Most paychecks use IRS wage bracket or percentage methods to calculate federal income tax withholding. Flat tax withholding is the exception: a single rate applied to a specific payment type, regardless of the employee's W-4 or income level. The federal supplemental wage rate, which sits at 22 percent for most bonuses and commissions, is the most common example, and it catches many employees off guard at bonus time. For HR and payroll teams, understanding where flat withholding applies, and when the IRS requires it, prevents the most common bonus-season payroll errors and the employee confusion that follows.
The Federal Supplemental Wage Rule The IRS defines supplemental wages as payments outside regular wages: bonuses, commissions, overtime pay, severance, retroactive pay, back pay, reported tips (in certain cases), awards, and prizes.
When supplemental wages are paid separately from regular wages, or when identified as supplemental on the same payment, two methods apply. The optional flat rate: 22 percent on supplemental wages up to $1 million per year per employee. The mandatory flat rate: 37 percent on the portion above $1 million.
The Aggregate Method as an Alternative If the employer combines the supplemental payment with regular wages and doesn't identify the supplemental portion separately, the aggregate method applies. Withholding is calculated on the combined amount using the regular wage bracket or percentage method, based on the employee's W-4.
The aggregate method can produce higher or lower withholding than the flat 22 percent, depending on the employee's pay level and W-4 elections. Most employers default to the optional flat rate because it's simpler and more predictable for employees.
Why Do Employees Complain About Bonus Withholding? Two reasons. At $100,000 of base salary, the employee's marginal tax rate is probably around 24 percent, higher than the 22 percent flat rate, so the bonus is under-withheld, and the employee owes at tax time. At lower incomes, the 22 percent flat rate over-withholds, and the employee feels the bonus was taxed too hard when in reality they'll get the money back as a refund.
State Flat Rates Several states apply a flat income tax rate statewide (Colorado, Illinois, Indiana, Kentucky, Massachusetts, Michigan, North Carolina, Pennsylvania, Utah). In those states, regular wage withholding uses the same flat percentage as supplemental withholding, simplifying the calculation.
Other states use flat rates only for supplemental wages, similar to the federal approach. California, Georgia, and others publish their supplemental withholding rates separately.
Handling Flat Tax Withholding Without Triggering Employee Complaints Communicate clearly at bonus time. A one-page explainer that shows the 22 percent federal plus applicable state flat rate plus 7.65 percent FICA, alongside the after-tax bonus amount, prevents the worst of the complaint volume in the weeks after payout.
Align bonus payouts with a refreshed W-4 prompt so employees can adjust withholding if the flat rate significantly under-withholds their actual tax liability. Reference the IRS Publication 15 for the current supplemental wage rules and the IRS Publication 15-T for withholding method details. Store supplemental wage withholding records alongside regular payroll documentation and W-2 form reconciliation.
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