A complete guide to federal withholding, FICA, state tax, and pre-tax deductions — with the exact 2025 formulas used by the calculator above.
Your paycheck starts as gross pay — your salary or hourly wages for the pay period, before anything is withheld. From there, pre-tax deductions (like a 401(k) contribution or health insurance premium) are subtracted first, then federal income tax, Social Security, Medicare, and state income tax are calculated on what's left. What remains after every deduction is your net pay, also called take-home pay. Getting the order right matters: pre-tax deductions must reduce your taxable wages before tax is calculated, not after — otherwise the tax withheld would be too high.
How is federal income tax withholding calculated?
Quick answerYour pay is annualized, the 2025 standard deduction is subtracted, and the remaining taxable income is run through the marginal tax brackets: 10% up to $11,925, 12% up to $48,475, 22% up to $103,350, 24% up to $197,300, 32% up to $250,525, 35% up to $626,350, and 37% above that (single filer thresholds; married filing jointly thresholds are roughly double through the middle brackets). The resulting annual tax is divided back down to your pay period.
- Step 1 — Annualize: Gross pay per period × pay periods per year = annual gross pay.
- Step 2 — Subtract pre-tax deductions and the standard deduction: Annual gross − 401(k) − health insurance − standard deduction ($15,000 single / $30,000 MFJ) = federal taxable income.
- Step 3 — Apply marginal brackets: Each dollar is taxed at the rate for its own bracket, not the top rate on the whole amount — this is why a raise rarely pushes your entire paycheck into a higher rate.
- Step 4 — De-annualize: Divide the resulting annual federal tax by the number of pay periods to get the amount withheld per paycheck.
What is FICA — Social Security and Medicare tax?
Quick answerFICA is the combined payroll tax for Social Security and Medicare. In 2025, employees pay 6.2% Social Security on wages up to the $176,100 wage base, and 1.45% Medicare with no wage limit — 7.65% combined below the cap. Unlike federal income tax, FICA is a flat rate on gross wages; it isn't reduced by the standard deduction, and a traditional 401(k) doesn't lower it either.
Once your wages exceed $200,000 (single) or $250,000 (married filing jointly), an Additional Medicare Tax of 0.9% applies to the excess, making the effective Medicare rate 2.35% above that threshold. There is no equivalent cap-lifting rule for Social Security — wages above $176,100 simply stop accruing the 6.2% tax.
How does state income tax work on a paycheck?
Quick answerNine states — Alaska, Florida, Nevada, New Hampshire, South Dakota, Tennessee, Texas, Washington, and Wyoming — charge no state income tax on wages, so state withholding is $0. The other 41 states plus Washington, D.C. each set their own brackets or flat rates; this calculator lets you enter your own estimated state rate rather than model all 42 systems in full detail.
How do pre-tax deductions like 401(k) and health insurance lower your taxes?
Quick answerA traditional 401(k) contribution and a Section 125 health insurance premium are both subtracted from gross pay before federal (and typically state) income tax is calculated, which lowers your taxable income and your tax bill. Health insurance premiums under a cafeteria plan also reduce the wages used for Social Security and Medicare; a traditional 401(k) does not — FICA still applies to the full gross wage.
A Roth 401(k) works differently: contributions come out after tax, so they don't reduce this paycheck's taxable income, but qualified withdrawals in retirement are tax-free. Choosing between traditional and Roth is a trade-off between a bigger paycheck now (pre-tax) and tax-free income later (Roth).
Pay frequency: weekly vs biweekly vs semi-monthly vs monthly
Quick answerWeekly pay produces 52 paychecks a year, biweekly produces 26, semi-monthly produces exactly 24 (twice a month, e.g. the 15th and last day), and monthly produces 12. Biweekly paychecks are a fixed, equal amount each time; semi-monthly paychecks are also fixed but land on calendar dates rather than every-other-Friday, so the gap between paydays varies slightly month to month.