Methodology

Last verified 2026-11-02 · Sources: IRS Publication 15-T (2026) · IRS Rev. Proc. 2025-32 · SSA contribution and benefit base · Report an error

Every rate on this site is transcribed from an official document, stored with its source and verification date, and applied by an engine that is tested against a second implementation. This page lists the sources, the steps and the limits.

Sources

Only documents that the site already uses are listed. A source is added when a figure taken from it is added, not before.

ssa.gov answers automated command-line requests with an HTTP 403 (it filters non-browser clients); the page loads in a browser. The wage base the calculator applies is the one transcribed from IRS Publication 15, so no figure on this site depends on the SSA page loading. michigan.gov also returns 403 to command-line requests while loading in a browser; the 4.25% rate and $5,900 exemption are stored with their source quotes in the Michigan data file, so no calculated figure depends on that page loading either.

The 2026 state tax changes page is generated from the same data/tax_year/2026/ files; each row names the document already stored with that state's figures. Where a state page cites more than one document, its own sources block lists the full set.

How it works

1. Transcribe the official table. tools/extract_p15t.py parses the downloaded Publication 15-T into data/tax_year/2026/federal_income_tax.json. It refuses to write a schedule with the wrong number of bands, bands that are not contiguous, a first band that does not start at zero, rates that are not monotonic, or a base amount that disagrees with the previous band's arithmetic.
2. Load behind a gate. engine/data.py (TaxYear) raises UnverifiedValue for any leaf whose status is not verified or derived. TaxYear.blockers() lists everything blocking publication, and a state with a blocker gets no page in the build.
3. Annualise and adjust. engine/federal.py (adjusted_annual_wage_amount) implements Worksheet 1A lines 1c-1i: wages multiplied by periods per year, plus W-4 step 4(a), minus step 4(b), minus the standard adjustment when the Step 2 box is not checked.
4. Find the band and apply the rate. find_band() and tentative_from_band() implement lines 2a-2g against the stored annual schedule. The result is divided by the periods per year (line 2h), Step 3 credits are subtracted, and the figure is rounded to the nearest dollar, half up (engine/money.py, line 4b: $2.50 becomes $3).
5. FICA, separately. engine/fica.py (compute_fica) applies 6.2% up to the $184,500 wage base measured on year-to-date wages, 1.45% with no cap, and 0.9% only to wages above $200,000 from the same employer. Filing status does not change the withholding amount; it changes the return, not the paycheck.
6. State lines. engine/state.py (compute_state) has one handler per state. States with no income tax return an explicit zero line rather than no line, and states whose own document defines a method (California DE 44 Method B, New York NYS-50-T-NYS Method II) are computed by that method, not by rate x gross.
7. Assemble the breakdown. engine/paycheck.py (compute_paycheck) returns an ordered list - gross, federal, Social Security, Medicare, additional Medicare, state items, net - and the tests assert that the deductions add up and that net equals gross minus them.
8. Prove the browser agrees. tools/golden_cases.py exports fixtures from the Python engine; web/engine.js is an independent implementation and tests/parity.mjs fails if the two disagree by a cent. The page you are reading could not render a different number from the one the calculator shows in your browser.
9. Render. build_site.py recomputes every figure in the copy at build time and attaches the verification line to each page. Nothing numeric in the HTML is typed by hand.

Two steps are manual and always will be. Deciding that a published state document has actually changed is a human re-read of the page each tax year, and the state document URLs in the data files were chosen by hand. Everything downstream of the transcription is mechanical.

Known limitations

• Estimates, not advice. The calculator reports withholding for one pay period from stored tables. It is not a tax return, it does not know about credits, itemising or other income, and it is not a substitute for your employer's payroll system or a professional.
• It depends on year-to-date wages. The Social Security wage base and the Additional Medicare threshold are measured on wages already paid this calendar year. With year-to-date left at zero the calculator behaves as if this is the first check of the year, which is right for January and wrong for December.
• Refresh lag is real. The stored figures were verified on 2026-10-30. A rate that changed after that date is not reflected until the data file is re-read against the document and the verification date moves. The date on each page is how you tell.