The 22% Flat Supplemental Withholding Rule
When payroll systems process a bonus separately from your regular paycheck, IRS regulations require employers to apply a statutory flat rate of 22% for federal income tax. If your regular marginal tax bracket is 12%, this means the bonus is over-withheld upfront, and you will get the difference back as a tax refund when you file.
The Aggregate Method: Why Combined Checks Get Squeezed
If your employer combines your bonus with your regular paycheck in a single direct deposit, the payroll software uses the 'aggregate method'. It annualizes that abnormally high check as if you earn that large amount every single pay period, temporarily bumping your withholding into higher tax brackets (such as 32% or 35%).
Worked Example: $10,000 Bonus Check Breakdown
On a $10,000 bonus check, you can typically expect to take home roughly $6,500 in spendable funds.
| Line Item | Withholding Rate | Deducted Amount | Remaining Balance |
|---|---|---|---|
| Gross Bonus | - | - | $10,000 |
| Federal Withholding | 22.00% | $2,200 | $7,800 |
| Social Security | 6.20% | $620 | $7,180 |
| Medicare | 1.45% | $145 | $7,035 |
| State Withholding (Est) | 5.00% | $500 | $6,535 |
| Net Take-Home Bonus | - | - | $6,535 |