What Is Federal Withholding on Your Paystub?

The line marked “Federal Income Tax” or “FIT” on your paystub is federal income tax withholding: your employer’s estimated prepayment of what you’ll owe the IRS for the year, deducted from each check and sent to the Treasury on your behalf. Payroll software arrives at that number by running your taxable wages through a formula built around your Form W-4, your pay frequency, and the progressive federal tax brackets. Federal withholding on your paystub is meant to land you close to zero at tax time, so April brings a modest refund or a modest balance due rather than a surprise.

Why That Money Comes Out Every Payday

The federal income tax system is pay-as-you-go. Instead of letting a year’s tax liability pile up and come due in one bill, federal law requires your employer to deduct an estimated income tax payment from every paycheck and remit it to the IRS.¹

Each paycheck’s withholding is an estimate, not a settled tax bill. Your real liability depends on your total income, deductions, and credits for the full year, and nobody knows those numbers until the year is over. The withholding system just tries to get close.

What Actually Controls the Amount

Everything starts with Form W-4, the Employee’s Withholding Certificate you filled out when you were hired.² Your employer plugs its entries into the withholding formula, so this form is the biggest single lever you have over that paystub number. The current W-4 works with dollar amounts and credits rather than the old “allowances” system it replaced in 2020.

Filing Status

Step 1(c) asks you to pick one of three boxes: single or married filing separately, married filing jointly or qualifying surviving spouse, or head of household.³ The choice sets both the standard deduction baked into the formula and the width of each tax bracket. At the same income, the joint boxes withhold less per check than single; head of household sits between them.

Multiple Jobs or a Working Spouse

Step 2 handles households with more than one source of wages. Skip it and each employer runs the calculation as if its paycheck is your only income, applying the lower brackets and the full standard deduction on its own. The predictable result is under-withholding across the household.

You have three ways to fix that: use the IRS Tax Withholding Estimator and enter a specific extra amount on line 4(c), fill out the Multiple Jobs Worksheet on the form, or, if there are only two jobs between you, check the Step 2(c) box. That box tells both employers to cut the bracket widths and standard deduction in half so neither one double-counts them.³

Dependents, Other Income, and Extra Withholding

The remaining steps fine-tune the result.

Step 3 is for credits. For 2026, you multiply each qualifying child under 17 by $2,200 and each other dependent by $500. The total is spread across your paychecks and subtracted from the calculated tax, reducing withholding dollar for dollar.³
Step 4(a) is for income you expect without its own withholding, such as interest, dividends, or rental income. Your employer adds it to your wages before running the formula.
Step 4(b) is for extra deductions above the standard deduction. It lowers the wage figure in the formula.
Step 4(c) is a flat dollar amount taken from every paycheck on top of whatever the formula produces. Useful when the other steps don’t quite cover your situation.

Once you submit a revised W-4, your employer has to implement the changes no later than the first payroll period ending on or after 30 days from the day they received it.⁴

How the Paycheck Number Gets Built

Payroll software uses one of two IRS-approved methods from Publication 15-T. Both aim for the same annual total.⁵

The wage bracket method is a table lookup. The employer finds your pay frequency, filing status, and wage range in a pre-built table and reads off the amount to withhold. Small employers doing payroll by hand tend to prefer it. It has wage ceilings, though, so higher-paid employees push their employers into the other method.

The percentage method is what most automated systems use because it handles any wage level. The steps go like this:

Annualize your taxable pay by multiplying it by the number of pay periods in a year (26 for biweekly, 24 for semi-monthly, 12 for monthly).
Add any Step 4(a) other income.
Subtract any Step 4(b) deductions plus a built-in standard deduction figure tied to your filing status and whether you checked the Step 2 box.
Run the resulting adjusted annual wage amount through the progressive bracket table for your filing status.
Divide that annual tax by the number of pay periods.
Subtract your Step 3 credit total, divided by the number of pay periods. Round to the nearest dollar. That’s the withholding for the check.

For context on the bracket step, 2026 rates start at 10% on the first $12,400 of taxable income for single filers and climb to 37% above $640,600. Joint filers hit 37% above $768,700.⁶

Pre-Tax Deductions Come Off First

Before the withholding formula runs, your employer subtracts pre-tax items from your gross pay. Traditional 401(k) contributions and benefits elected through a Section 125 cafeteria plan (health insurance premiums, FSA and HSA payroll contributions) are the common ones.⁷ Those amounts shrink the wage base the formula sees. If you put $500 per paycheck into a traditional 401(k), withholding is calculated on $500 less than your gross, which is why raising your retirement contribution costs your take-home less than the contribution itself.

Bonuses and Other Supplemental Wages

Bonuses, commissions, overtime pay, and severance are “supplemental wages” and can be withheld under different rules than your regular paycheck.⁸ If your employer identifies the supplemental payment separately, it picks one of two approaches. The flat 22% method withholds 22% for federal income tax regardless of your W-4. Most employers choose this route for bonuses because it’s simple. It often over-withholds people in the 10% or 12% brackets and under-withholds people at 32% and above. The aggregate method combines the supplemental payment with your regular wages for the pay period, runs the whole amount through the normal formula, and withholds the difference from the supplemental piece.

One hard rule sits above both: once supplemental wages from a single employer cross $1 million in a calendar year, the excess is withheld at 37% regardless of your W-4 or filing status.⁸

The Other Federal Lines on the Stub

Federal income tax withholding is not the only federal deduction on your paystub. Social Security and Medicare (together, FICA) sit on their own lines and use flat rates rather than brackets. Social Security is 6.2% of gross wages up to a 2026 wage base of $184,500, after which it stops for the rest of the year.⁹ Medicare is 1.45% of all gross wages with no cap, and an additional 0.9% kicks in on wages above $200,000 in a calendar year regardless of filing status.¹⁰ Your W-4 does not affect these numbers.

Checking Whether Yours Is Right

Every dollar withheld during the year counts as a prepayment. After year-end, your employer reports the total on Box 2 of your W-2, and that number flows onto your Form 1040 to be compared against your actual tax. If Box 2 exceeds what you owe, you get a refund; if it falls short, you owe the balance.

A large refund means you loaned the government money interest-free all year. A large balance due can trigger a penalty. Getting the W-4 close to right is what keeps either from happening.

The IRS Tax Withholding Estimator is a free online tool that walks through your income, deductions, and credits and tells you whether you are on track. It can produce a pre-filled W-4 to hand to your employer.¹¹ Mid-year is a good moment to run it: you have real paystubs to feed in, and there’s still enough of the year left to absorb a correction without gutting your December take-home. Life changes such as marriage, a new child, a home purchase, or a second job move the variables enough to be worth a fresh check.

What Happens If Too Little Comes Out

If you owe $1,000 or more when you file, the IRS may charge an underpayment penalty, calculated at a rate the IRS publishes each quarter.¹² You avoid it by meeting either of two safe harbors:¹³

Your total withholding and estimated payments cover at least 90% of the tax shown on your 2026 return, or
They equal at least 100% of the tax shown on your 2025 return, provided that return covers a full 12 months.

If your 2025 adjusted gross income was over $150,000 ($75,000 if married filing separately in 2026), the prior-year safe harbor rises to 110% of your 2025 tax.¹³ Meeting either safe harbor shields you from the penalty even if you owe a sizable balance in April.

1
Office of the Law Revision Counsel. 26 U.S. Code 3402 – Income Tax Collected at Source
2
Internal Revenue Service. About Form W-4, Employee’s Withholding Certificate
3
Internal Revenue Service. Form W-4 (2026) Employee’s Withholding Certificate
4
Internal Revenue Service. Topic No. 753, Form W-4, Employees Withholding Certificate
5
Internal Revenue Service. Publication 15-T (2026), Federal Income Tax Withholding Methods
6
Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026, Including Amendments From the One, Big, Beautiful Bill
7
Office of the Law Revision Counsel. 26 USC 125 – Cafeteria Plans
8
Internal Revenue Service. Publication 15 (2026), (Circular E), Employer’s Tax Guide
9
Social Security Administration. Contribution and Benefit Base
10
Internal Revenue Service. Topic No. 751, Social Security and Medicare Withholding Rates
11
Internal Revenue Service. Tax Withholding Estimator
12
Internal Revenue Service. Underpayment of Estimated Tax by Individuals Penalty
13
Internal Revenue Service. 2026 Form 1040-ES, Estimated Tax for Individuals