How Do Employers Calculate Federal Tax Withholding?

Employers calculate federal tax withholding in four steps: they collect your filing status and dependent information from Form W-4, subtract pre-tax benefits like 401(k) and health premiums from your gross pay, run the remaining taxable wages through the IRS withholding tables or formulas in Publication 15-T, and then apply any credits and extra withholding you asked for on the W-4. For 2026, the rates built into those tables range from 10% to 37% across seven income brackets, with a standard deduction starting at $16,100 for a single filer.¹

Step One: What Your W-4 Tells the Employer

Form W-4 is where you tell your employer three things that drive the entire calculation: your filing status, credits for dependents, and any optional adjustments.² Filing status (single, married filing jointly, or head of household) selects which set of brackets and which standard deduction the payroll system applies. Picking the wrong one is the fastest way to end up owing money in April.

Step 3 of the form handles dependents. For 2026, each qualifying child under 17 reduces the annual withholding figure by $2,200, mirroring the child tax credit you’ll claim on your return. Step 4 covers optional adjustments. You can report other income that has no withholding attached to it, such as investment income, so your employer withholds extra to cover it. You can also claim deductions beyond the standard deduction to shrink the amount withheld, or simply request a flat additional dollar amount per paycheck.²

If You Never Turn in a W-4

When no valid W-4 is on file, the employer must withhold as if you are single with no adjustments.³ The same default applies to any altered or homemade substitute form the employer can’t accept.⁴ The single-with-no-adjustments default produces heavier withholding than most other combinations, so skipping the W-4 usually means lending more money to the government interest-free until you file. The rule sits in 26 U.S.C. § 3402.⁵

Claiming Exempt

If you had zero federal tax liability last year and expect none this year, you can write “Exempt” on the W-4 and have no federal income tax withheld. The exemption expires every February, so you have to file a fresh W-4 each year to keep it. If you don’t, the employer reverts to single with no adjustments.

When the IRS Overrides Your W-4

If the IRS decides an employee has been under-withheld, it can send the employer a “lock-in letter” setting a minimum withholding arrangement. After that, the employer cannot go below the amount the IRS specified, even if you submit a new W-4 asking for less. A new W-4 that produces more withholding than the lock-in figure still gets honored; anything lower is blocked.⁴

Step Two: Turning Gross Pay Into Taxable Wages

Gross pay is everything you earned in the pay period: salary, hourly wages, bonuses, commissions. Not all of it is subject to federal income tax withholding. Before running the calculation, the employer subtracts certain pre-tax benefit contributions.

The most common subtraction is traditional 401(k) or 403(b) contributions. Money you put into those accounts comes out of gross pay before federal income tax is calculated, which shrinks the taxable base immediately. Health insurance premiums paid through a Section 125 cafeteria plan work the same way. Those salary reductions aren’t treated as wages for federal income tax purposes, and they’re generally exempt from Social Security and Medicare taxes as well.⁶ Flexible spending account and health savings account contributions also come off the top.

Fringe Benefits That Get Added Back

Some employer-provided benefits go the other direction and increase your taxable wages. Any fringe benefit is taxable unless the law specifically excludes it. The ones that show up most often:

Group life insurance coverage above $50,000 — the cost of the excess coverage gets added to taxable wages.
Transit passes, van pooling, and qualified parking above $340 per month in 2026 (a separate $340 monthly limit applies to each). Anything over the limit is taxable.
Nonstatutory stock options at exercise: the spread between market value and the price paid is taxable wages.
Personal use of a company car, non-qualifying employer-provided lodging, or a cell phone given primarily as a perk.

Employers report the taxable value of these on your W-2, and the withholding calculation must reflect them.⁷

Step Three: Running the Numbers Through Publication 15-T

Once taxable wages for the pay period are set, the employer turns to IRS Publication 15-T, which contains two methods for converting those wages into a withholding amount.⁸ The two methods produce the same result from the same inputs. The difference is mechanical.

Wage Bracket Method

The employer finds the table matching your pay frequency and filing status, locates the row for your taxable wage range, and reads across to the withholding amount. Simple, but the tables don’t cover every possible wage level, especially at higher incomes.

Percentage Method

Most payroll software uses the Percentage Method because it handles any wage level. The sequence:

Annualize the pay: multiply the period’s taxable wages by the number of pay periods in a year (52 weekly, 26 biweekly, 24 semimonthly, 12 monthly).
Subtract the standard deduction built into the table. For 2026, that’s $16,100 for a single filer, $32,200 for married filing jointly, and $24,150 for head of household.¹
Slice the remainder across the 2026 brackets, applying each rate to its portion.
Subtract the annualized dependent credits from Step 3 of the W-4.
Divide the annual figure by the number of pay periods to get the amount withheld from this paycheck.

The 2026 federal income tax brackets for a single filer:

10% up to $12,400
12% from $12,401 to $50,400
22% from $50,401 to $105,700
24% from $105,701 to $201,775
32% from $201,776 to $256,225
35% from $256,226 to $640,600
37% over $640,600

For married filing jointly, each threshold roughly doubles.¹ Employers have to use the current year’s Publication 15-T because these figures shift with inflation. Running last year’s tables on this year’s paychecks will under- or over-withhold every employee on the payroll.

Bonuses and Other Supplemental Pay

Bonuses, commissions, overtime, severance, and back pay are treated as “supplemental wages,” and the rules differ from regular payroll. When a bonus is paid separately from your regular paycheck and your year-to-date supplemental wages are $1 million or less, the employer can use a flat 22% federal income tax rate instead of running the full bracket calculation.⁹ That’s why so many bonus checks look like roughly 22% was pulled out.

Once supplemental wages exceed $1 million in a calendar year, the portion above $1 million must be withheld at 37%, the top rate, regardless of what your W-4 says.⁹

When a bonus is combined with a regular paycheck rather than issued separately, the employer can use the “aggregate method.” That means adding the supplemental and regular wages together, computing withholding on the combined total as a normal paycheck, and subtracting what would have been withheld on the regular wages alone. The difference is the withholding on the bonus.¹⁰ The aggregate method often withholds more than the flat 22% because the combined check briefly pushes you into a higher bracket for that single period, even if your annual income won’t actually land there.

What About Social Security and Medicare?

Federal income tax isn’t the only line pulled from a paycheck. FICA taxes fund Social Security and Medicare, and they run on a separate track from the calculation above.

For Social Security, the employer withholds 6.2% of your wages up to $184,500 in 2026.¹¹ Once year-to-date earnings hit that cap, Social Security withholding stops for the rest of the year. The employer pays a matching 6.2% from its own funds.

Medicare has no wage cap. The employer withholds 1.45% on all wages and matches it with another 1.45%. An additional 0.9% Medicare surtax applies once your wages with that employer exceed $200,000 in a calendar year. The employer withholds the extra 0.9% on wages above $200,000 regardless of your filing status, and does not match it.¹²

If the numbers on your pay stub don’t look right, the two things worth checking first are the filing status and dependent entries on your most recent W-4, and whether any pre-tax benefit contributions changed this pay period. Those inputs move withholding far more than anything else on the paycheck.

1
Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026
2
Internal Revenue Service. Form W-4, Employee’s Withholding Certificate
3
Internal Revenue Service. FAQs on the 2020 Form W-4
4
Internal Revenue Service. Withholding Compliance Questions and Answers
5
Office of the Law Revision Counsel. 26 USC 3402 – Income Tax Collected at Source
6
Internal Revenue Service. FAQs for Government Entities Regarding Cafeteria Plans
7
Internal Revenue Service. Employer’s Tax Guide to Fringe Benefits – Publication 15-B
8
Internal Revenue Service. Publication 15-T (2026), Federal Income Tax Withholding Methods
9
Internal Revenue Service. Publication 15 (2026), (Circular E), Employer’s Tax Guide
10
eCFR. 26 CFR 31.3402(g)-1 – Supplemental Wage Payments
11
Social Security Administration. Contribution and Benefit Base
12
Internal Revenue Service. Questions and Answers for the Additional Medicare Tax