Withholding is not a separate tax. It is your employer collecting part of your expected annual federal tax, paycheck by paycheck, so that most of the year's tax is already paid by December. When the amount withheld does not match the final tax, the difference shows up as a refund or as an amount owed, and that difference is often misunderstood as a bonus or a penalty.
The calculator estimates annual federal tax and splits it by pay period. Withholding on a real stub can differ from that estimate for the reasons in this guide.
Withholding in one paragraph
Each pay period, your employer looks at your pay for that period, your filing status and adjustments from your W-4, and the payroll tax tables for the year, and withholds an amount intended to add up, over all your paychecks, to roughly your annual tax. Social Security and Medicare are withheld separately, at their own rates, and state tax is withheld under state tables. Withholding is therefore a pay-period estimate of an annual figure, not a tax calculated only on that one paycheck in isolation.
Why two identical salaries can have different withholding
Two employees earning the same salary can see different federal withholding because their W-4s differ, because one has pre-tax deductions and the other does not, because one is paid biweekly and the other monthly, or because one has a second job or a working spouse accounted for on the W-4 and the other does not. Pay frequency matters for the per-paycheck amount even when the annual amount is similar: the same annual tax divided by 26 biweekly paychecks is smaller per paycheck than the same tax divided by 12 monthly paychecks.
A bonus, overtime, or a first paycheck in a new job can also look different. A bonus is often withheld at a flat supplemental rate, and a first paycheck can reflect a partial pay period or a new W-4 taking effect, rather than a full period at the usual rate.
Withholding is not the final tax
At the end of the year, your actual tax is calculated on your full annual income, after deductions and credits that withholding tables may not fully capture. If withholding over the year was more than that final tax, you get a refund of the excess. If it was less, you owe the difference. A refund is therefore not extra money from the government, it is your own over-withheld pay coming back. An amount owed is not a new tax either, it is the part of the annual tax that was not collected during the year.
That is also why a paycheck calculator and a pay stub can both be correct in different ways. The calculator estimates annual tax from salary, filing status, and deductions. The stub reflects the employer's table, your W-4, and the pay period in question. Small differences between them are normal, and large, persistent differences are worth checking against the W-4.
What to check if withholding looks wrong
Start with the W-4 on file, then with pre-tax deductions, then with pay frequency and pay period dates. Check whether a recent change, a new job, a bonus, or a change in filing status has taken effect yet. Check state withholding separately, because a federal figure that looks right can sit next to a state figure that uses a different table. This guide is general information, not advice on what to put on a W-4, and a payroll department or tax professional can check a specific stub.
FAQs
Is a tax refund free money?
No. It is usually tax you overpaid during the year through withholding, returned after your final tax is calculated.
Why did my withholding change when my salary did not?
A new W-4, a change in pre-tax deductions, a bonus, a different pay period, or a mid-year table update can all change withholding without a salary change.
Does withholding include Social Security and Medicare?
Those are withheld as well, but they are payroll taxes with their own rates and limits, separate from federal income tax withholding.