Guide
How 401(k) Contributions Change Your Take-Home Pay
A traditional 401(k) contribution reduces the income the IRS taxes — but Social Security and Medicare are still calculated on your full gross wages.
By Buğra SözeriPublished
A traditional 401(k) contribution is often described as “pre-tax,” which is true but incomplete — it’s pre-tax for federal (and usually state) income tax purposes only. Social Security and Medicare, collectively FICA, are still calculated on your full gross wages, contribution included. Understanding that split explains why a $500 contribution doesn’t shrink your paycheck by exactly $500.
What a 401(k) contribution actually exempts
Per the IRS’s guidance on 401(k) contribution limits, traditional (pre-tax) 401(k) contributions reduce the income subject to federal income tax in the year they're made. They do not reduce wages for Social Security (6.2%) or Medicare (1.45%) purposes — per IRS Publication 15, FICA tax applies to gross wages before the 401(k) deferral is subtracted.
Worked example
Take a $80,000 salary, single filer, with a $5,000 annual traditional 401(k) contribution and no state tax:
| Line item | Wage base used |
|---|---|
| Federal income tax | $75,000 (gross minus 401(k)) |
| Social Security (6.2%) | $80,000 (full gross) |
| Medicare (1.45%) | $80,000 (full gross) |
Only the federal income tax calculation uses the reduced $75,000 figure. FICA is computed on the full $80,000 either way, which means the 401(k) contribution saves you federal tax at your marginal bracket rate, but doesn’t reduce your FICA bill at all.
Why the paycheck doesn't drop by the full contribution
If you're in the 22% marginal federal bracket, a $500 contribution reduces your federal tax by roughly $500 × 0.22 = $110. Since $500 leaves your paycheck as a 401(k) deferral but $110 of federal tax you would have otherwise paid doesn’t get withheld, your take-home pay actually drops by about $390 — not the full $500. This is the core appeal of pre-tax retirement contributions: part of the contribution is effectively subsidized by the tax you didn’t pay on it this year.
What it doesn't do
A 401(k) contribution never reduces your Social Security wage base, so it has no effect on your future Social Security benefit calculation (which is based on FICA-taxed wages). It also doesn’t reduce state tax in every state — most states follow the federal treatment, but confirm for your specific state. For how a bonus interacts with the same withholding system, see our bonus withholding guide.
Frequently asked questions
- Does a 401(k) contribution reduce my Social Security tax?
- No. Traditional 401(k) contributions are excluded from federal taxable income but are still part of your Social Security and Medicare wage base — you pay FICA on the full gross amount you earned, including what went into the 401(k).
- So why does my paycheck drop by less than the full contribution amount?
- Because the contribution reduces your federal-taxable income, which lowers your federal income tax withholding. If you're in the 22% federal bracket, a $500 contribution lowers your federal tax by roughly $110, so your take-home pay drops by about $390, not the full $500.
- Does this work the same for a Roth 401(k)?
- No — a Roth 401(k) contribution is made with after-tax dollars, so it doesn't reduce federal-taxable income at all. It still counts toward Social Security and Medicare wages the same as a traditional 401(k), since FICA applies before either type of contribution is deducted.
- Is there a limit to how much I can contribute?
- Yes — the IRS sets an annual elective-deferral limit that changes periodically with inflation adjustments. Check the current year's limit on the IRS retirement-topics page before assuming a specific number.
Sources & references
Authoritative references cited by this piece. Verified by Buğra Sözeri on the dates shown and re-checked at every deploy.
- IRS — Retirement Topics: 401(k) and Profit-Sharing Plan Contribution Limits — IRS reference on how traditional 401(k) contributions are excluded from federal taxable wages(as of )
- IRS Publication 15 — Employer's Tax Guide — FICA wage-base rules confirming 401(k) contributions remain subject to Social Security and Medicare tax(as of )
- SSA — Social Security Contribution and Benefit Base — Annual Social Security wage base referenced in the worked example(as of )
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Published September 25, 2026