What Is Tax Withholding? How Your W-4 Decides What's Taken Out

Tax withholding is your employer's running estimate of what you'll owe for the year, taken out of your paycheck a little at a time and sent to the IRS and your state before you ever see it. The form that sets the estimate is the W-4, and it is only ever an estimate: get it wrong and you either hand the government an interest-free loan until you file, or end up owing a bill you didn't plan for.

This guide walks through how withholding actually works, what each part of the W-4 does, and why the amount withheld and your real tax bill don't always match.

Disclosure: we make Salary Left, the free expense tracker linked below, and the salary calculator that estimates the annual tax figures below.

Withholding is a prepayment, not your tax bill

Your actual federal income tax for the year is calculated once, at filing time, from your full year's income, deductions and credits. Withholding tries to collect that amount gradually, a slice from every paycheck, using IRS withholding tables and the answers on your W-4.

Those two numbers, withheld so far and actually owed, are rarely identical. At filing time the IRS compares them:

• Withheld more than you owed → you get the difference back as a refund.
• Withheld less than you owed → you owe the balance, and if the shortfall is large enough, a penalty on top.

Neither outcome means you did anything wrong. It means the running estimate didn't land exactly on the final number, which is normal, especially if your income changed during the year.

The five steps of the W-4

The W-4 was redesigned in 2020 to stop using withholding "allowances" and ask for information in plainer terms instead. It has five steps, but most people only need to touch the first and third:

Steps 2 and 4 are optional and only apply in specific situations, but skipping Step 2 when you genuinely have two jobs, or a working spouse, is the single most common reason people end up under-withheld.

A worked example: why the numbers can drift

Say a single filer earns $75,000 from one job. Based on that income alone, here's roughly what they'd owe for the year, which is the number withholding is trying to hit:

Now suppose partway through the year they pick up a second part-time job paying $15,000. Each employer's payroll system withholds as though its paycheck were the person's only income, so neither one accounts for the other. The combined household income has moved into a situation the two separate W-4s don't see, which is exactly what Step 2 exists to correct. Without it, the two jobs together can under-withhold by a meaningful amount, something that only shows up as a surprise balance the following April.

When to update your W-4

Your W-4 doesn't update itself. It's worth revisiting after:

• Starting a new job — you fill one out automatically, but it's easy to rush through Step 3 and 4 and leave money on the table or under-withhold.
• Picking up a second job, or a spouse starting one — revisit Step 2 on at least one of the W-4s involved.
• A child is born, turns 17, or stops being a dependent — Step 3 changes directly.
• Marriage, divorce, or a change in filing status.
• A large, irregular bonus — bonuses are often withheld at a flat 22% federal rate regardless of your actual bracket, which can over- or under-collect depending on your real income.
• You got a big refund or owed a big balance last year — either one is a sign the current W-4 isn't matching your situation.

You can submit a new W-4 to your employer at any time; it isn't limited to once a year.

It isn't the same as FICA

Withholding and FICA are deducted from the same paycheck but work completely differently. Federal income tax withholding is the variable, W-4-driven estimate this guide covers. FICA (Social Security and Medicare) is a flat 7.65% of wages with no form to fill out and no way to adjust it. Our guide to FICA covers how that side works.

If you're self-employed

A W-4 only exists because an employer is doing the withholding. If you're a 1099 contractor or run your own business, there's no employer in the relationship, so nothing is automatically withheld from what you're paid. Instead, you estimate your own tax and pay it in quarterly instalments. Our self-employment tax guide works through exactly how that's calculated.

Sources and limits

• Form W-4 and the 2026 instructions, including the Step 3 credit amounts and income thresholds: the IRS's 2026 Form W-4.
• For your own exact withholding, the IRS's Tax Withholding Estimator (irs.gov/W4App) accounts for your full situation more precisely than any general guide.
• Underpayment penalties and estimated tax: IRS Publication 505, Tax Withholding and Estimated Tax.

This is general information, not tax advice. Withholding tables, flat bonus rates and penalty rules have exceptions this guide doesn't cover; for your specific paycheck, check the sources above or a tax professional.