The “FITWH” line on your paycheck is federal income tax withholding — the amount your employer subtracts from your gross pay each period and sends to the IRS as a prepayment toward your annual federal income tax bill.1Office of the Law Revision Counsel. 26 U.S. Code 3402 – Income Tax Collected at Source It’s a pay-as-you-go system, which is why you don’t get a single enormous bill in April. FITWH covers only federal income tax. The separate Social Security and Medicare deductions on your stub are FICA taxes, not the same thing.2Internal Revenue Service. Understanding Employment Taxes When the system works, the total withheld across the year lands close to your actual tax liability, and you finish with a small refund or a small balance due.
How Your Employer Decides the Amount
Two things drive the number: the Form W-4 you filled out when you were hired, and the withholding tables in IRS Publication 15-T.3Internal Revenue Service. About Publication 15-T, Federal Income Tax Withholding Methods Your W-4 tells payroll your filing status, whether you have dependents, whether you have other income or a second job, and whether you want any extra dollars taken out. Publication 15-T turns that into a specific figure for your wages and pay frequency.
Filing status matters because it sets your standard deduction and the tax brackets your employer applies. For 2026, the standard deduction is $16,100 for single filers, $32,200 for married couples filing jointly, and $24,150 for heads of household.4Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026 Payroll prorates that deduction across your pay periods before running the tax calculation, so a married-filing-jointly employee earning the same gross wages as a single filer will see less withheld each check.
Adjusting Your Withholding With the W-4
You can submit a new W-4 any time your situation changes — a marriage, a new child, a side business, or picking up a second job. Your employer must start using it no later than the first payroll period ending on or after the 30th day from when you turned it in.5Internal Revenue Service. About Form W-4, Employee’s Withholding Certificate
The current W-4 walks through several steps. You pick your filing status, claim qualifying dependents by entering the dollar value of the resulting child tax credits, and, in Step 2, account for a second job or a working spouse so you don’t end up under-withheld. Step 4 lets you report other income that has no withholding (freelance work, investment gains) and lets you request an extra flat amount per paycheck if you know your tax picture is complicated.5Internal Revenue Service. About Form W-4, Employee’s Withholding Certificate
If you’re not sure what to put down, the IRS Tax Withholding Estimator at irs.gov will do the arithmetic. Have your recent pay stubs handy for every job, your spouse’s stubs if you file jointly, and any records of self-employment income or deductions you plan to itemize.6Internal Revenue Service. Tax Withholding Estimator The tool spits out a W-4 configuration designed to get your refund or balance due close to zero.
If You Never Submit a W-4
Your employer can’t guess. If no W-4 is on file, the IRS requires them to withhold as though you’re single with no other adjustments, which is roughly the highest default withholding for someone with no dependents.7Internal Revenue Service. Withholding Compliance Questions and Answers You’d probably see more taken out than necessary and recover the excess as a refund. Filing a W-4 that reflects your real situation moves that money back into your paycheck.
Claiming Exempt
You can check a box on the W-4 that stops federal income tax withholding entirely, but only if you had no federal income tax liability last year and expect none this year. In practice, this fits people with very low incomes. The box sits below Step 4(c).8Internal Revenue Service. Publication 15-T (2026), Federal Income Tax Withholding Methods
Exempt status expires. A W-4 claiming exempt runs out on February 15 of the following year, and if you don’t submit a fresh one, your employer has to switch you to the single-with-no-adjustments default.9Internal Revenue Service. Topic No. 753, Form W-4, Employees Withholding Certificate If you’re genuinely exempt, put a reminder on your calendar to renew.
Why Bonuses Get Withheld Differently
Bonuses, commissions, back pay, and other supplemental wages don’t follow your regular W-4 math. When your employer pays them separately or identifies them separately on your stub, they can withhold a flat 22% instead.10Internal Revenue Service. Publication 15 (2026), (Circular E), Employer’s Tax Guide That’s why a bonus check often looks like it lost a bigger bite than a regular paycheck of the same size.
If your supplemental wages from one employer top $1 million in a calendar year, the excess is withheld at 37%, the top marginal rate, no matter what your W-4 says.10Internal Revenue Service. Publication 15 (2026), (Circular E), Employer’s Tax Guide Because these flat rates rarely match anyone’s actual bracket, big bonuses tend to create either a larger refund or a larger balance due at filing time.
Reconciling FITWH When You File
After the year ends, your employer sends you a Form W-2. Box 2 shows the total FITWH taken from your pay. When you file your Form 1040, that number is credited against your actual tax liability. Withholding higher than your liability comes back as a refund; withholding lower than your liability produces a balance due.11Internal Revenue Service. Tax Withholding Estimator FAQs For 2026 wages, the usual January 31 W-2 deadline falls on a Sunday, so employers have until February 1, 2027, to get the form to you.12Internal Revenue Service. General Instructions for Forms W-2 and W-3 (2026)
A large refund means too much was withheld; you gave the government an interest-free loan. A large balance due means too little came out, and you might owe an underpayment penalty on top of the tax itself.
Staying Out of the Underpayment Penalty
The IRS charges a penalty if your withholding and any estimated tax payments come in too far below what you actually owe. You’re safe if your balance due after withholding and credits is less than $1,000.13Internal Revenue Service. Underpayment of Estimated Tax by Individuals Penalty
Two safe harbors cover you above that line. Your total payments need to hit at least 90% of the current year’s tax or 100% of last year’s tax, whichever is smaller. If your adjusted gross income last year was over $150,000 (or $75,000 if you file married separately), the prior-year figure rises to 110%.13Internal Revenue Service. Underpayment of Estimated Tax by Individuals Penalty
If you have meaningful income outside your paycheck — freelance work, rental income, investment gains — regular FITWH may not be enough on its own. You have two ways to close the gap. Ask for extra withholding on Step 4(c) of your W-4, or make quarterly estimated tax payments using Form 1040-ES. Estimated payments are required when you expect to owe $1,000 or more after subtracting withholding and refundable credits.14Internal Revenue Service. 2026 Form 1040-ES, Estimated Tax for Individuals Checking your withholding once a year, and again after any major life or income change, is the simplest way to keep the FITWH line doing what it’s supposed to do.