How Bonus Tax Actually Works: Flat Rate vs Aggregate Method

You got a $15,000 bonus. After taxes, $9,200 landed in your account.

Your math says 38.7% was withheld. Your colleague got the same bonus and kept $11,700. The internet says bonuses are taxed at 22%.

All three of these facts can be true simultaneously. Here's why.

The two withholding methods employers use for bonuses

The IRS allows employers to choose between two methods for withholding federal income tax on supplemental wages (bonuses, commissions, overtime, severance).

Method 1: Percentage (Flat) Method Withhold a flat 22% for federal income tax on the supplemental wage, regardless of the employee's regular salary or W-4 elections. (37% applies above $1 million in supplemental wages in a calendar year.)

A $15,000 bonus under this method: $3,300 federal income tax withheld. Plus Social Security (6.2%, capped at wage base) and Medicare (1.45%). Total federal withholding: ~$4,275.

Method 2: Aggregate Method Combine the bonus with the employee's most recent regular paycheck. Calculate withholding on the combined amount as though it were a single paycheck, then subtract the withholding already paid on the regular paycheck.

For a $120,000 base salary employee receiving a $15,000 bonus:

• Regular biweekly gross: $4,615
• Bonus added: $15,000
• Combined: $19,615
• Withholding on combined amount: significantly higher, because the spike in income pushes into higher annualized bracket
• Minus regular paycheck withholding: the net on the bonus paystub can be 32–38%

Same employee. Same bonus. Different withholding method. Different net deposit.

The important clarification: withholding ≠ actual tax

Neither method changes your actual tax liability for the year.

At year-end, your W-2 reports total wages and total withholding. Your tax return calculates actual tax owed based on total income — bonus plus salary plus all other income — against the tax tables. Any over-withholding comes back as a refund.

The 38% withheld on the bonus doesn't mean your bonus was taxed at 38%. It means your employer overcollected during that pay period, and you'll true up at filing.

For employees with accurately calibrated withholding overall, the aggregate method means a smaller refund (or a payment) in April that would otherwise not have appeared — not an extra tax.

When timing a bonus across tax years actually matters

The year in which the bonus is paid does affect your tax bill — not the withholding method.

If your income is unusually high this year (RSU vesting, investment gains, home sale), receiving the bonus in January of the following year could shift it into a lower-income year — and a lower marginal rate.

Conversely, if you expect higher income next year (promotion, new income source), accelerating the bonus into December of the current year locks in the lower rate.

The math: a $20,000 bonus for someone at the 22%/24% bracket boundary saves approximately $400 if timed into the 22% year. That increases to $2,000+ for someone near the 32%/35% boundary.

Year-end strategies that reduce taxable bonus income

Several mechanisms reduce the taxable portion of a bonus:

401k contribution increase: If you haven't hit the annual limit ($23,500 in 2026), increase your 401k contribution percentage before the bonus pay period. Pre-tax contributions reduce the taxable bonus dollar for dollar.

HSA contribution: If enrolled in a qualifying high-deductible health plan, make or increase your HSA contribution before year-end. The 2026 individual limit is $4,300; family is $8,550.

Charitable giving: A cash donation to a qualifying charity in the same year as the bonus creates an itemized deduction — or a Qualified Charitable Distribution from an IRA offsets income without the itemization requirement.

None of these strategies change the withholding method your employer uses. They reduce the taxable income the bonus is calculated against, which matters when you file.

How to calculate your true bonus tax

The question isn't what percentage was withheld. It's: what is your marginal rate on the bonus, and how does the bonus affect other income-based thresholds?

For most W-2 employees:

1. What is your projected total income for the year including the bonus?
2. What marginal bracket does that place you in?
3. How much of the bonus falls in that bracket vs. the bracket below?

For employees with IRMAA exposure, ACA subsidy phaseouts, or large capital gains, the bonus can trigger secondary costs beyond the marginal income tax rate.

The US Bonus Tax Calculator computes both the flat-rate and aggregate withholding on a bonus, the actual marginal tax impact, and the net after-tax amount using your salary and filing status.