackets). Sources (Rate schedule): Individual Income Tax Rate Schedule (checked 2026-09-27); Idaho Code 63-3024 (checked 2026-09-27); SECT63 3024 (checked 2026-09-27)
5. The income total every income test uses Other ordinary income + retirement distributions (before any exclusion) + conversion + federally taxable Social Security + long-term gains. 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.
6. 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, 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, 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.
7. 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.
8. 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.
   Idaho's values · engine step (compute_state_tax_detail). Sources (Military retirement pay): EFO00088 09 29 2025 (checked 2026-09-27); Sect63 3022a (checked 2026-09-27)
9. 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.
   Idaho's values · engine step (military_exempt_one). Sources (Military retirement pay): EFO00088 09 29 2025 (checked 2026-09-27); Sect63 3022a (checked 2026-09-27)
10. One military cap per joint return joint returns On a joint return the cap is one amount for the couple, over the pay of the spouses who pass the age test, reduced by the household's Social Security where the rule says so, then the income test.
    Idaho's values · engine step (compute_state_tax_detail). Sources (Military retirement pay): EFO00088 09 29 2025 (checked 2026-09-27); Sect63 3022a (checked 2026-09-27)
11. Social Security exempt None of the federally taxable benefit enters the base.
    Idaho's values · engine step (ss_amount_in_state_base). Sources (Social Security): unsourced: No official Idaho source could be fetched: tax.idaho.gov and legislature.idaho.gov (and every *.idaho.gov host tried) refuse connections from this host. No secondary source for Idaho's Social Security exemption was verified either.
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.
    Idaho's values · engine step (compute_state_tax_detail). Sources (IRA, 401(k) and private pensions): unsourced: No official page states in terms that Idaho has no general retirement exclusion; Idaho Code 63-3022A (see Public pensions and Military retirement pay) lists the only retirement benefits Idaho deducts.
13. 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.
    Idaho's values · engine step (compute_state_tax_detail). Sources (Roth conversions): unsourced: No official Idaho source could be fetched: tax.idaho.gov and legislature.idaho.gov (and every *.idaho.gov host tried) refuse connections from this host.
14. 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.
    Idaho's values · engine step (compute_state_tax_detail). Sources (Capital gains): unsourced: No official Idaho source could be fetched: tax.idaho.gov and legislature.idaho.gov (and every *.idaho.gov host tried) refuse connections from this host.
15. Public pension not covered by Social Security only when the caller says the pension was not covered by Social Security Only when the caller says the public pension was not covered: the smaller of the public pension still in the base and the cap, reduced above the income start (by a percentage per step, or linearly over a range). Where it is an election instead of the Social Security exclusion, the engine keeps whichever excludes more.
    Idaho's values · engine step (compute_state_tax_detail). Sources (Public pensions): Idaho Code 63-3022A (archived copy) (checked 2026-09-27); EFO00088 09 29 2025 (checked 2026-09-27)
16. Non-covered pension: age test and shared cap only when the caller says the pension was not covered by Social Security Where the state sets them: only a recipient at or above the age qualifies (each spouse's own public pension in per-person mode); the cap is reduced by the Social Security received (gross when supplied) and by the military pay the same deduction already excluded.
    Idaho's values · engine step (compute_state_tax_detail). Sources (Public pensions): Idaho Code 63-3022A (archived copy) (checked 2026-09-27); EFO00088 09 29 2025 (checked 2026-09-27)
17. 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.
    Idaho's values · engine step (compute_state_tax_detail). Sources (Deductions, exemptions and credits): Individual income tax rate schedule (checked 2026-09-27); SECT63 3024 (checked 2026-09-27); 2025 Idaho Form 40 instructions (archived copy) (checked 2026-09-27); Rev. Proc. 2025-32 (2026 inflation adjustments) (checked 2026-09-27)
18. 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.
    Idaho's values · engine step (compute_state_tax_detail). Sources (Age 65+ subtractions and credits): 2025 Idaho Form 40 instructions (archived copy) (checked 2026-09-27); Idaho Code 63-3004 (archived copy) (checked 2026-09-27); 2025 Schedule 1-A (Form 1040), Part V Enhanced Deduction for Seniors (checked 2026-09-27); Rev. Proc. 2025-32 (2026 inflation adjustments) (checked 2026-09-27)
19. 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.
20. No tax at or below zero State taxable income at or below zero returns a tax of 0 (credits are not refundable and nothing below applies).
    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.
21. Separate-return scaling married filing separately For the halved-joint rule, taxable income is doubled before the schedule and the result halved.
    Idaho's values · engine step (compute_state_tax_detail). Sources (Rate schedule): Individual Income Tax Rate Schedule (checked 2026-09-27); Idaho Code 63-3024 (checked 2026-09-27); SECT63 3024 (checked 2026-09-27)
22. 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.
    Idaho's values · engine step (apply_state_brackets). Sources (Rate schedule): Individual Income Tax Rate Schedule (checked 2026-09-27); Idaho Code 63-3024 (checked 2026-09-27); SECT63 3024 (checked 2026-09-27)
23. 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.

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.

What the engine does not calculate, and known approximations

• The 62-and-disabled route is not modeled (no disability input). The engine reads a public pension flagged as not covered by Social Security as a qualifying CSRS or Idaho police/firefighter pension; otherwise public pensions are taxed like other retirement income. (Public pensions)
• Idaho's capital gains deduction for certain Idaho property is not modeled; engine taxes LTCG as ordinary income. (Capital gains)

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

Verification

Statute and instructions cited Rate and the 0% band cited to Idaho Code 63-3024; one golden row.

• Golden scenarios (hand-computed expected tax, asserted to within $1): XID1.
• State-specific unit tests: test_military_tiers, test_military_per_return_caps, test_hoh_federal_deduction_states, test_id_aged_and_senior_deduction, test_id_civil_service_deduction.
• Structural tests covering every state: 2.