Why Your 401(k) Lowers Income-Tax Wages but Not FICA in 2026

When an employee contributes to a traditional 401(k) through payroll, the elected salary deferral generally is not included in federal income-tax wages for the year of contribution. The same employee deferral generally remains included in wages subject to Social Security and Medicare tax. The IRS retirement-plan contribution FAQ shows this difference in a federal withholding table. It also states that designated Roth employee deferrals remain subject to both federal income tax and FICA.

This explains a common pay-stub surprise. A worker increases a traditional 401(k) election by $100 per check, but net pay falls by less than $100 after the federal income-tax withholding calculation changes. At the same time, the Social Security and Medicare lines do not fall merely because of the 401(k) election. A 'pre-tax' label on a retirement contribution refers to particular tax bases; it does not mean the contribution is excluded from every tax.

The employer match is a different item. The IRS FAQ says employer matching and nonelective contributions generally are not wages subject to the employee's federal income-tax withholding or FICA when contributed under the described qualified plan rules. They are not a second deduction from the worker's paycheck. Plan design and vesting determine when a participant owns the match under the plan's rules.

This article describes ordinary employee contributions to a qualified 401(k). Government and nonprofit plans, special compensation, excess deferrals and certain other arrangements can follow different details. Read the employer plan materials and the IRS guidance for the exact plan type.

Suppose a worker has $3,000 of gross cash wages in a biweekly period and chooses a $300 traditional 401(k) deferral. Ignore other benefits and special adjustments for this example. The simplified wage base for federal income-tax withholding is $2,700. The simplified Social Security and Medicare wage base remains $3,000, assuming all wages are covered and the Social Security annual cap has not been reached.

At the ordinary employee rates, Social Security withholding on $3,000 is $186, and Medicare withholding is $43.50. Both figures are based on $3,000, not $2,700. Federal income-tax withholding is not simply a fixed percentage of $2,700. The employer follows Form W-4 and the applicable IRS Publication 15-T withholding method, which can use filing status, payroll period, other W-4 entries and rounding.

Imagine an illustrative $250 of federal income-tax withholding on that check and no state tax or other deductions. The net deposit would be $3,000 minus $300 minus $186 minus $43.50 minus $250, or $2,220.50. The $250 is invented solely to show how the lines add; it is not a calculated IRS withholding result for the worker. A real stub can have health premiums, state withholding, local taxes and a different W-4 outcome.

If the worker changes the election to $400, the traditional deferral grows by $100. The federal wage base for withholding in the simplified example becomes $2,600; Social Security and Medicare wages remain $3,000. The precise federal withholding difference must be calculated from the worker's actual W-4 and payroll method. The worker should not conclude from a smaller net-pay drop that the missing money was never saved; the additional $100 went to the retirement plan.

Check the contribution line and the retirement account statement. A pay stub shows what payroll says it deducted, while the plan account should show the deposited amount subject to processing time. If the figures do not match after the normal posting window, ask payroll or the plan administrator.

A designated Roth 401(k) employee contribution is generally included in federal income-tax wages in the contribution year. It also remains in Social Security and Medicare wages. A traditional employee deferral generally reduces federal income-tax wages in that year but remains in FICA wages. The choice affects the timing of income taxation, not whether ordinary employee FICA applies to the elected wages. The IRS Roth account guide explains the current-tax distinction.

Using the same $3,000 gross and $300 employee deferral, a worker choosing designated Roth has a simplified $3,000 federal income-tax wage base for the period, compared with $2,700 with a traditional deferral. Both choices have $3,000 in the simplified Social Security and Medicare wage bases. Federal income-tax withholding may therefore differ even though the retirement account receives the same nominal $300 and the FICA withholding is the same under the ordinary assumptions.

Neither choice guarantees a larger lifetime after-tax retirement balance. Future tax rates, withdrawal rules, the worker's current bracket, state taxes, the plan's investments and the timing of distributions all matter. Avoid choosing solely from one paycheck's net deposit. If the plan offers both contribution types, read its election instructions carefully; some portals label them 'pre-tax' and 'Roth' without explaining the wage-base difference.

Employer matching contributions are not the same as an employee's designated Roth election. Some plans may offer Roth treatment for certain employer contributions under applicable rules, but that is plan-specific and has separate tax consequences. This article's examples cover ordinary employee salary deferrals only. If the plan's statement labels a match unexpectedly, ask the administrator for its tax treatment.

At year end, Form W-2 Box 1 reports federal income-tax wages. Box 3 reports Social Security wages up to the annual wage base, and Box 5 reports Medicare wages without that Social Security cap. The IRS retirement-plan FAQ says employee pre-tax salary deferrals are omitted from Box 1 but included in Boxes 3 and 5 under the ordinary rules. The IRS W-2 instructions explain reporting and the appropriate Box 12 codes for elective deferrals.

Suppose a worker has $78,000 of ordinary covered cash wages for the year and contributes $7,800 to a traditional 401(k), with no other adjustments. A simplified W-2 could show $70,200 in Box 1 and $78,000 in both Boxes 3 and 5. The difference of $7,800 corresponds to the traditional deferral. That is an illustration, not a prediction of any particular W-2. Health benefits, HSA contributions, taxable fringe benefits, tips and other items can also affect the boxes.

Now suppose the worker earns $210,000 and contributes $10,000 traditional. Box 1 might be $200,000 in the simple model, Box 5 might be $210,000, and Box 3 is limited to the 2026 Social Security wage base of $184,500 for one employer's ordinary covered wages. The difference between Box 1 and Box 3 no longer equals the contribution. The SSA table confirms the 2026 wage base. A person with multiple employers or special wage types needs a more careful reconciliation.

Box 12 records employee elective deferrals using plan-specific codes. The exact code matters, and the amount may combine regular and applicable catch-up contributions under the W-2 instructions. Do not add Box 12 traditional deferrals back to Box 1 as though the employer forgot wages; use the tax form's reporting rules. If you are comparing a pay stub with a W-2, compare like wage bases and account for all benefit deductions, not just the retirement line.

The general employee elective deferral limit for a 401(k) is $24,500 in 2026, before applicable catch-up contributions and plan-specific restrictions. The IRS 401(k) contribution-limits page confirms the figure and says employee elective deferrals generally are aggregated across plans in which the person participates. An employer match has a different place in the overall plan contribution limits; it does not use up the employee's basic elective deferral limit in the same way.

Suppose a worker contributes $14,000 at Employer A, changes jobs, then asks Employer B to withhold $15,000 for the rest of 2026. The combined employee elective deferrals would be $29,000, which exceeds the general $24,500 limit by $4,500 before considering whether a catch-up exception applies. Employer B may not know Employer A's contributions. The worker needs to track the combined amount and follow the IRS correction procedure if an excess occurs.

The worker's age, the plan's catch-up rules and special rules can change the allowed total. Do not apply a catch-up amount to someone simply because they turn 50 near year end; check the age and plan requirements for the relevant year. Likewise, a plan may impose a lower administrative limit or stop deductions based on its own records, but that does not necessarily account for another employer's plan.

If you want a steady contribution over the year, compare your elected percentage or dollar amount with the number of remaining paychecks. A midyear raise can increase percentage-based deferrals. Bonuses may or may not be eligible for deferral under plan terms. A 27-pay-date calendar can alter the number of deductions. Ask payroll how the election applies to bonuses and the final check, and verify year-to-date totals in the plan portal.

An employer match can increase retirement savings without appearing as cash in the employee's net deposit. A pay stub might show it as an informational line, and a retirement portal may display it beside employee contributions. The IRS FAQ generally distinguishes employer matching and nonelective contributions from employee salary deferrals for withholding purposes.

For example, if a worker defers $300 and the employer contributes $150 under a particular match formula, the worker's gross wages generally fall by the $300 employee deferral for federal income-tax wage purposes under the ordinary traditional arrangement. The employer's $150 is not another $150 removed from the worker's paycheck. The plan account may show $450 deposited, subject to timing and vesting. The match formula itself comes from the plan document, not from a universal tax rule.

Some employers calculate matching contributions per pay period. Others have a year-end 'true-up' provision, and some do not. If you hit the employee limit early and stop deferring for later checks, the match you receive can differ under a per-period formula. Ask the plan administrator how the match is calculated and whether it offers a true-up. Do not infer the answer from a generic calculator or a blog headline.

Vesting determines whether employer contributions are fully yours under the plan schedule if you leave. Employee salary deferrals are a different category. Read the summary plan description before treating an unvested match as guaranteed compensation. This distinction matters when comparing job offers: the advertised match percentage and the amount actually retained can differ.

Start with actual gross pay for the period, not annual salary alone. Enter the traditional 401(k) employee amount in the correct field and verify whether the calculator subtracts it from federal taxable wages while keeping it in ordinary FICA wages. If the calculator also asks about Roth contributions, put those in the Roth field; a Roth deferral generally should not reduce current federal income-tax wages.

Then compare the result with a real stub. If the federal withholding differs, check the W-4, pay frequency, bonus treatment and any additional withholding. Publication 15-T allows different employer calculation methods under the rules. An annual tax estimate divided by 26 is not guaranteed to match each payroll withholding. State taxes can also treat benefits differently, so do not assume a federal wage-base adjustment gives the same state result everywhere.

If the Social Security line differs, check year-to-date covered wages against the 2026 wage base and whether you have changed employers. If Medicare differs, check for Additional Medicare withholding when a single employer's Medicare wages exceed $200,000. These are separate questions from the 401(k) election. The IRS FICA topic explains the ordinary rates and Additional Medicare trigger.

A calculator is most useful when its assumptions are visible. Record the gross amount, contribution type, year-to-date wages, filing status and W-4 details used. When a result changes, you can identify which input caused it. If the site provides only an annualized estimate, treat the per-check amount as a planning figure and verify actual withholding with payroll.

Ask whether your election is traditional or designated Roth and how it applies to regular wages, bonuses and overtime. Ask where the employee deferral and employer match appear on the stub and plan statement. Ask whether the match is calculated per pay period and whether the plan offers a year-end true-up. These details come from the employer's plan, not the IRS wage-base table.

Check the gross wage line, employee deferral, federal income-tax withholding, Social Security withholding and Medicare withholding separately. Do not expect an increase in traditional contributions to reduce every tax line. Keep a running total of employee deferrals across employers and compare it with the applicable IRS limit before the last pay date. A job change makes this especially important.

At year end, compare the retirement deduction total from stubs with W-2 Box 12 and the plan statement. Compare Box 1 with Boxes 3 and 5, remembering that other benefits and the Social Security cap can also create differences. If you find a mismatch, request the employer's explanation or a corrected form rather than changing reported figures by intuition.

Finally, judge the retirement election in the context of your budget and longer-term plan. A larger traditional deferral can lower current federal income-tax wages, but it also lowers cash available for immediate expenses. A Roth election can have a different current-tax effect. The most useful decision accounts for your actual cash needs, plan match, contribution limit and future tax expectations—not just the size of the next bank deposit.