Payroll withholding is your employer paying your taxes in installments on your behalf, one paycheck at a time. Each payday it estimates your eventual annual tax bill, takes a proportional slice, and sends it to the IRS and your state. Your Form W-4 tells it how big that slice should be; the IRS Publication 15-T tables turn your W-4 into an exact dollar figure. This guide explains the whole machine in plain English — and includes a quick estimator to see how an extra-withholding amount changes your refund.
The U.S. tax system is pay-as-you-go: you cannot wait until April to pay a year's worth of tax at once. So your employer estimates and prepays it for you every payday. The key word is estimate — withholding is a forecast of your tax, not the exact figure. The annual reconciliation on your tax return settles the difference, which is why you get a refund (over-withheld) or owe a balance (under-withheld).
| Withholding | How it's set | 2026 detail |
|---|---|---|
| Federal income tax | Your W-4 + Pub 15-T tables | Graduated, depends on filing status |
| Social Security | Fixed rate | 6.2% up to $184,500 |
| Medicare | Fixed rate | 1.45% (+0.9% over $200k) |
| State income tax | State W-4 equivalent | Varies; 0% in 9 states |
| Local tax | City/county rules | e.g. NYC, some PA/OH cities |
Only the federal income tax line is shaped by your W-4. FICA is mechanical and identical for everyone at the same wage.
Behind the scenes, payroll software follows the IRS Publication 15-T method. In simplified terms it: (1) annualizes your current paycheck (multiplies by the number of pay periods), (2) subtracts the standard deduction built into the tables for your filing status, (3) finds the tax on that annualized amount using the percentage-method brackets, (4) subtracts your Step 3 dependent credits, (5) divides back down to one pay period, and (6) adds any Step 4c extra amount. The result is the federal income tax withheld from that check. Change any W-4 input and step 1–6 produce a different number.
A refund simply means you prepaid more than you owed; the IRS returns the surplus. It feels like a windfall but it is an interest-free loan you made to the government all year. A common smart move: if you routinely get a $3,000 refund, that is about $115 per biweekly check you could have had along the way. Adjusting your W-4 to withhold less converts that lump-sum refund into a bigger paycheck — useful if you would rather invest or pay down debt than wait for April. The estimator above shows the reverse: how an extra Step 4c amount grows your refund at the cost of take-home now.
The opposite problem is owing too much at filing. The IRS can charge an underpayment penalty if you did not prepay enough during the year. You generally avoid it under the safe harbor: pay at least 90% of the current year's tax, or 100% of last year's (110% if your prior-year AGI was over $150,000). This is why people with side income often set a higher Step 4c amount or make quarterly estimated payments — withholding from a W-2 job alone may not cover the extra tax.
Each payday your employer estimates your annual tax from your W-4 and the Pub 15-T tables and withholds a slice. FICA is withheld separately at 7.65%. Year-end reconciliation produces a refund or balance due.
Filing status, multiple jobs (Step 2), dependents (Step 3), other income/deductions (Step 4), and extra withholding (Step 4c). It no longer uses allowances.
Because you withheld more than your final tax. It is your own money returned; you can adjust your W-4 to keep more per check instead.
FICA is withheld from your check but calculated separately as a fixed 7.65%. Your W-4 only affects federal income tax withholding.