Employers know how much tax to withhold from your paycheck by combining two things: the entries you make on IRS Form W-4 and the withholding tables the IRS publishes each year in Publication 15-T. Federal law requires every employer paying wages to deduct income tax using methods the IRS prescribes.1Office of the Law Revision Counsel. 26 USC 3402 – Income Tax Collected at Source Layered on top of that income-tax calculation are fixed-rate Social Security and Medicare deductions that don’t depend on your W-4 at all. The whole system is built to spread your annual tax bill across the year, so nothing you see on your pay stub is final until you file your return.
What Your W-4 Tells the Payroll System
When you’re hired, your employer gives you Form W-4, the Employee’s Withholding Certificate. Everything on it feeds the calculation, and your entries are the single biggest factor in how much income tax leaves each check.
The first choice is filing status: single, married filing jointly, married filing separately, or head of household. Filing status determines which set of tax brackets your employer uses, and the difference is real. A married-filing-jointly filer keeps more per paycheck than a single filer at the same wage because the joint brackets are wider.
The form also asks about dependents. If you have qualifying children under 17 or other dependents, you enter the expected dollar value of the credits you’ll claim, and that amount directly reduces the tax withheld. You can report income from a second job, a working spouse, or interest and investment earnings, which pushes withholding up so you aren’t short in April. And you can request a flat extra dollar amount withheld each pay period if you know standard withholding won’t be enough.
Accuracy matters. Filing a W-4 that falsely reduces your withholding carries a $500 civil penalty per false statement if you had no reasonable basis for the claim.2Office of the Law Revision Counsel. 26 USC 6682 – False Information With Respect to Withholding That’s on top of any tax you’d owe.
You can submit a revised W-4 at any time. Once your employer receives it, they must apply the changes no later than the start of the first payroll period ending on or after 30 days from receipt.3Internal Revenue Service. Topic No. 753, Form W-4, Employees Withholding Certificate Most payroll departments process changes faster than that, but 30 days is the legal ceiling.
Pre-Tax Deductions Come Off First
Before the withholding tables come into play, certain deductions reduce your taxable wage. Contributions to a traditional 401(k), health insurance premiums paid through your employer’s plan, flexible spending accounts, and health savings accounts all come off the top. If you earn $2,000 per pay period but contribute $200 to a 401(k) and $50 to an HSA, your employer calculates income tax withholding on $1,750, not $2,000. That’s one of the main reasons two coworkers with the same salary and identical W-4 entries can see different federal tax numbers on their pay stubs.
Not every deduction is pre-tax. Roth 401(k) contributions, for example, come out after taxes are calculated. Knowing which is which explains a lot of pay-stub math.
The Withholding Tables Do the Actual Math
With your W-4 data in hand and pre-tax deductions subtracted, the payroll system consults IRS Publication 15-T, which contains the federal income tax withholding tables and computational procedures.4Internal Revenue Service. Publication 15-T (2026), Federal Income Tax Withholding Methods Two main approaches exist, and which one your employer uses depends mostly on how their payroll system is set up.
The percentage method is designed for automated payroll software. The system runs a worksheet that adjusts your wages based on your W-4 entries, then applies rates from a table that mirrors the progressive federal brackets. It works at any income level and with any version of the W-4, which makes it the default for most mid-size and large employers.
The wage bracket method is simpler and aimed at employers who process payroll manually. You find the row matching the employee’s wage range for that pay period, read across to the filing-status column, and the table gives a flat dollar amount to withhold. The trade-off: these tables only cover wages up to roughly $100,000 annualized. Above that, the employer switches to the percentage method.
Both methods account for pay frequency. The same annual salary produces different per-period withholding amounts depending on whether you’re paid weekly, biweekly, semimonthly, or monthly, because the bracket boundaries shift with the period length. The tables are updated each year to reflect inflation adjustments to the brackets.
Why Bonuses Look Taxed Harder
Supplemental wages like bonuses, commissions, and severance get different treatment from regular pay. Employers can use a flat 22% withholding rate instead of running the payment through the standard tables.5Internal Revenue Service. Publication 15 (2026), Employer’s Tax Guide That’s why a bonus check often looks like it was taxed at a higher rate than your regular paycheck. If supplemental payments to a single employee exceed $1 million in a calendar year, the rate on the excess jumps to 37%. Some employers instead combine the bonus with your regular pay for that period and run the total through the percentage method, which can produce a higher or lower amount depending on your income. Either way, the withholding is an estimate; your actual liability is settled when you file.
Social Security and Medicare Are Separate
Income tax isn’t the only deduction. Federal law requires employers to withhold FICA taxes from every paycheck, and these don’t depend on your W-4.6Office of the Law Revision Counsel. 26 USC 3102 – Deduction of Tax From Wages
- Social Security: 6.2% of your wages, up to $184,500 in 2026. Once you hit that cap, the deduction stops for the rest of the year.7Office of the Law Revision Counsel. 26 USC 3101 – Rate of Tax8Social Security Administration. Contribution and Benefit Base
- Medicare: 1.45% of all wages, with no cap.
- Additional Medicare Tax: An extra 0.9% once your wages from that job pass $200,000 in the calendar year. Your employer withholds it based on that single-job threshold regardless of your filing status, though the actual thresholds that apply on your return are $200,000 single, $250,000 married filing jointly, and $125,000 married filing separately. No employer match on this one.
When the IRS Overrides Your W-4
In rare cases, the IRS tells your employer to ignore what you put on your W-4. This comes through a lock-in letter. If the IRS determines your withholding is too low, it sends notice to your employer requiring a higher rate.9Internal Revenue Service. Understanding Your Letter 2801C Once a lock-in is in effect, the employer must disregard any new W-4 you submit that would lower withholding. You have 30 days from the letter’s date to challenge the determination before it takes effect.
Claiming Exempt From Income Tax Withholding
If your income is low enough that you won’t owe any federal income tax, you can tell your employer to skip income tax withholding entirely. Two things must be true to claim exempt on your W-4: you had zero federal income tax liability last year, and you expect zero liability this year.10Internal Revenue Service. Employee’s Withholding Certificate FICA taxes still come out. The exemption expires annually, and you must submit a new W-4 claiming it by February 15 of the following year.3Internal Revenue Service. Topic No. 753, Form W-4, Employees Withholding Certificate Miss that date and your employer starts withholding as if you were a single filer with no adjustments, which is the highest default rate. They won’t refund what was withheld during the gap.
Checking Whether Your Withholding Is Right
Even a perfectly filled-out W-4 produces only an estimate. The tables assume steady pay all year, which breaks down when you change jobs, get a mid-year raise, pick up significant investment income, or claim deductions the standard tables don’t know about. The IRS offers a free Tax Withholding Estimator that walks through your actual situation and tells you whether to adjust.11Internal Revenue Service. Tax Withholding Estimator It can generate a pre-filled W-4 to hand to your employer.
Getting it wrong by a large margin has consequences. If you owe $1,000 or more when you file, the IRS may charge an underpayment penalty. You avoid it by meeting one of two safe harbors: your total withholding and estimated payments cover at least 90% of your current-year tax, or they cover 100% of last year’s tax (110% if your adjusted gross income exceeded $150,000).12Office of the Law Revision Counsel. 26 USC 6654 – Failure by Individual to Pay Estimated Income Tax The 100%-of-last-year target is useful when your income is unpredictable, because the number is fixed.
The best time to check is after any major life change and again in the fall, when enough paychecks remain to absorb a correction. Waiting until December can mean a painfully large adjustment on your last few checks of the year.