Methodology & sources

Federal — IRS Publication 15-T (2026)

The federal engine implements Worksheet 1A and the Percentage Method Tables for Automated Payroll Systems (Publication 15-T, §1, p. 11) verbatim, for all six schedule sets: single/married-filing-separately, married filing jointly, and head of household, each with and without the Step 2 checkbox. The printed tables' base amounts are used exactly as printed — including the two places where the printed base differs by cents from a continuous recomputation (Single, Step 2 checked, at $108,938 and $328,350 of adjusted annual wages); the printed bases govern, because they are what the IRS published. FICA uses the Social Security Administration's 2026 wage base of $184,500 and the statutory 6.2% / 1.45% / 0.9% rates. Supplemental wages use the Publication 15 flat rates: 22% up to $1,000,000 of cumulative supplemental wages, 37% above.

Rounding contract: full precision is kept through the worksheet; only the final per-check federal amount is rounded to cents. Withholding is always labeled as withholding — an estimate of prepayment — never as your tax bill.

States — each from its own publication

All 51 jurisdictions (50 states + DC) are pinned to their own official employer's guide, formula publication, or statute for tax year 2026. Where a state offers both wage-bracket tables and a percentage method, the percentage/formula method is implemented and the tables were used as a cross-check. State inputs mirror each state's own withholding form. Local taxes are computed only where they are part of the pinned state method set (Maryland counties, Indiana counties, New York City, Yonkers, Philadelphia, California SDI, the Oregon statewide transit tax); the big deferred sets — Ohio municipalities, Pennsylvania Act 32 earned income tax, Michigan city taxes — are named as not-yet-computed on the pages they affect.

Where a publication contradicts itself

Most jurisdictions' publications agree with themselves. These do not, and the choice this calculator made is disclosed here rather than hidden:

• North Dakota — the 2026 booklet's worked example (weekly $1,800, single) prints $14.00 a week, but its own Section 2 percentage table computes $701.51 a year, which is $13 a week under the worksheet's rounding. The example appears to be a stale carryover. This calculator follows the table.
• New York — the annual exact-calculation method and the per-period computation tables in NYS-50-T-NYS differ by cents because the tables' per-period amounts and scaled bases are independently rounded ($7.99 vs $8.01 a week on the publication's own example inputs). This calculator uses the annual method throughout.
• New Mexico — FYI-104's wage-bracket table and its Table 7 percentage method differ by 2 cents on its own example pattern ($21.80 vs $21.78). This calculator uses Table 7; the bracket tables are not reproducible from published parameters.
• Oregon — the formula publication contradicts itself in four places; the worked example confirms the formula pages' figures (including the $8,750 federal-withholding cap), and those are the ones used here.
• Mississippi — Publication 89-700 publishes wage-bracket tables only, with no formula and no worked example. This calculator computes from the exemption and standard-deduction structure the tables are built from, and the result was verified against the printed Table A at mid-table wages before shipping.
• New Jersey — NJ-WT's printed worked examples sit on an image-only page of the official PDF. The New Jersey cases in the test suite are hand-computed from NJ-WT's rate tables and method steps.
• Maine — the withholding tables publish single and married sets only; head-of-household filers are computed with the single standard deduction and brackets, and the state page says so.
• Vermont — one W-4VT rule is not modeled: extra Vermont withholding equal to 30% of any extra federal withholding you request. The state page says so.
• South Carolina — the 1.99%/5.21% return rates set by Act 110 were never adopted for withholding; the in-force withholding method (0%/3%/6% with a $5,000 allowance) is what is computed here.

The test suite

The calculator ships with a case suite of 264 assertions: the federal worked examples and a 126-case boundary grid over all six Publication 15-T schedules, each state's published worked example where one exists, hand-derived cases for the states that publish none, and end-to-end cases for the nine no-tax states. The build refuses to emit the site unless the suite is green, and the same engine files the suite tests are the files the site ships. One designed case — a regression anchor against a real pay stub — is intentionally not run: no real stub data was used or invented.

Update discipline

Withholding publications change mid-year more often than they used to (in 2026 alone, Utah changed rates June 1, Ohio changed tables August 1, Georgia's rate changed in May, and Maine revised its booklet in August). Federal tables are replaced verbatim when the new Publication 15-T posts in December; state publications are re-checked every January and on a quarterly watch in between. Source check dates are printed on every state page.

Source list — all 51 jurisdictions

Federal: IRS Publication 15-T (2026). Social Security wage base: SSA contribution and benefit base.