How Do Employers Calculate Tax Withholding: W-4, FICA, and State

Employers calculate tax withholding in three steps: collect the employee’s Form W-4, determine taxable wages for the pay period, and then apply the IRS formulas in Publication 15-T for federal income tax while adding flat-rate Social Security and Medicare on top. The W-4 tells the employer what filing status and credits to use. Pre-tax deductions like 401(k) contributions and health premiums come out before income tax is figured. FICA taxes follow their own rates and caps and don’t depend on the W-4 at all.

Start With the Employee’s W-4

Every calculation starts with Form W-4. The form gives the employer the filing status (single, married filing jointly, or head of household) that selects which set of tax brackets applies, plus a handful of fields that adjust the result up or down.

  • Child tax credit: for households earning $200,000 or less ($400,000 or less if married filing jointly), each qualifying child under 17 reduces annual withholding by $2,200.
  • Other dependents: $500 per year each.
  • Other income: interest, dividends, or side income the employee wants covered through payroll.
  • Deductions above the standard deduction.
  • Extra withholding: a flat dollar amount added to every check.

The $2,200 per-child figure on the 2026 W-4 reflects the increase from $2,000 under the One, Big, Beautiful Bill Act signed in 2025.1Internal Revenue Service. Form W-4 (2026) Employee’s Withholding Certificate

If an employee doesn’t turn in a valid W-4, federal regulations require the employer to withhold as if the employee were single with the standard withholding allowance prescribed by the IRS.2eCFR. 26 CFR 31.3402(f)(2)-1 That usually means more tax withheld than necessary. Employers must keep W-4 records for at least four years after the tax becomes due or is paid, whichever is later.3Internal Revenue Service. How Long Should I Keep Records?

The IRS can also send an employer a “lock-in letter” specifying the withholding arrangement to use for an employee whose W-4 doesn’t produce enough tax. Once the letter’s effective date passes, the employer must follow it and can only accept a new W-4 that increases withholding above the lock-in amount.4Internal Revenue Service. Withholding Compliance Questions and Answers Ignoring a lock-in letter makes the employer personally liable for the tax that should have been collected.

Figure Out Taxable Wages for the Pay Period

Taxable gross wages equal the employee’s total compensation minus the pre-tax deductions the law allows to be excluded from income. This is the number the withholding formula runs against, so misidentifying the deductions is where a lot of payroll errors originate.

Pre-Tax Deductions That Reduce the Base

Several common benefits come out before federal income tax is calculated:

Fringe Benefits That Add to the Base

Any fringe benefit an employer provides is taxable unless a specific provision in the tax code excludes it.8Internal Revenue Service. Employer’s Tax Guide to Fringe Benefits (2026) Excluded items include on-premises athletic facilities, de minimis perks like occasional meals, group-term life insurance up to $50,000 of coverage, educational assistance up to $5,250 per year, and commuter benefits up to $340 per month.

When a benefit exceeds its exclusion cap, the excess is added back into taxable wages. Group-term life insurance above $50,000 of coverage is the one that catches employees most often. Personal use of a company vehicle, stock options exercised above their grant price, and cell phones provided as a morale perk rather than a business tool also land in the taxable column.

Pay Frequency Matters

Once the taxable number is set, the employer picks the row that matches how often the employee is paid: weekly, biweekly, semimonthly, or monthly. The IRS tables and formulas are built around specific pay intervals, so a $5,000 monthly paycheck runs through different per-period bracket math than an equivalent weekly one.

Run the Federal Income Tax Formula

With taxable wages set and the W-4 in hand, the employer applies one of two calculation methods from IRS Publication 15-T.9Internal Revenue Service. Publication 15-T Federal Income Tax Withholding Methods (2026) Both methods produce the same answer when done right; the choice is about what fits the payroll setup.

Wage Bracket Method

The simpler option. The employer finds the employee’s adjusted wage in a table organized by pay period and filing status, and the intersection gives the exact dollar amount to withhold. The IRS publishes separate tables for W-4s filed from 2020 onward and for pre-2020 forms. The catch is that the tables cover wages only up to a cap, described by the IRS as generally less than $100,000 in annual terms. Above the last bracket in the applicable table, the employer must switch to the Percentage Method.

Percentage Method

This method works at any income level and is what most payroll software uses. The steps:

  • Start with taxable wages for the pay period and subtract the per-period standard deduction that matches the filing status. For 2026, the annual standard deductions are $16,100 for single filers, $32,200 for married filing jointly, and $24,150 for head of household. Publication 15-T converts these into per-period amounts.10Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026
  • Run the adjusted wage through the graduated brackets (10%, 12%, 22%, 24%, 32%, 35%, 37%), calculating the tax owed in each bracket the wage passes through.
  • Subtract the per-period value of any child or dependent credits from the W-4.
  • Add any extra withholding the employee requested in Step 4(c) of the W-4.

The result is the federal income tax to withhold for that pay period. Payroll systems automate this, but the employer is still responsible for using the correct year’s tables and the right W-4 data.

Handle Bonuses and Other Supplemental Pay

Bonuses, commissions, and severance are classified as supplemental wages and follow separate rules. When a supplemental payment is identified separately from regular wages, the employer can withhold a flat 22%. If the employee receives more than $1 million in supplemental wages during the calendar year, the excess is subject to a flat 37% regardless of what the W-4 says.11Internal Revenue Service. Publication 15 (2026), (Circular E), Employer’s Tax Guide

The alternative is to combine the supplemental payment with the most recent regular paycheck, treat the total as a single wage payment, and run the standard formula. This aggregate approach sometimes withholds more than the flat rate, which is why bonus checks often feel smaller than expected.

Add Social Security and Medicare

Federal income tax is only one piece. Employers also withhold FICA and match the same amounts from company funds.

Social Security tax is 6.2% on wages up to the annual wage base, which is $184,500 for 2026.12Social Security Administration. 2026 Cost-of-Living Adjustment (COLA) Fact Sheet Once year-to-date wages reach the cap, the employer stops withholding for the rest of the year.

Medicare tax is 1.45% with no wage cap.13Internal Revenue Service. Topic No. 751, Social Security and Medicare Withholding Rates On top of that, employers withhold an Additional Medicare Tax of 0.9% on wages exceeding $200,000 in a calendar year, and the employer does not match this portion. The $200,000 withholding trigger is the same regardless of filing status, even though the actual liability threshold is $250,000 for married couples filing jointly.14Internal Revenue Service. Questions and Answers for the Additional Medicare Tax That mismatch means some married employees end up owing more at filing, while others get a refund.

Deposit and Report on Time

Calculating the right amount only counts if the money reaches the Treasury on schedule. The IRS assigns each employer to a monthly or semiweekly deposit schedule based on total tax liability during a four-quarter lookback period. Monthly depositors (those who reported $50,000 or less) deposit by the 15th of the following month. Semiweekly depositors (more than $50,000) follow a Wednesday/Friday split tied to each payday. Any monthly depositor that accumulates $100,000 or more in liability on a single day becomes a semiweekly depositor immediately and for the following year.15Internal Revenue Service. Deposit Requirements for Employment Taxes (Notice 931)

Total withholding is reported quarterly on Form 941, due April 30, July 31, October 31, and January 31.16Internal Revenue Service. Instructions for Form 941

Late deposit penalties scale with delay: 2% for 1–5 days late, 5% for 6–15 days, 10% beyond 15 days, and 15% after an IRS notice demanding payment. The tiers do not stack.17Internal Revenue Service. Failure to Deposit Penalty Separately, any person responsible for collecting and paying over employment taxes who willfully fails to do so can be assessed a trust fund recovery penalty equal to the full unpaid amount, and the IRS can pursue owners, officers, or bookkeepers personally.18Office of the Law Revision Counsel. 26 USC 6672

State Withholding Comes on Top

Most states impose their own income tax withholding in addition to the federal calculation, with top marginal rates ranging from around 2.5% to over 14%. A handful of states have no individual income tax. Each state publishes its own tables and forms; some accept the federal W-4, and others require a separate state certificate. Employers with workers in multiple states have to track which state’s rules apply to each employee based on where the work is performed, and state penalties for withholding errors run on a separate track from the federal system.