Supplemental Wage Withholding: Flat 22% vs. Aggregate Method

When your employer withholds federal income tax from a bonus, commission, or other extra pay, the choice comes down to supplemental wage withholding under the flat rate versus the aggregate method. The flat rate takes a straight 22% off the top. The aggregate method stacks the payment onto your regular wages for that pay period and runs the combined total through the normal W-4 tables. Your employer picks which one to use, and the pick changes how much lands in your account on payday. It doesn’t change what you actually owe when you file.¹

What Counts as Supplemental Wages

The IRS uses a broad definition. Bonuses, commissions, overtime pay, severance pay, accumulated sick leave payouts, back pay, prizes and awards, retroactive pay increases, and payments for nondeductible moving expenses all qualify. Reported tips count once an employee reports more than $20 in tips for a month.¹

Non-cash awards count too. A vacation package or electronics given as a performance reward has a fair market value, and that value is treated as supplemental wages subject to withholding, showing up on your W-2 like a cash bonus.²

The Flat 22% Method

Under the flat rate method, your employer withholds exactly 22% of the supplemental payment for federal income tax, no matter your filing status, W-4 elections, or bracket. A $10,000 bonus produces $2,200 in federal income tax withholding. A $500 commission produces $110. The math never varies.¹

Two conditions must be met before an employer can use this method. The supplemental payment has to be identified separately from regular wages on the payroll records. And the employer must have withheld federal income tax from the employee’s regular wages during either the current or the immediately preceding calendar year. If either condition fails, the aggregate method is mandatory.¹

The 22% rate was permanently extended by P.L. 119-21, so it isn’t a temporary provision waiting to sunset.¹

When 22% Lands Close to Reality

The flat rate roughly matches actual liability for single filers earning about $48,000 to $103,000 and married-filing-jointly filers earning about $96,000 to $206,000, since those ranges sit inside the 22% federal bracket for 2026.³ If you’re in that zone, the flat rate is unlikely to leave you with a big surprise at tax time.

When 22% Falls Short

If your marginal rate is 32% or 35%, a 22% withholding on a large bonus will fall meaningfully short of what you actually owe on that money. One under-withheld paycheck won’t trigger a penalty by itself. Repeated bonuses across the year that all get the 22% treatment can push you into underpayment territory by April.

The Aggregate Method

The aggregate method treats your supplemental payment as if it were part of your regular paycheck. Your W-4 information, filing status, and dependent adjustments all drive the calculation, so the withholding reflects your personal tax situation rather than a fixed rate.

How the Calculation Works

Your employer adds the supplemental payment to the regular wages for the same payroll period. If the bonus is paid between regular paydays, the employer uses the regular wages from the current or preceding payroll period. That combined total is treated as a single wage payment for one payroll period, and the employer looks up withholding in the standard IRS tables in Publication 15-T.¹

The employer then subtracts the tax already withheld from the regular wages. What’s left is pulled from the supplemental payment.¹ A simplified example: your regular biweekly pay is $3,000, with normal withholding of $280. Your employer pays a $5,000 bonus in the same period. Withholding gets calculated on $8,000 as if that were a normal biweekly paycheck, which might come out to $1,350. Subtract the $280 already assigned to the regular wages, and $1,070 comes out of the bonus.

This is where the aggregate method can feel heavy. Stacking the bonus on top of regular pay pushes the pay-period calculation into a higher bracket. The tables don’t know it’s a one-time bonus; they treat it as if you earn $8,000 every two weeks. The withholding often exceeds what the flat rate would produce. You get the difference back as a refund when you file.

When the Aggregate Method Is Required

Two situations force the employer’s hand. If the employer didn’t withhold federal income tax from your regular wages during the current or preceding calendar year, the flat rate isn’t available. And if the supplemental wages aren’t separately identified on the payroll records, the employer has to withhold as if the combined total were a single regular-period payment.¹ Beyond those rules, some employers simply prefer the aggregate method because it fits their payroll software.

Who Picks the Method

The employer decides. Employees have no right to demand one method over the other, and most payroll departments pick whichever approach their software handles cleanly. Large employers with sophisticated systems often default to the aggregate method. Smaller operations often prefer the flat rate for its simplicity. If you want to know which one applies to your bonus, ask payroll.

The $1 Million Ceiling

Once your supplemental wages from a single employer cross $1 million in a calendar year, both regular methods stop applying to the excess. Every dollar above $1 million must be withheld at 37%, matching the top individual federal rate for 2026. The employer applies this without regard to your Form W-4.¹³

The first $1 million can still ride at the 22% flat rate or under the aggregate method. Only the portion above $1 million triggers the mandatory 37%. And supplemental wages from businesses under common control must be aggregated for this threshold, so you can’t split bonuses across related subsidiaries to reset the count.¹

Don’t Forget Social Security and Medicare

Federal income tax withholding is only part of what comes out. Supplemental wages are also subject to Social Security tax at 6.2% and Medicare tax at 1.45%, the same rates that apply to regular pay. Your employer pays a matching amount.⁴

Social Security tax stops once your total wages for the year exceed $184,500 in 2026.⁵ Medicare tax has no wage cap. Once total wages exceed $200,000 in the year, your employer must also withhold the Additional Medicare Tax of 0.9% on everything above that mark, with no employer match on the additional portion.⁴

Rough estimate for a $10,000 bonus while you’re still under the Social Security wage base: about $2,200 federal income tax under the flat rate, $620 Social Security, $145 Medicare. That’s $2,965 before state taxes, leaving roughly $7,035. Most states with an income tax add their own supplemental withholding on top, at rates that vary widely.

If You’re Under-Withheld, Here’s How To Catch Up

If you’re above the 22% bracket and your employer uses the flat rate on a substantial bonus, plan on owing money at filing time. Two tools help you avoid the gap becoming a penalty.

You can submit a revised Form W-4 with an additional dollar amount on Line 4(c), which tells your employer to withhold extra from each remaining paycheck.⁶ Or you can make quarterly estimated payments to the IRS using Form 1040-ES.

The IRS generally won’t charge an underpayment penalty if you meet one of three safe harbors: you owe less than $1,000 after subtracting withholding and credits; your total withholding and estimated payments equal at least 90% of your current-year tax; or they equal at least 100% of your prior-year tax liability.⁷ The prior-year safe harbor is usually the easiest to hit because you already know the number.

The Method Changes Your Cash Flow, Not Your Tax Bill

Both the flat rate and the aggregate method are just withholding estimates. Your actual tax liability gets calculated when you file your return. Over-withholding comes back as a refund. Under-withholding gets settled with your payment. So while the choice of method affects how much of your bonus you see today, it does not decide what you owe for the year. Track the running gap between what’s been withheld and what you’ll actually owe, and use W-4 adjustments or estimated payments to keep the two in the same neighborhood by December.

1
Internal Revenue Service. Publication 15 (2026), (Circular E), Employer’s Tax Guide
2
Internal Revenue Service. Publication 525, Taxable and Nontaxable Income
3
Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026, Including Amendments From the One, Big, Beautiful Bill
4
Internal Revenue Service. Topic No. 751, Social Security and Medicare Withholding Rates
5
Social Security Administration. Contribution and Benefit Base
6
Internal Revenue Service. Form W-4, Employee’s Withholding Certificate
7
Internal Revenue Service. Topic No. 306, Penalty for Underpayment of Estimated Tax