The “one-time tax” on your salary is the federal income tax your employer withholds from any pay that falls outside your regular paycheck, such as a bonus, commission, severance, or back pay. The IRS calls these payments supplemental wages, and the standard withholding is a flat 22% for federal income tax. It is not a separate tax and not a higher tax rate. It is an advance payment toward the same income tax bill you settle when you file your return.
What Kinds of Payments Get Taxed This Way
Supplemental wages are any compensation paid on top of your regular salary or hourly wages. The IRS lists bonuses, commissions, overtime pay, severance pay, back pay, retroactive raises, accumulated sick leave payouts, and payments for nondeductible moving expenses as common examples.1Internal Revenue Service. Publication 15 (Circular E), Employer’s Tax Guide Reported tips fall in this category too.2eCFR. 26 CFR 31.3402(g)-1 – Supplemental Wage Payments
What ties these payments together is that they sit outside your normal pay cycle or amount. Your base salary is predictable, so ordinary payroll withholding tables can be applied cleanly. A bonus or commission is not, which is why a separate withholding rule exists. The label on your paystub might read “supplemental,” “bonus tax,” or “one-time tax,” but the underlying calculation is the same.
How the Withholding Is Calculated
Federal rules give your employer two options for calculating the federal income tax on supplemental wages. Your employer picks; you do not. That is why two coworkers with identical bonuses can see different amounts withheld.
The Flat 22% Method
The simpler method is a flat 22% on any supplemental payment, provided your total supplemental wages for the calendar year stay at or below $1 million. No other flat percentage is permitted. Once your supplemental wages for the year exceed $1 million, the excess is withheld at 37%, regardless of what your Form W-4 says.1Internal Revenue Service. Publication 15 (Circular E), Employer’s Tax Guide These rates were permanently extended by legislation in 2025 and mirror the third and top federal income tax bracket rates.
The arithmetic is quick. A $10,000 bonus withheld at 22% sends $2,200 to federal income tax, and the rest moves through to the rest of your deductions. That is why so many bonus checks show a round-looking federal tax figure. Large payroll departments default to this method for the same reason.
The Aggregate Method
The aggregate method treats the supplemental payment as if it were part of your regular paycheck. Your employer adds the supplemental amount to your normal wages for that pay period, calculates the federal income tax on the combined total as if it were a single regular payment, then subtracts the tax already being withheld on your regular wages. What is left is the withholding on the supplemental portion.3eCFR. 26 CFR 31.3402(g)-1 – Supplemental Wage Payments
This can come out higher or lower than 22%. A modest salary can produce aggregate withholding below the flat rate. A salary that already sits in the 32% or 35% bracket can produce aggregate withholding well above it. Workers who receive a bonus on the same check as their regular pay often see the aggregate method applied by default, which explains bonus checks that look taxed harder than expected.
Social Security and Medicare Come Off Too
The “one-time tax” line usually refers only to federal income tax, but supplemental wages also carry the same Social Security and Medicare taxes as your regular pay.
- Social Security: 6.2% of wages up to $184,500 in 2026. Once your combined regular and supplemental wages for the year reach that ceiling, no further Social Security tax is withheld.4Social Security Administration. Contribution and Benefit Base
- Medicare: 1.45% on all wages, with no cap. If your total wages for the year exceed $200,000, your employer must also withhold an additional 0.9% Medicare tax on wages above that threshold.5Internal Revenue Service. Social Security and Medicare Withholding Rates
A large bonus paid late in the year is often the payment that pushes you across the $200,000 mark and triggers the additional Medicare withholding for the first time. Combined with 22% federal income tax withholding, the total deduction on that one check can look severe. Your employer has no discretion once the threshold is crossed.
State Withholding on Top
Federal is only half the story. About half the states set their own flat withholding rate for supplemental wages, distinct from the rate used on regular pay. State supplemental rates range from roughly 1.5% to over 10%. States without an income tax withhold nothing extra, but in a high-tax state, adding state supplemental withholding to 22% federal can push combined income tax withholding past 30%, before Social Security and Medicare are added.
Your paystub should show state withholding on a separate line. If the amount looks off, your state’s department of revenue website lists the current supplemental rate. Like the federal figure, it is an estimate that gets reconciled on your state return.
Why Your Bonus Feels Overtaxed
This is the most common complaint about the one-time tax line, and it is almost always a misreading of what the withholding represents. The federal income tax is progressive. Your first dollars are taxed at 10%, later dollars at 12%, then 22%, and so on up to 37%. In 2026, a single filer does not reach the 22% bracket until taxable income exceeds about $50,400, and the 24% bracket begins around $105,700.
The flat 22% withholding on a bonus assumes you sit somewhere in the middle of that structure. For many workers, that is a reasonable estimate. For some, it is not. If your full-year effective tax rate ends up below 22%, you overpaid on that check and will get the difference back at filing. If your income puts you in the 32% or 35% bracket, 22% was actually too little and you will owe more in April. Neither outcome means your bonus was taxed at a special rate. The IRS does not tax bonuses differently from salary. It withholds differently to keep payroll manageable.
Withholding is a deposit toward your tax bill. It is not the bill.
What to Do If 22% Will Not Be Enough
When the flat withholding does not match your actual bracket, you can end up owing at filing time, and possibly owing a penalty. The IRS charges an underpayment penalty if you owe $1,000 or more when you file and your total withholding plus estimated payments did not meet one of these safe harbors:6Office of the Law Revision Counsel. 26 USC 6654 – Failure by Individual to Pay Estimated Income Tax
- 90% of the current year’s tax. If your withholding and estimated payments cover at least 90% of what you owe for 2026, no penalty applies.
- 100% of last year’s tax. If your withholding at least matches the total tax on your 2025 return, you are safe even if you owe a large balance this year. That threshold rises to 110% if your adjusted gross income exceeded $150,000, or $75,000 if married filing separately.
If you receive a large bonus midyear and suspect 22% will fall short, you have two practical options. You can submit an updated Form W-4 to raise withholding on your remaining regular paychecks. The IRS Tax Withholding Estimator at irs.gov walks through the math. Or you can make a quarterly estimated tax payment using Form 1040-ES to cover the gap directly. Either route keeps you inside a safe harbor.
How It Settles at Filing Time
At year-end, your employer reports all your wages, regular and supplemental combined, on your Form W-2. Box 1 shows your total taxable compensation, and Box 2 shows the total federal income tax withheld across every paycheck and bonus during the year.7Internal Revenue Service. 2026 General Instructions for Forms W-2 and W-3 There is no separate line for supplemental wages.
Your Form 1040 uses the Box 1 amount as income and credits the Box 2 amount as tax already paid. The return calculates your actual liability based on your full year of income, deductions, and credits, then compares that liability to what was withheld. If withholding exceeded liability, you get a refund. If it fell short, you pay the difference. That reconciliation is where the one-time tax on your paystub finally gets settled. It was never a final tax to begin with.