Bonus Check Calc

Methodology

Last reviewed by the Bonus Check Calc Editorial Team

The calculator, the worked examples on every state page and the test suite all run the same JavaScript module. This page describes what it computes, in order, and what it deliberately leaves out.

1. Taxable bonus

The bonus is reduced by any 401(k) deferral you enter before federal and state income tax are figured, because elective deferrals are not federal taxable wages. Social Security, Medicare and the state payroll contributions use the full bonus. Pennsylvania is the exception for state tax: its guide (REV-415) treats employee 401(k) deferrals as taxable compensation, so Pennsylvania's 3.07% applies to the whole bonus. Other states are assumed to follow the federal exclusion.

2. Federal income tax

Flat method: 22% of the taxable bonus. If your year's supplemental wages including this bonus exceed $1,000,000, the excess is withheld at 37% (Publication 15, section 7).

Aggregate method: Publication 15-T Worksheet 1A is applied twice — once to your regular taxable pay for the period, once to regular pay plus bonus — and the difference is the bonus withholding. Worksheet 1A annualizes the period's wages (×52, 26, 24 or 12), subtracts $12,900 (married filing jointly) or $8,600 (other statuses) unless the W-4 Step 2 box is checked, applies the 2026 annual percentage table for the filing status, and divides back by the number of periods. W-4 Steps 3 and 4 are not modeled. For a single employee paid $3,000 biweekly, a $5,000 bonus is withheld $1,166.31 under this method versus $1,100.00 at 22%.

3. Social Security and Medicare

Social Security is 6.2% of the bonus up to the part of the $184,500 wage base not yet used by your year-to-date wages. Medicare is 1.45% of the bonus plus 0.9% of the part of your year-to-date wages plus bonus that is above $200,000 and not already above it before the bonus.

4. State income tax — rule types

  • Flat supplemental rate (for example Alabama 5%, California 10.23% for bonuses, Michigan 4.25%, New York 11.70%): rate × taxable bonus. Where a state allows either its flat rate or aggregation, we use the flat rate — the method most employers use for a separate bonus check — and say so on the state page.
  • Tied to federal (Kansas, New Mexico; Iowa and Vermont in effect): the state rule changes when you switch the federal method.
  • Aggregate formula (Colorado, Connecticut, Delaware, Hawaii, Kentucky, Mississippi, Oklahoma, South Carolina, Utah, West Virginia): the state's own annualized formula is applied to regular pay with and without the bonus, and the difference is withheld. Delaware's method adds the bonus to annualized pay without annualizing it, exactly as its Employer's Guide example does.
  • Special rules: New Jersey runs the bonus through the pay-period rate table with no allowances; Massachusetts applies 5%, or 9% on the part above $1,107,750; Maryland applies its 6.50% top rate plus the county's local table; Wisconsin picks a flat percentage from your annual salary; Arizona uses your Form A-4 percentage.
  • Estimates: for DC and Louisiana no current official withholding table could be verified. DC is estimated with the DC rate schedule on top of your annualized pay less the federal standard deduction; Louisiana with its flat 3% rate. Both are labeled "Estimate," and you can enter your employer's rate instead.

5. Local taxes and payroll contributions

New York City (4.25%) and Yonkers (1.95975% resident, 0.50% nonresident) use the New York supplemental rates. Indiana adds the county rate from Departmental Notice #1. For states where local taxes vary by city or school district (Ohio, Pennsylvania, Michigan, Kentucky, Missouri, Alabama, Delaware), you can enter your own rate; we do not guess one. California SDI (1.3%, no cap), New Jersey's four worker contributions (with their 2026 wage bases), New York Paid Family Leave (0.432%, $411.91 annual maximum) and the WA Cares premium (0.58%) are included.

6. Dates and expiring figures

Every rule has a validity window taken from its source. The pay date you enter selects the rule, which matters for Georgia (5.19% before May 11, 2026) and Utah (schedules effective June 1, 2026). If no verified rule covers a date — for example a 2027 pay date before the 2027 guides are published — the line is marked "Needs your figure" and shows zero until you enter the official rate. Nebraska's 2027 rate is already verified.

7. Tax estimate

The "withholding is not your final tax" panel annualizes your regular pay and compares federal tax with and without the bonus, using the 2026 brackets and standard deduction from Revenue Procedure 2025-32. It assumes wage income only, no itemized deductions and no credits.

8. Gross-up

In gross-up mode the calculator searches for the smallest gross bonus whose net reaches your target, using the same rules, so caps, tables and the aggregate method are all respected.

9. Rounding and what is left out

Each line is rounded to the cent; Mississippi's formula rounds to whole dollars as its flowchart requires. Not included: health, dental, HSA and other pre-tax deductions besides 401(k); W-4 Steps 3 and 4; garnishments; paid-leave and disability programs we could not verify for 2026 (for example Washington, Massachusetts, Oregon, Colorado, Connecticut, Minnesota, Maine, Delaware and Rhode Island programs, and New York disability insurance); local taxes you do not enter; and your employer's own rounding conventions.