The model
We use the annualised percentage method: your pay is converted to an annual figure, pre-tax contributions (traditional 401(k), payroll HSA) and the 2026 federal standard deduction are subtracted, and the 2026 federal brackets are applied. FICA is computed separately — Social Security at 6.2% up to the 2026 wage base of $184,500, Medicare at 1.45% plus the 0.9% additional Medicare tax on wages above $200,000. State income tax follows each state's own 2026 rules. The result is divided by your pay periods.
Two details most calculators get wrong, handled correctly here: traditional 401(k) contributions reduce income-tax wages but not FICA wages; payroll HSA contributions through a cafeteria plan reduce both.
Data sources
- Federal brackets and standard deduction: IRS Rev. Proc. 2025-32 (2026 inflation adjustments)
- Social Security wage base: Social Security Administration, 2026 announcement
- FICA rates and additional Medicare threshold: IRS Publication 15
- State rules: each state's Department of Revenue withholding documentation, cited on every state page
Data is reviewed every January when new tables are published, and the “Updated” date on each page reflects the last review.
What the estimate does not include
Employer-specific deductions (health, dental and vision premiums, group life, FSA), wage garnishments, Roth contributions, state payroll premiums (Washington PFML/WA Cares, California SDI, Massachusetts PFML, New Jersey UI/TDI/FLI — each noted on its state page), municipal earnings taxes outside the modeled county systems (e.g. Kansas City and St. Louis), and school-district surtaxes (Iowa). Actual employer withholding also depends on your W-4 entries, so a real paycheck can differ from the estimate by a few dollars per period.
Accuracy policy
Every state's calculation is covered by automated tests with hand-computed expected values, and cross-checked against major payroll providers' calculators before publishing. Found an error? Tell us — accuracy reports get priority over everything else.