How Does a 401(k) Contribution Affect Your Paycheck?

Key takeaways

• A traditional pre-tax 401(k) deferral is generally excluded from wages subject to federal income-tax withholding.
• Traditional employee deferrals are generally still included in wages subject to Social Security and Medicare taxes.
• A designated Roth 401(k) contribution is included in current taxable income, so it normally does not reduce current federal withholding.
• Your take home pay usually falls by less than the traditional contribution itself because of the possible income-tax withholding reduction.
• An employer match adds retirement savings without being deducted from your net paycheck, although plan eligibility and vesting rules may apply.

Traditional and Roth contributions affect payroll differently

With a traditional 401(k), you direct part of your compensation into the plan before current federal income tax is calculated. The IRS says these elective deferrals generally are not included in Form W-2 box 1 wages, but remain in boxes 3 and 5 as Social Security and Medicare wages. That is why a traditional contribution may lower federal income-tax withholding without lowering the employee’s usual 6.2% Social Security or 1.45% Medicare withholding.

A designated Roth 401(k) contribution is made after current income tax. It remains part of gross income and wages for federal withholding and FICA purposes. Its potential benefit comes later: a qualified Roth distribution may be excluded from gross income if the applicable requirements are met. Retirement tax treatment can be complex, so the paycheck effect alone should not determine which option is appropriate.

A simplified paycheck example

Suppose a paycheck has $2,000 of gross wages and you elect a 5% contribution, or $100. In a simplified illustration, assume that $100 would otherwise fall in a 12% federal marginal bracket and ignore state tax, credits and the IRS withholding-table mechanics.

This is not a payroll quote. Actual federal withholding is calculated from Form W-4 and IRS Publication 15-T, not simply by multiplying one contribution by a marginal rate. State income-tax treatment can also differ. FICA generally remains based on the full $2,000 in either column.

2026 contribution limits and payroll timing

For 2026, the IRS employee elective-deferral limit for 401(k), 403(b), most governmental 457 plans and the federal Thrift Savings Plan is $24,500. The general catch-up limit for participants age 50 or older is $8,000. A higher catch-up limit of $11,250 applies for eligible participants ages 60 through 63. These limits can interact with plan terms, participation in more than one employer plan and special rules.

A percentage election also moves with eligible compensation. A 6% election on a paycheck containing overtime or a bonus may produce a larger deduction than on an ordinary paycheck, unless the plan excludes that compensation or your payroll election says otherwise. Changes may take one or more payroll cycles to appear. Review the plan portal and summary plan description rather than assuming an election is immediate.

How to check the result on your pay stub

• Confirm gross pay and the compensation used to calculate the contribution.
• Find the current 401(k) deduction and verify whether it is labeled traditional, pre-tax or Roth.
• Compare federal taxable wages with Social Security and Medicare wages; they need not be identical.
• Check year-to-date contributions against your own records, especially if you changed employers.
• Separate your contribution from any employer match, which may appear only in an employer-paid or informational section.

Where this comes from

• IRS: 2026 retirement contribution limits (opens in a new tab)
• IRS Topic 424: 401(k) plans (opens in a new tab)
• IRS: employment-tax treatment of retirement contributions (opens in a new tab)
• IRS: designated Roth accounts (opens in a new tab)