How it works

The calculator follows the same steps a payroll system follows, in the same order, using only official numbers.

1. Gross pay

Salary divided by paychecks per year, or each hourly rate times its hours (overtime at 1.5× or 2×).

2. Pre-tax deductions

A traditional 401(k) or 403(b) lowers the pay that federal income tax is figured on, but not Social Security or Medicare. Health, dental and vision premiums and HSA contributions through a cafeteria plan lower both.

3. Federal income tax

We follow IRS Publication 15-T (2026), Worksheet 1A for automated payroll systems: annualize the paycheck, add Step 4(a) income, subtract Step 4(b) deductions and the IRS standard amount ($12,900 married filing jointly, $8,600 otherwise, or zero when the Step 2 box is checked), look up the annual table, divide by paychecks per year, subtract Step 3 credits, and add Step 4(c). Older (2019 and earlier) W-4s use the allowance method from the same worksheet. Show the math lists every line.

4. Social Security and Medicare

Social Security is 6.2% of pay until your pay for the year reaches the wage base ($184,500 in 2026). We figure it on year-to-date pay so cents do not drift. Medicare is 1.45% of all pay, plus 0.9% withheld on pay above $200,000 in the year.

5. State rules

Each state is added only after its rules are checked against the state's own published guidance, and where that guidance prints worked examples, the calculator must reproduce them before the state goes live (each one is listed on our how we verify page). Every state uses its own method: some annualize your pay and apply brackets, some use per-paycheck tables, some take a flat rate after allowances, and a few (Maryland, Indiana, New York City, Yonkers) add county or city tax. Each state's page shows the method, the official guide it comes from, the date we checked it, and a changelog of every version we have used. Each state's own rules also decide which pre-tax deductions lower state wages; for example, California does not exclude HSA contributions and Pennsylvania does not exclude 401(k) contributions.

Some states also take employee payroll programs, such as disability insurance, paid family and medical leave, long-term care, unemployment or transit taxes (for example California, New York, New Jersey, Washington, Massachusetts, Colorado, Oregon and Rhode Island). Each state page lists them with their rate, cap and official source.

6. Dates decide the rates

Every rate has the date it takes effect. The pay date you enter picks the rates in force on that day, so January and mid-year changes apply automatically. If an agency has announced new rates but we have not finished checking them, the page says so and uses the last checked rates.

7. Rounding

Amounts are shown to the cent. Employers may round withholding to whole dollars, so a real paycheck can differ by a few cents.

Testing

Every release runs the IRS worksheet worked by hand, edge cases (Social Security cap, Additional Medicare, exempt W-4s) and each state's programs. Every build also runs the worked examples printed in the states' own instructions, and stops if any answer drifts. Each of those examples, with its document, page and result, is listed on our how we verify page. A release that fails any check is not published.