The Federal Income Tax line on your paystub is your employer sending money to the IRS on your behalf, a little each payday, toward the income tax you'll owe for the year. It's a prepayment, not a final number. When you file your tax return next spring, you settle up — if too much was withheld, you get a refund; if too little, you owe the difference.
That's the key thing to understand: the amount withheld each paycheck is an estimate. It's the IRS's best guess, based on your W-4, of what your yearly tax will be, spread across your paychecks. Your actual tax bill isn't known until you add up your whole year and file.
How the estimate is built
Payrollix figures your federal withholding from three inputs:
- How much you're paid this period, annualized. The system looks at this paycheck as if you earned it every pay period all year.
- Your W-4 — your filing status, any dependents you claimed (as dollar amounts), and any extra withholding or adjustments you entered. The current W-4 doesn't use "allowances" anymore, so old advice about "claiming 0 or 1" no longer applies.
- The 2026 tax brackets and standard deduction ($16,100 for a single filer, $32,200 married filing jointly), which are built into the system.
From those, it calculates the tax on your annualized pay and divides it back down to this one check. Change any input and the withholding changes.
Why the amount jumps between paychecks
Two identical gross paychecks can have different federal withholding. The usual reasons:
- You updated your W-4. Adding a dependent, changing filing status, or entering extra withholding all move the number on the next run. This is the one you control.
- Your pay changed. A raise, extra hours, or a slow period changes the annualized estimate, and the withholding tracks it.
- You crossed into a higher bracket for the year. As your year-to-date pay climbs, more of it falls in higher-rate brackets, so the withholding on later checks can tick up.
- You got a bonus or other supplemental pay. Bonuses are withheld at a flat supplemental rate rather than run through the normal brackets, so a bonus check has a noticeably different withholding percentage than your regular pay. That's expected, and it evens out when you file.
This is not the same as FICA
People lump all the tax lines together, but federal income tax and FICA are different animals:
- FICA (Social Security 6.2% + Medicare 1.45%) is a fixed percentage of your wages. It doesn't care about your W-4, your filing status, or your dependents. It funds two specific programs.
- Federal income tax is a variable estimate based on your W-4 and your total pay, and it funds the general federal government. It's the line you can actually influence by adjusting your W-4.
If your refund or bill last year was bigger than you'd like, the fix is your W-4, not the FICA lines — those you can't change. Update it in your profile and the next paycheck reflects it.
Related: Profile & W-4 · Gross pay vs. net pay · Employer-paid taxes explained.