Methodology

The calculation order

1. Gross pay for the period. Hourly pay is rate times hours, plus overtime hours at 1.5 times the rate. Salary is annual divided by periods.
2. Pre-tax deductions come off. Section 125 items (health premiums, HSA, FSA) reduce both income tax wages and FICA wages. A traditional 401(k) reduces income tax wages only.
3. FICA on gross less Section 125: Social Security at 6.2% to the wage base, Medicare at 1.45% with no cap, Additional Medicare at 0.9% above the filing-status threshold.
4. Federal income tax by the IRS percentage method: annualise, subtract the standard deduction, apply the bracket table, subtract dependent credits, divide back down, add extra W-4 withholding, floor at zero.
5. State income tax from that state own rules: brackets or flat rate, its own base, its own standard deduction and exemption or credit.
6. State insurance withholding, each with its own rate and wage cap.
7. Local income tax where a verified rate exists.
8. Post-tax deductions and Roth 401(k). What remains is net pay.

Sources

• Federal brackets and standard deduction: IRS Rev. Proc. 2025-32, reconciled against IRS Publication 15-T.
• Social Security wage base, Medicare and Additional Medicare: Social Security Administration 2026 announcement.
• State brackets, deductions, exemptions, supplemental rates and insurance withholding: for the ten highest-traffic states and the no-income-tax states, each state own revenue or employment department withholding guide. For the remaining states and DC, the Tax Foundation 2026 compilation of state statutes, forms and instructions. The source link and verification date for each state appear on that state page.

Assumptions

• Standard deduction only. Itemised deductions are not modelled.
• A current-year W-4 with no allowances, since allowances were removed in 2020.
• One work state, and residence in the state of work. Multi-state allocation and reciprocity agreements are not applied.
• Year-to-date wages are annualised rather than tracked, so the Social Security cap and the Additional Medicare threshold are spread evenly across the year instead of appearing as a step.
• The two-jobs checkbox on the W-4 is not modelled. Enter the second income under other income instead.
• Head of household uses single brackets with the head of household standard deduction, because the 2026 head of household withholding table is not yet verified. This slightly overstates withholding, and the calculator says so when you select it.

What is deliberately excluded

Local wage taxes, which are excluded in every state. That matters most in Maryland counties, Ohio and Pennsylvania municipalities including Philadelphia, Indiana counties, Michigan cities, and New York City and Yonkers. Washington paid family leave, although the WA Cares long-term care premium is included. Employee unemployment contributions in Pennsylvania and Alaska. Supplemental bonus rates in the states whose 2026 rate is not yet verified. Every one of those is flagged as a warning in the results panel on the affected state rather than silently omitted.

Verification

The engine is tested on ten reference cases against ADP and SmartAsset, and each must land within one dollar per pay period. The cases cover bracketed and flat states, a no-tax state, the Social Security cap, the Additional Medicare threshold, overtime, a 401(k) against an HSA of the same size, and a bonus under the flat supplemental method. Where ADP and SmartAsset disagree, the state own withholding guide decides and the reason is recorded in that state data file.

Update cadence

Federal and FICA figures are updated in the first two weeks of January when the IRS revenue procedure and Publication 15-T are published. State files are worked through in the same window, and each verification date on the site moves only when that state file has actually been re-read against the source.