Yes — traditional 401(k) contributions reduce your adjusted gross income dollar for dollar, up to the annual contribution limit. Roth 401(k) contributions do not. If you earn $100,000 and defer $24,500 into a traditional 401(k), the IRS treats your income as $75,500 for federal income tax purposes.1Internal Revenue Service. 401(k) Plan Overview Put the same $24,500 into a Roth 401(k) and your AGI stays at $100,000, because Roth contributions are made with after-tax dollars.2Internal Revenue Service. Roth Comparison Chart
How the Reduction Actually Happens
Traditional 401(k) deferrals don’t show up as a deduction you claim on your tax return. The money is excluded from your taxable wages before your employer reports them. Your W-2 shows the already-reduced figure in Box 1, “Wages, tips, other compensation.”3Internal Revenue Service. 2026 General Instructions for Forms W-2 and W-3
So if you earned $100,000 and contributed $20,000 to a traditional 401(k), Box 1 shows about $80,000. Your full deferral is reported separately in Box 12 under Code D. Roth contributions, if you made any, appear under Code AA instead and are included in Box 1. When you file Form 1040, you carry the Box 1 amount as your wage income. There is no additional 401(k) deduction to claim, because the reduction already happened at the W-2 level.
The tax you avoid now isn’t erased — it’s postponed. Withdrawals from a traditional 401(k) in retirement are taxed as ordinary income.1Internal Revenue Service. 401(k) Plan Overview The tradeoff assumes your tax rate later will be lower than it is today.
How Much You Can Shelter in 2026
The amount you can strip from your AGI through pre-tax deferrals depends on your age. For 2026, the base elective deferral limit is $24,500.4Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 Older workers can add catch-up contributions:
- Age 50 and over: an additional $8,000, for a total of $32,500.
- Ages 60 through 63: an additional $11,250 under a SECURE 2.0 Act provision, for a total of $35,750.
Every pre-tax dollar contributed within those limits comes out of your AGI. If you split contributions between traditional and Roth in the same plan, only the traditional portion lowers AGI.
Employer matching contributions don’t affect your AGI in the year they’re made. The match goes directly into your account and is not reported as current income on your W-2.1Internal Revenue Service. 401(k) Plan Overview You’ll owe tax on that money when you take distributions later.
What a 401(k) Does Not Reduce
This is where people get caught off guard. Traditional 401(k) contributions reduce your federal income tax, but they do not reduce your Social Security or Medicare taxes. Your employer calculates FICA withholding on the full amount of your pay, including the dollars you deferred.1Internal Revenue Service. 401(k) Plan Overview
You can see the split on your W-2 itself. Box 1 (income tax wages) excludes your 401(k) deferrals, but Box 3 (Social Security wages) and Box 5 (Medicare wages) include them.5Internal Revenue Service. Retirement Plan FAQs Regarding Contributions – Are Retirement Plan Contributions Subject to Withholding for FICA, Medicare or Federal Income Tax Earn $100,000 and defer $20,000, and income tax applies to $80,000 while your 6.2% Social Security tax and 1.45% Medicare tax still apply to the full $100,000. Those higher Social Security wages can raise your eventual benefit calculation.
State income tax treatment varies. In most states the pre-tax deferral flows through and reduces state taxable income too, but a few tax it. Check how your state handles Box 1 versus state wages on your W-2.
Why the AGI Reduction Matters Beyond Income Tax
The direct tax savings are the obvious payoff. The bigger one is often hidden: many tax benefits use AGI (or Modified AGI, which for most W-2 employees is the same number) as a gatekeeper. Dropping your AGI by a few thousand dollars can carry you below a threshold you’d otherwise exceed.
Medical Expense Deduction
You can only deduct medical expenses that exceed 7.5% of your AGI.6Internal Revenue Service. Publication 502 (2025), Medical and Dental Expenses Lower AGI, lower floor. At $80,000 AGI, the floor is $6,000; at $60,000, it’s $4,500. That’s an extra $1,500 of medical costs becoming deductible in a heavy medical year.
Child Tax Credit
The Child Tax Credit begins to phase out at a Modified AGI of $200,000 for single filers and $400,000 for married couples filing jointly.7Internal Revenue Service. Child Tax Credit Taxpayers just over the line can slide back under by increasing traditional deferrals.
Net Investment Income Tax
A 3.8% surtax applies to certain investment income when Modified AGI exceeds $200,000 (single) or $250,000 (joint).8Internal Revenue Service. Questions and Answers on the Net Investment Income Tax Deferrals that bring MAGI below the threshold eliminate the tax. Partial reductions still help, because the surtax applies to the lesser of your net investment income or the amount your MAGI exceeds the threshold.
Traditional IRA Deductibility
If you’re covered by a workplace plan, your traditional IRA deduction phases out at specific AGI ranges. For 2026, a single filer loses the full deduction between $81,000 and $91,000 in AGI. For married couples filing jointly where the contributing spouse is covered, the range is $129,000 to $149,000.4Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 Bigger 401(k) contributions can keep your AGI inside the deductible range.
Saver’s Credit
The Retirement Savings Contributions Credit gives low- and moderate-income workers a credit worth 10%, 20%, or 50% of their retirement contributions, up to $2,000 per person. The credit tier depends entirely on AGI. For 2026, the ceilings are:9Internal Revenue Service. 2026 Amounts Relating to Retirement Plans and IRAs, as Adjusted
- 50% credit: AGI up to $48,500 (married filing jointly), $36,375 (head of household), or $24,250 (single).
- 20% credit: AGI up to $52,500 (married filing jointly), $39,375 (head of household), or $26,250 (single).
- 10% credit: AGI up to $80,500 (married filing jointly), $60,375 (head of household), or $40,250 (single).
Because traditional 401(k) contributions reduce the very AGI that determines eligibility, the same deferral that shelters your income can sometimes qualify you for a credit on that deferral.
If You’re Self-Employed
Without a W-2, the wage-exclusion mechanism doesn’t apply to you. Solo 401(k) contributions instead reduce AGI through a direct deduction on Schedule 1 of Form 1040, line 16.10Internal Revenue Service. Calculating Your Own Retirement Plan Contribution and Deduction The deduction does not go on Schedule C, which means it lowers your income tax but not your self-employment tax.
You can contribute in two roles: as employee (up to $24,500 in elective deferrals for 2026) and as employer (up to 25% of net self-employment income). The combined total can’t exceed $72,000, or $80,000 with the standard age-50 catch-up.11Internal Revenue Service. Retirement Topics – 401(k) and Profit-Sharing Plan Contribution Limits
Watch the Limit If You Changed Jobs
Contributing more than the annual limit creates an excess deferral, and the IRS treats it harshly. The excess is included in your taxable income for the contribution year, and if you don’t correct it in time, it gets taxed a second time when you eventually withdraw it.12Internal Revenue Service. Consequences to a Participant Who Makes Excess Deferrals to a 401(k) Plan
You have until April 15 of the following year to withdraw the excess plus any earnings on it. The deadline doesn’t move if you file a tax extension. This most often catches people who change jobs mid-year and start contributing at a new employer without accounting for what they already deferred at the old one. Your second employer can’t see the first employer’s payroll records, so combining the totals is your job.