What Are Supplemental Wages?
The IRS uses the term supplemental wages to describe payments made to employees that are not part of their regular wage or salary. Bonuses are the most common form, but supplemental wages also include commission payments, overtime pay, tips, severance pay, back pay, and certain awards or prizes paid by an employer.
The distinction between regular wages and supplemental wages matters because the IRS allows employers to use different withholding methods for each type. As a result, the federal withholding on a bonus payment can look quite different from what an employee might expect based on their regular paycheck withholding.
Why Bonus Withholding Looks Different
On a regular paycheck, employers calculate withholding using the employee's W-4 elections and the IRS withholding tables. The tables attempt to estimate what a full year of wages would produce in annual tax, then spread that liability across each pay period. The result is paycheck withholding that tracks reasonably closely with annual tax liability for employees with straightforward situations.
Bonuses disrupt this calculation. If an employer simply added a large bonus to the next regular paycheck, the withholding tables would calculate withholding as if the employee earns that elevated amount every pay period — producing dramatically high withholding on that paycheck. To avoid this distortion, the IRS provides separate methods for withholding on supplemental wage payments.
The Flat-Rate (Percentage) Method
The most common withholding approach for bonuses is the flat-rate method, sometimes called the optional flat rate. Under this approach, the employer withholds a flat percentage of the supplemental wage payment — a fixed rate set by the IRS for supplemental wages up to a certain threshold, and a higher flat rate for very large supplemental payments above that threshold.
The flat rate applicable to most bonuses has historically been 22% for supplemental wages below a high-income threshold, and 37% for amounts that exceed a separate higher threshold. These rates are subject to change through legislation or IRS regulation, and the applicable rate for a given year should be verified with current IRS guidance (Publication 15).
The flat-rate method is simpler to calculate than the alternative and is widely used. The withholding is applied directly to the bonus amount without considering the employee's other wages or W-4 information.
The Aggregate Method
The alternative is the aggregate method. Under this approach, the employer adds the supplemental payment to the employee's most recent regular wages, then calculates withholding on the combined total using the normal withholding tables — as if the combined amount were a single regular paycheck. The withholding already applied to the regular wages in that pay period is subtracted, and the remainder is withheld from the bonus payment.
The aggregate method attempts to approximate the tax that would apply if the bonus were part of a regular paycheck with the same effective pay period rate. It takes the employee's W-4 elections into account. The result may be higher or lower than the flat-rate method depending on the employee's regular wage level and filing status.
| Method | How Withholding Is Calculated | Uses W-4 Info? |
|---|---|---|
| Flat-Rate | Fixed % applied directly to the supplemental payment | No |
| Aggregate | Bonus added to recent regular wages; standard tables applied to total | Yes |
Withholding ≠ final tax liability. Whichever method an employer uses, the amount withheld from a bonus payment is not necessarily the amount of tax ultimately owed on that bonus. Year-end tax liability depends on total annual income, deductions, credits, and other factors. Bonus withholding is an estimate applied at the time of payment — the actual tax due is determined when you file your return.
When Are Both Methods Available?
The IRS generally allows employers to use either method when supplemental wages are paid separately from regular wages. If regular and supplemental wages are combined in a single payment and not identified separately, the employer must use the aggregate method — it cannot apply the flat rate to a portion of an undifferentiated combined payment.
Most employers that issue bonus payments as separate checks use the flat-rate method for simplicity. Some payroll systems apply the aggregate method by default. The method used by a specific employer determines what withholding appears on the bonus pay stub, which is why bonus withholding varies widely between employers even for identical bonus amounts.
FICA on Bonus Payments
Bonus payments are generally subject to Social Security and Medicare withholding (FICA) in the same way as regular wages, up to the applicable Social Security wage base for the year. An employee who has already reached the Social Security wage base ceiling before receiving a bonus will not have additional Social Security tax withheld on that bonus, but Medicare withholding continues with no income cap.
An employee earning $80,000 per year receives a $10,000 year-end bonus paid as a separate check.
Using the flat-rate method: The employer applies 22% federal withholding directly to the $10,000 bonus = $2,200 withheld. Medicare and Social Security taxes are withheld additionally on top of this.
Using the aggregate method: The employer adds $10,000 to the most recent regular paycheck (say, $3,077 bi-weekly), calculates withholding on $13,077 using the withholding tables and the employee's W-4, then subtracts what was already withheld from the regular portion. The remainder is the bonus withholding. This amount may be higher or lower than $2,200 depending on the employee's W-4 elections and filing status.
In both cases, the amount withheld is an estimate applied at time of payment. The actual federal tax on the bonus is determined at year-end based on total annual income.
These figures are illustrative and do not constitute tax calculations. Withholding rates can change — verify with current IRS guidance.
Why Calculator Estimates May Vary
Bonus tax calculators typically apply the flat-rate method by default, as it is the most common and straightforward. If your employer uses the aggregate method, or if your W-4 elections result in different withholding than the flat rate, the calculator output will not match your actual bonus check withholding. This is expected — calculators estimate based on a general methodology, not your specific employer's payroll system configuration.