How Is a 401(k) Deducted From Your Paycheck?

A 401(k) is deducted from your paycheck through your employer’s payroll system: you sign a salary reduction agreement telling payroll how much to withhold each pay period, and that amount is pulled out of your wages and sent to your retirement account before you ever see it. For a traditional 401(k), the money comes out of your gross pay before federal and state income tax is calculated, so your taxable income drops immediately. For a Roth 401(k), income tax is calculated on your full pay first and the contribution comes out of what’s left, so your paycheck is smaller now but qualified withdrawals in retirement are tax-free. In 2026, you can defer up to $24,500 this way, with more room if you’re 50 or older.¹

Setting Your Contribution Amount

It starts with a salary reduction agreement. That’s the form telling your employer how much to withhold from each check. You pick either a flat dollar amount per pay period or a percentage of your gross wages. Percentage elections are more common because the contribution scales automatically when you get a raise. The election stays in place every pay cycle until you file a new one, and most plans let you change it whenever you want.

Some plans let you split contributions between a traditional pre-tax account and a Roth account. If yours does, you make separate elections for each, and the combined total still has to stay under the annual federal limit.

Pre-Tax vs. Roth: How Each Changes Your Take-Home Pay

The account type controls when the tax gets calculated, and that decides which direction your paycheck moves.

With a traditional pre-tax 401(k), payroll subtracts your contribution from gross pay before calculating federal and state income tax. If you earn $2,000 per pay period and contribute $200, income tax is calculated on $1,800 instead of $2,000. You pay less tax now and owe income tax on every dollar you withdraw in retirement.²

With a Roth 401(k), payroll calculates and withholds income tax on your full gross pay first, then deducts your contribution from what’s left. Your current paycheck is smaller than it would be with a pre-tax contribution of the same size, but qualified withdrawals in retirement come out completely tax-free.³

Your 401(k) Doesn’t Cut Your FICA Taxes

This one trips people up. Pre-tax 401(k) contributions lower federal and state income tax withholding, but they don’t reduce Social Security or Medicare tax. Both are calculated on your full gross pay no matter how much you defer.⁴

Earn $80,000, defer $10,000 into a traditional 401(k), and you’ll pay income tax on $70,000 but Social Security and Medicare tax on the full $80,000. The 6.2% Social Security tax and 1.45% Medicare tax apply to every dollar of your gross wages, whether it goes into your pocket or your retirement account. The upside is that those FICA-taxed wages count toward your future Social Security benefit, so deferring into a 401(k) doesn’t shrink your eventual Social Security checks.

How Much Can Come Out in 2026

Federal law caps how much you can defer each year. The 2026 standard limit is $24,500.¹ Your employer’s payroll system tracks your year-to-date deferrals and stops withholding once you hit that ceiling.

Catch-up contributions let older workers save more. If you’re 50 or older, you can contribute an additional $8,000, for a total of $32,500. If you’re between 60 and 63, a higher catch-up of $11,250 applies under SECURE 2.0, bringing the total to $35,750. These amounts apply for the calendar year you reach the qualifying age.¹

A separate, higher ceiling covers your deferrals plus any employer contributions combined. For 2026 that total annual addition limit is $72,000, or $80,000/$83,250 with the applicable catch-up.⁵ Most people won’t hit it, but it matters if your employer offers a generous match or profit sharing.

Work multiple jobs? You’re responsible for making sure your combined deferrals across all plans stay under the annual limit. If they exceed it, you have to request a return of the excess from one of the plans by April 15 of the following year to avoid being taxed on that money twice.⁶

One boundary worth naming: if you earned more than $160,000 from your employer in the prior year, you’re a highly compensated employee for 2026, and nondiscrimination testing can force the plan to cap your contributions or refund part of your deferrals after year-end.⁵ You might elect 10% of pay and later receive a refund because lower-paid coworkers didn’t contribute enough to keep the plan compliant.

Employer Match: Not a Paycheck Deduction

Many employers match a portion of what you contribute. A common formula is 50 cents on the dollar up to 6% of your pay, though structures vary. The match is separate from your payroll deduction. It doesn’t come out of your paycheck. Your employer funds it on top of your wages and usually deposits it into your account at the same time your own deduction is transferred.

Matching contributions aren’t included in your taxable income when they’re made, and they’re exempt from FICA. You won’t see the match reducing your paycheck, but you should see it credited to your 401(k) account after each pay period.

One catch: employer contributions often come with a vesting schedule. Vesting means you gradually earn ownership of the match. Plans use either cliff vesting, where you go from 0% to 100% after a set number of years (up to three), or graded vesting, where ownership increases each year until you’re fully vested (up to six years).⁷ Leave before you’re fully vested and you forfeit the unvested portion of the match. Your own contributions are always 100% yours.

Automatic Enrollment: Why a Deduction May Appear Without You Electing One

If your employer set up its 401(k) after December 29, 2022, federal law now requires the plan to automatically enroll eligible employees. Under SECURE 2.0, the default contribution rate falls between 3% and 10% of pay, and it automatically increases by 1% each year until it reaches at least 10%, with a cap of 15%.⁸ You can opt out or choose a different rate, but if you do nothing, deductions start automatically.

Businesses with fewer than 10 employees, companies less than three years old, church plans, and government plans are exempt. Plans that existed before the cutoff can offer automatic enrollment voluntarily but aren’t required to.

The practical effect: you might see a 401(k) deduction on a paystub without having filled out a contribution election. If you were auto-enrolled and didn’t realize it, check the stub. You can change the amount or stop the deduction at any time.

Reading Your Paystub and W-2

Your 401(k) deduction shows up as a separate line on every paystub, usually labeled something like “401K,” “Ret Plan,” or “401K Roth” depending on the account type. Contribute to both, and you’ll typically see two lines.

At year’s end, these amounts land on your W-2 in Box 12. Traditional pre-tax 401(k) contributions use code D, and designated Roth contributions use code AA.⁹ Your Box 1 wages (subject to federal income tax) already reflect the pre-tax reduction, but Boxes 3 and 5 (Social Security and Medicare wages) show your full gross pay because 401(k) deferrals don’t reduce FICA.⁴

Double-check your W-2 each January. If Box 12 shows the wrong code, or the total doesn’t match your final paystub’s year-to-date figure, flag it with payroll before filing your return. These errors trigger IRS notices that are far more annoying to resolve after filing.

When the Money Actually Reaches Your Account

The deduction leaving your paycheck doesn’t mean the money instantly lands in your retirement account. The employer holds it briefly before transferring it to the plan’s financial institution. Federal law requires employers to deposit withheld contributions as soon as they can reasonably be separated from the company’s general assets.¹⁰

For small plans with fewer than 100 participants, a seven-business-day safe harbor applies. If the employer deposits your contribution within seven business days of the pay date, it’s presumed timely.¹⁰ Larger employers generally need to move faster because their payroll systems can process transfers sooner. If you notice a long gap between when your paycheck issues and when contributions appear in your account, ask HR about the timing.

When a Deduction Is Missed or Wrong

Payroll mistakes happen. A new deferral election doesn’t get entered, or auto-enrollment was supposed to start and didn’t. When your employer fails to withhold the contribution you elected, the IRS has a correction procedure.

The employer must make a corrective contribution equal to 50% of the missed deferral amount, adjusted for investment earnings from the date the deferral should have been made through the correction date. You’re immediately 100% vested in that corrective contribution, and it’s subject to the same withdrawal restrictions as regular elective deferrals.¹¹

The catch is that the corrective contribution doesn’t make you fully whole. You get 50% of what you would have deferred, not 100%. The sooner you catch the error, the smaller the gap. Review your paystubs regularly, especially after submitting a new election or starting a new job.

Loan Repayments on Your Paycheck

If you borrow from your 401(k), repayments are typically deducted from your paycheck alongside your regular contributions. Loan repayments are a separate line and don’t count toward your annual deferral limit, because you’re repaying money you already contributed rather than making a new contribution.

Federal rules require loan repayments to follow a substantially level amortization schedule with payments due at least quarterly. Most employers align repayments with the normal payroll cycle, so you’ll see a deduction every pay period. Loans generally can’t exceed the lesser of $50,000 or 50% of your vested account balance, and they must be repaid within five years unless the loan was used to buy your primary home.¹²

Leaving your job with an outstanding balance is where this gets messy. The plan can require you to repay the full balance, and if you can’t, the remaining amount is treated as a distribution: income tax on the unpaid balance, plus a 10% early withdrawal penalty if you’re under 59½. You can avoid the tax hit by rolling the outstanding amount into an IRA or another eligible plan by the due date of your tax return for that year, including extensions.¹³

State Income Tax Treatment

Nearly every state with an income tax follows the federal treatment of pre-tax 401(k) contributions, so your deferral reduces state taxable income the same way it reduces federal taxable income. Pennsylvania is the notable exception. It taxes 401(k) contributions at the state level even when they’re pre-tax federally, so if you work in Pennsylvania your 401(k) deduction won’t lower your state income tax bill. Residents of states with no income tax, like Texas or Florida, won’t see any state-level impact either way.

1
Internal Revenue Service. Retirement Topics – 401(k) and Profit-Sharing Plan Contribution Limits
2
eCFR. 26 CFR 1.401(k)-1 – Certain Cash or Deferred Arrangements
3
Office of the Law Revision Counsel. 26 USC 402A – Optional Treatment of Elective Deferrals as Roth Contributions
4
Internal Revenue Service. Retirement Plan FAQs Regarding Contributions – Are Retirement Plan Contributions Subject to Withholding for FICA, Medicare or Federal Income Tax
5
Internal Revenue Service. Notice 2025-67 – 2026 Amounts Relating to Retirement Plans and IRAs
6
Office of the Law Revision Counsel. 26 USC 402 – Taxability of Beneficiary of Employees Trust
7
Internal Revenue Service. Retirement Topics – Vesting
8
Internal Revenue Service. 401(k) Plan Overview
9
Internal Revenue Service. Common Errors on Form W-2 Codes for Retirement Plans
10
eCFR. 29 CFR 2510.3-102 – Definition of Plan Assets – Participant Contributions
11
Internal Revenue Service. Fixing Common Plan Mistakes – Correcting a Failure to Effect Employee Deferral Elections
12
eCFR. 26 CFR 1.72(p)-1 – Loans Treated as Distributions
13
Internal Revenue Service. Retirement Topics – Plan Loans