Methodology — the paycheck engine

Federal withholding — the IRS Pub. 15-T percentage method

The federal line is computed the way a real payroll computes it: Worksheet 1A of IRS Publication 15-T (2026), the percentage method for automated payroll systems. The pay period is projected to a full year; the Form W-4 entries are applied exactly as the worksheet prescribes — Step 4(a) other income added, Step 4(b) deductions subtracted, and, when the Step 2 multiple-jobs box is not checked, the worksheet's own adjustment ($12,900 married filing jointly, $8,600 otherwise); the tentative withholding is read from the publication's annual percentage schedules (the STANDARD schedules, or the Step 2 Checkbox schedules when the box is checked), divided by the number of pay periods, reduced by the Step 3 amount per period, and increased by any Step 4(c) extra amount. The federal figure is therefore the withholding on this paycheck for the Form W-4 modeled — not an estimate of the year-end tax bill, which can differ (a second job, a bonus at the supplemental rate, credits claimed on the return).

State tax — the annualized method

State income tax is still computed as an estimated annual liability: the pay period is projected to a full year, taxed on the state's official 2026 tables, then divided by the number of pay periods. State per-period withholding tables are not reproduced, so a real paycheck's state withholding can differ from the figure shown. That difference is a documented, deliberate limit, published here rather than hidden.

FICA, computed cumulatively

Social Security is 6.2% up to the 2026 wage base of $184,500.00, applied to combined wages (this job plus any declared other-employer wages): the cap follows the worker across the year instead of restarting each period. Medicare is 1.45% with no cap.

Free entry — the hours you type are the hours computed

On the paycheck pages you can type any pay. In hourly mode you enter the rate and the hours of one pay period — regular hours and overtime hours separately. Overtime hours are paid at 1.5x the rate, and the period's gross pay is derived from exactly those entries: rate x regular hours + 1.5 x rate x overtime hours. The calculator does not guess a schedule, does not spread hours across weeks and does not apply a weekly 40-hour rule for you: the split between regular and overtime hours is yours to declare, as it appears on your pay stub. In annual salary mode, the salary is divided by the number of pay periods per year.

One computation, two executions — the parity bench

The paycheck pages run the engine twice over: the reference engine computes the precomputed cases when the site is built, and the same computation runs in your browser when you type a free entry. Both executions read the same data files and implement the same rules, including the Pub. 15-T worksheet above. Their equality is measured, not asserted: a differential bench compares the full structured result of both engines, line by line, with a $0.00 tolerance on every published line. The bench covers 7,352 systematic and seeded-random cases — all 51 jurisdictions, all filing statuses and pay frequencies, income levels from low wages to above the Social Security wage base, Form W-4 variants, tips and local modules — and re-derives all 1,068 precomputed cases shipped on the paycheck pages, identically. Any divergence is a defect that fails the bench.

What is not modeled

• Pre-tax deductions: 401(k), HSA and cafeteria-plan amounts are not inputs — gross pay is fully taxable in v1.
• Itemized deductions and federal credits (child tax credit, EITC, education credits), the AMT and the net investment income tax.
• State per-period withholding tables — state tax is annualized (see above). The federal Pub. 15-T method IS reproduced.
• Forms W-4 from 2019 or earlier (withholding allowances) — the engine models the 2020-or-later Form W-4.
• Local wage taxes beyond the modules encoded in the data pack (New York City, Yonkers, Maryland counties, Indiana counties).

Tips and the §224 deduction

Declared tips are wages — for federal tax, FICA, state tax and the state payroll programs. When the caller declares eligibility, the engine applies the IRC §224 deduction (OBBBA's “no tax on tips”: up to $25,000, phased out over modified AGI of $150,000 single / $300,000 joint at $100 per full $1,000 — the engine never adjudicates the tipped-occupation list itself). In the federal withholding computation, the allowed §224 amount is entered at Step 4(b) of the Pub. 15-T worksheet — the Form W-4 slot for expected deductions — so it lowers withholding during the year. At state level the treatment is per-state and primary-sourced: in the seven states whose base is federal taxable income, five let §224 flow through (CO, IA, ID, MT, ND), South Carolina adds it back, and in New Mexico it cannot propagate by construction.

State data status — stated, never hidden

Of the 51 jurisdictions in the pack, 48 are verified line by line against their primary source (status primary_verified). Arizona's 2026 standard deduction and DC's 2026 amounts are documented statutory derivations (Arizona: A.R.S. §43-1041(H) ties the standard deduction to IRC §63; DC: D.C. Code §47-1801.04), each flagged on every result for that jurisdiction. Vermont's parameters are compiled and cross-checked but not yet primary-verified, and every Vermont result is flagged as compilation-checked, next to the figures. No state is ever presented as more verified than its file says.

Filing statuses and head of household

All four filing statuses are computed. Where a state publishes a distinct schedule for a status, that schedule is what the engine uses — New York's and California's head-of-household schedules included. Where the state's own publication groups statuses under one schedule (New Jersey's Tables A and B, Oregon's two rate charts, the flat-rate states), the data file declares that equivalence in writing, with its primary basis, and the golden bench pins it. Vermont is the one declared exception: its TY2026 schedules were not yet published, so its file declares a provisional fallback, flagged on every Vermont result. The engine never substitutes a schedule silently. Head-of-household standard deductions are exact where the state publishes them.