Methodology

Federal calculations

Federal income tax withholding follows IRS Publication 15-T's percentage method for automated payroll systems (Worksheet 1A) for 2020-or-later Forms W-4: wages are annualized, adjusted by the W-4 Step 2–4 entries, run through the annual rate schedules, and prorated back to the pay period. Social Security is withheld at 6.2% of wages up to the $184,500 wage base (2026); Medicare at 1.45% with the additional 0.9% withheld on wages over $200,000.

• IRS Publication 15-T (2026) — withholding method and rate schedules
• SSA contribution and benefit base — Social Security wage base
• IRS Topic 560 — Additional Medicare Tax withholding
• IRS Notice 2025-67, Rev. Proc. 2025-19, Rev. Proc. 2025-32 — 2026 401(k), HSA, and FSA limits

State calculations

Each state is implemented from its official withholding publication — California from EDD DE 44 Method B (exact calculation), New York from Publication NYS-50-T-NYS (1/26) with the NYC and Yonkers publications for local taxes — including employee-paid insurance programs (CA SDI, NY PFL/DBL, WA PFML and WA Cares, AK employee unemployment insurance). Where a state treats benefits differently from the IRS (California taxes HSA contributions), we follow the state rule. Every state page cites its sources and shows the date we last verified the data.

Conventions and rounding

• All internal math uses whole cents — no floating-point dollar arithmetic.
• Amounts round half-up at the same points the official worksheets round.
• Annualization: per-period wages are annualized, taxed on annual tables, and divided back — the approach the IRS and state publications themselves use for automated systems. Results can differ from a competitor by a few cents where they use per-period tables; the official worksheets are our reference, not competitors.
• If we don't have verified data for a year or state, the calculator says so rather than guessing.

Verification

The calculation engine has 150+ automated tests, including fixtures hand-computed from the official worksheets and the worked examples printed in the state publications themselves (for example, California DE 44 Examples E and F reproduce to the cent). Tax data files carry the source URL and verification date, and are re-verified each January.

What the estimates don't include

• Local income taxes outside NYC and Yonkers (e.g., Ohio and Pennsylvania municipalities) — noted on the result when relevant.
• Pre-2020 Forms W-4 (withholding allowances).
• Year-end reconciliation: we estimate per-paycheck withholding, not your final tax bill or refund.

Estimates are for informational purposes only — not tax, legal, or financial advice.