District of Columbia state income tax: engine rules, 2026

How the tax is computed

Every step the engine takes for District of Columbia, 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 District of Columbia'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.
   District of Columbia's values · engine step (select_brackets). Sources (Rate schedule): 2026 D-40ES Estimated Payment for Individual Income Tax (booklet) (checked 2026-09-27); D.C. Code §47-1806.03 - Tax on residents and nonresidents: imposition and rates (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).
   District of Columbia's values · engine step (select_brackets). Sources (Rate schedule): 2026 D-40ES Estimated Payment for Individual Income Tax (booklet) (checked 2026-09-27); D.C. Code §47-1806.03 - Tax on residents and nonresidents: imposition and rates (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.
   District of Columbia's values · engine step (select_brackets). Sources (Rate schedule): 2026 D-40ES Estimated Payment for Individual Income Tax (booklet) (checked 2026-09-27); D.C. Code §47-1806.03 - Tax on residents and nonresidents: imposition and rates (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.
   District of Columbia's values · engine step (compute_state_tax_detail). Sources (Military retirement pay): 47 1803.02 (checked 2026-09-27)
9. Social Security exempt None of the federally taxable benefit enters the base.
   District of Columbia's values · engine step (ss_amount_in_state_base). Sources (Social Security): 2025 D-40 Booklet (checked 2026-09-27); D.C. Code §47-1803.02 - Gross income: items included and excluded (checked 2026-09-27); 2026 D40ES Book wLinks04012026 (checked 2026-09-27)
10. 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.
    District of Columbia's values · engine step (compute_state_tax_detail). Sources (IRA, 401(k) and private pensions): 2025 D-40 Booklet (checked 2026-09-27)
11. 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.
    District of Columbia's values · engine step (compute_state_tax_detail). Sources (Roth conversions): 2025 D-40 Booklet (checked 2026-09-27)
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.
    District of Columbia's values · engine step (compute_state_tax_detail). Sources (Capital gains): 2025 D-40 Booklet (checked 2026-09-27); 2026 D40ES Book wLinks04012026 (checked 2026-09-27)
13. 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.
    District of Columbia's values · engine step (compute_state_tax_detail). Sources (Deductions, exemptions and credits): 2026 D-40ES Estimated Payment for Individual Income Tax (booklet) (checked 2026-09-27); D.C. Code §47-1801.04 - General definitions (basic standard deduction) (checked 2026-09-27); 2025 D-40 Booklet (checked 2026-09-27)
14. 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.
15. 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.
16. Separate-return scaling married filing separately For the halved-joint rule, taxable income is doubled before the schedule and the result halved.
    District of Columbia's values · engine step (compute_state_tax_detail). Sources (Rate schedule): 2026 D-40ES Estimated Payment for Individual Income Tax (booklet) (checked 2026-09-27); D.C. Code §47-1806.03 - Tax on residents and nonresidents: imposition and rates (checked 2026-09-27)
17. 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.
    District of Columbia's values · engine step (apply_state_brackets). Sources (Rate schedule): 2026 D-40ES Estimated Payment for Individual Income Tax (booklet) (checked 2026-09-27); D.C. Code §47-1806.03 - Tax on residents and nonresidents: imposition and rates (checked 2026-09-27)
18. 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

• DC and federal government survivor benefits for survivors aged 62+ remain excludable (§47-1803.02, D-40 line 12); the engine does not model survivor benefits. (Public pensions)

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

Worksheet-tested An engine 1.2.0 test follows the 2026 D-40ES head-of-household deduction; golden rows pin the schedule.

• Golden scenarios (hand-computed expected tax, asserted to within $1): F06, X05, DCMFS1.
• State-specific unit tests: test_dc_is_included, test_dc_hoh_deduction, test_dc_aged_additional_deduction.
• Structural tests covering every state: 2.
• State forms and worksheets followed in the test derivations: D-40ES (2026).