Bonus tax withholding: the percentage method formula for supplemental wages

TL;DR

- Most employers withhold federal tax on a bonus using the flat Percentage Method: a straight 22% of the bonus amount, rising to 37% once cumulative supplemental wages exceed $1,000,000 in a calendar year. - The alternative Aggregate Method combines the bonus with a regular paycheck and taxes the total at ordinary bracket rates, then backs out what would have been withheld on wages alone -- a calculation that often lands closer to a filer's true marginal rate. - FICA applies to a bonus the same way it applies to regular wages: 6.2% Social Security up to the 2026 wage base of $178,000 in combined wages, plus 1.45% Medicare with no cap, plus an additional 0.9% Medicare surtax on combined wages above $200,000. - Whether the flat 22% withholding over- or under-shoots a filer's actual liability depends entirely on where their marginal tax bracket sits relative to 22%, which is a comparison the withholding method itself does not make. - A depending-on-bracket gap between the two methods commonly runs from a four-figure refund for filers below the 22% bracket to a similarly sized balance due for filers in the 32% or higher brackets.

A bonus check often looks smaller than expected, and the reason is almost always the withholding method rather than the tax itself. The IRS classifies bonuses, commissions, and other one-time payments as "supplemental wages," and it gives employers two distinct ways to calculate how much federal tax to hold back before the money reaches an employee's account. Modeling both methods side by side -- alongside the FICA taxes that apply regardless of which one is used -- clarifies whether a bonus's smaller-than-expected size reflects extra withholding that will come back at tax time, or an accurate preview of the actual liability.

Informational calculation reference only. All equations, tools, and outputs on this page are intended strictly for educational modeling and mathematical illustration. They do not constitute certified financial, legal, or tax advice. For specific scenarios, consult a certified public accountant (CPA) or a fiduciary financial advisor.

The mathematical formula behind the calculation

Employers choose between two federal withholding methods for supplemental wages, and FICA taxes stack on top of either one identically.

The Percentage Method, used by most employers for a bonus paid as its own check, withholds a flat rate regardless of the recipient's actual bracket:

FederalWithholding_{Percentage} = Bonus \times 0.22

with the rate rising to 0.37 on any portion of cumulative supplemental wages that exceeds $1,000,000 within the same calendar year. That higher rate applies only to the excess above the threshold, not to the full bonus once the threshold is crossed.

The Aggregate Method, used when a bonus is folded into a regular paycheck, taxes the combined total using ordinary progressive brackets, then subtracts what would have been withheld on the regular wages alone:

FederalWithholding_{Aggregate} = Tax(Salary + Bonus) - Tax(Salary)

Because this method applies the filer's actual marginal bracket to the bonus dollars, it tends to track true tax liability more closely than the flat rate does -- particularly for filers whose marginal bracket sits well above or below 22%.

FICA withholding is calculated the same way under either method, since Social Security and Medicare are not part of the supplemental wage election:

$$ FICA = \min(Bonus,\ SSWageBaseRemaining) \times 0.062 + Bonus \times 0.0145 + 0.009 \times \max(0,\ CombinedWages - \$200{,}000) $$

Here, $SSWageBaseRemaining$ is whatever portion of the 2026 Social Security wage base of $178,000 has not already been reached by the employee's regular wages earlier in the year, and $CombinedWages$ refers to total wages plus bonus for the year to date. Once regular wages alone reach $178,000, no further 6.2% Social Security tax applies to the bonus, but the 1.45% Medicare rate continues uncapped, and the additional 0.9% Medicare surtax applies only to the portion of combined wages above the $200,000 threshold.

Step-by-step practical calculation example

Consider a single filer earning a $75,000 annual salary who receives a $10,000 bonus paid as a separate check, with the employer using the Percentage Method.

Step 1: Apply the flat federal rate. $10,000 bonus times 0.22 equals $2,200 withheld under the Percentage Method.

$$ FederalWithholding_{Percentage} = \$10{,}000 \times 0.22 = \$2{,}200 $$

Step 2: Estimate the Aggregate Method for comparison. At $75,000 in regular income, this filer's bonus dollars fall within the 22% marginal bracket after the standard deduction, so the Aggregate Method produces a very similar figure -- close to $2,200 -- meaning the two methods roughly agree at this income level.

Step 3: Calculate FICA on the bonus. Combined year-to-date wages of $85,000 remain well under the $178,000 Social Security wage base, so the full 6.2% applies: $10,000 times 0.062 equals $620. Medicare adds $10,000 times 0.0145, or $145, with no additional Medicare surtax since combined wages stay under $200,000.

$$ FICA = \$10{,}000 \times 0.062 + \$10{,}000 \times 0.0145 = \$620 + \$145 = \$765 $$

Step 4: Total withholding and net bonus. Combined federal and FICA withholding of $2,200 plus $765 equals $2,965, leaving a net bonus of roughly $7,035 before any state withholding.

Strategic applications for financial modeling

Bracket-gap estimation before bonus season. Comparing a filer's marginal bracket against the flat 22% rate ahead of an expected bonus payout can indicate, directionally, whether that bonus will produce an over-withholding refund or an under-withholding balance due, without needing to wait for the actual paycheck.

Pre-tax contribution timing. Increasing a 401(k) contribution percentage specifically for the pay period containing the bonus reduces the taxable wages the flat rate or aggregate calculation applies to, converting withheld tax into retirement savings for that pay cycle. Reverting the contribution rate afterward avoids over-contributing beyond the annual limit or shrinking regular paychecks more than intended.

Multi-bonus cumulative modeling. Employees who receive several bonuses across a year should model cumulative supplemental wages against the $1,000,000 threshold, since the 37% rate applies once that cumulative total is crossed, not merely to any single bonus payment in isolation.

W-4 adjustment as an offsetting lever. Because the flat withholding rate on the bonus itself cannot be changed by the employee, adjusting W-4 withholding on regular paychecks for the remainder of the year is the primary lever available for bringing total annual withholding closer to actual liability when a bonus creates a known over- or under-withholding gap.

Common pitfalls and variable mistakes

Assuming the bonus is taxed at a permanently higher rate. The 22% figure is a withholding rate, not the bonus's actual tax rate -- the amount ultimately owed on that income depends on the filer's marginal bracket once it is combined with the rest of the year's income on the tax return.

Ignoring the Social Security wage base when it has already been reached. Modeling the full 6.2% Social Security rate on a bonus paid late in the year, after regular wages have already crossed the $178,000 threshold, overstates FICA withholding for that payment.

Applying the 37% supplemental rate too early. The higher rate applies only to cumulative supplemental wages above $1,000,000 in the calendar year, so modeling it against a single moderate bonus payment produces a significantly inflated withholding estimate.

Treating the Aggregate Method estimate as exact. Because the Aggregate Method depends on the specific pay period's regular wages and the filer's exact bracket position, a modeled estimate is directionally useful but will not always match an employer's payroll system output down to the dollar.

Overlooking state supplemental withholding. Federal Percentage Method calculations do not include state-level supplemental wage withholding, which varies by state and can meaningfully change the net bonus figure beyond what the federal-only formulas above capture.

Modeling both withholding methods alongside FICA turns the smaller-than-expected bonus check into an explainable figure rather than a surprise, and clarifies whether the gap between withholding and actual liability will show up as a refund or a balance due. The Bonus Tax Calculator applies these same Percentage Method, Aggregate Method, and FICA formulas automatically across salary levels and bonus amounts.

Disclaimer: While we strive for absolute mathematical precision, actual real-world financial outcomes may vary based on institutional fees, localized tax brackets, changes in federal legislation, or fluctuating market indexes.