Gross-Up Calculator

When every withholding is a flat percentage of the payment, the gross-up formula is:

Gross = (Net + after-tax deductions) ÷ (1 − total withholding rate)

Take a $1,000 net bonus for an employee in Texas (no state income tax) with $50,000 of 2026 wages so far. The rates are 22% federal + 6.2% Social Security + 1.45% Medicare = 29.65%, so the gross is $1,000 ÷ 0.7035 = $1,421.46. Check it line by line:

The gross-up multiplier is 1.4215, and the employer's cost is $1,530.20 once its own Social Security and Medicare match are added. The common mistake is to multiply instead of divide: $1,000 × 1.2965 = $1,296.50, which leaves only $912.09 after withholding because the added $296.50 is taxed as well.

The single-rate formula breaks when the payment crosses a cap. With $182,000 already paid in 2026, a $15,000 net bonus needs a gross of $19,818.90: only $2,500 of it is still under the $184,500 Social Security wage base ($155.00 withheld), and $1,818.90 lands above the $200,000 Additional Medicare threshold ($16.37 withheld). The calculator handles these breaks by searching for the exact gross instead of using one rate.

Pub 15 (2026) section 7 gives employers these federal options for supplemental wages. The rates stayed at 22% and 37% because P.L. 119-21 made the 2017 tax rates permanent.

Under the aggregate methodyou add the bonus to the regular wages for the period, figure withholding on the total with the Pub 15-T tables, and subtract what was withheld from the regular wages. Pub 15's own example: an employee paid $2,000 monthly has $65 withheld; a $1,000 bonus added to that pay produces $179 on $3,000, so the bonus gets $114. The flat method would take 22% × $1,000 = $220. The aggregate method depends on the employee's W-4 and pay, which is why this calculator uses the flat method for bonuses: its result is the same for every employee.

Social Security and Medicare apply under either method. Social Security is 6.2% of wages up to $184,500 for 2026 (the wage base on the SSA contribution and benefit base table), Medicare is 1.45% of all wages, and employers must withhold the 0.9% Additional Medicare Tax on wages above $200,000 in the calendar year regardless of filing status. For regular paychecks, Pub 15-T Worksheet 1A annualizes the wages, adds Step 4(a) income, subtracts Step 4(b) deductions plus $8,600 ($12,900 for married filing jointly) unless the Step 2 box is checked, applies the annual rate schedule, divides by the pay periods, subtracts Step 3 credits per period, and adds Step 4(c). Our guide to Form W-4 for 2026 explains each step.

States set their own bonus rules. Some publish a flat supplemental rate, some require the aggregate method, and nine states have no wage income tax. The table shows the rule for each state and the rate this calculator applies by default, plus employee-paid disability or paid-leave contributions. Each rule comes from the state's current withholding guide or agency page (linked on the state name), verified September 2026. Where a state has no flat rate, the default is its top rate, so the result errs toward a slightly larger gross.

Local taxes are separate: New York City and Yonkers, Pennsylvania local earned income taxes, Ohio municipal taxes, Maryland county taxes, Indiana county taxes, and Michigan city taxes all apply to bonuses. Enter the local rate in the calculator if the employee owes one.

A bonus is ordinary income. Withholding is a prepayment, and the employee settles the real tax on Form 1040. Take a single employee earning $60,000 who gets a $5,000 bonus in 2026. After the $16,100 standard deduction, taxable income is $48,900, still inside the 12% bracket that ends at $50,400 (Rev. Proc. 2025-32). The bonus costs $600 in federal income tax, but the flat method withheld $1,100, so about $500 comes back at filing, all else equal. An employee in the 24% bracket or higher is under-withheld at 22% and may owe; extra withholding on Step 4(c) of the W-4 closes that gap.

The same goes for FICA extras. Employers withhold Additional Medicare Tax on wages above $200,000 from them, but the tax itself starts at $250,000 for married couples filing jointly, $125,000 for married filing separately, and $200,000 for everyone else, and it is reconciled on Form 8959. Employees with two employers who together withhold Social Security on more than $184,500 get the excess back as a credit on their return.

For the employer, the grossed-up amount is wages: it goes on the quarterly Form 941, and the withheld tax follows your payroll tax deposit schedule. Model the employee's side of a regular paycheck with the paycheck calculator.