Do 401(k) Contributions Reduce AGI? Traditional vs. Roth

Yes — traditional pre-tax 401(k) contributions do reduce your adjusted gross income, dollar for dollar. Roth 401(k) contributions do not. If you earn $100,000 and defer $24,500 into a traditional 401(k), the IRS treats your wage income as $75,500 for federal income tax purposes.¹ Defer that same amount into a Roth 401(k), and your AGI stays at $100,000 because the money was already taxed before it went in.²

How the Traditional Reduction Actually Works

You won’t find a line on Form 1040 where you claim a 401(k) deduction. The reduction happens earlier, at the payroll level. Your employer excludes your pre-tax deferral from the taxable wages it reports in Box 1 of your W-2, so the money never enters your gross income to begin with.³ When you transfer Box 1 to your return as wage income, the reduction is already baked in.

The tax isn’t erased, just postponed. Distributions from a traditional 401(k) in retirement are taxed as ordinary income.¹ The bet is that your rate then will be lower than your rate now.

How Much You Can Shelter in 2026

The ceiling on the AGI reduction is the elective deferral limit. For 2026, that base limit is $24,500.⁴ Older workers can push higher through catch-up contributions:

At age 50 and over, an additional $8,000, bringing the total to $32,500.
At ages 60 through 63, an additional $11,250 under a SECURE 2.0 provision, bringing the total to $35,750.

Every dollar within those limits contributed on a pre-tax basis is a dollar off your AGI. If you split your deferrals between traditional and Roth in the same plan, only the traditional portion drops your AGI.⁴

Why Roth 401(k) Contributions Leave AGI Alone

Roth 401(k) contributions come out of pay that has already been taxed. Your employer withholds income tax on the full amount you earned, then routes the Roth deferral into the plan. The money stays in your AGI for the year.²

The tradeoff arrives in retirement. Qualified withdrawals of both contributions and earnings come out tax-free, provided you’re at least 59½ (or disabled, or the account is being distributed after death) and the account has been open for at least five tax years counted from your first Roth contribution to the plan.⁵ Earnings withdrawn before both conditions are met are taxed as ordinary income.

The choice comes down to when you’d rather pay. If you need a lower AGI this year — for a credit, a deduction phase-out, or a surtax threshold — traditional is the lever. If your current bracket is low and you expect it to rise, Roth locks in today’s rate.

What the Reduction Does Not Touch

Traditional 401(k) deferrals cut your federal income tax, but they do not cut your Social Security or Medicare taxes. FICA is calculated on your full pay, including the dollars you deferred.¹ On your W-2, Box 1 excludes your deferrals, but Box 3 (Social Security wages) and Box 5 (Medicare wages) include them.⁶ So on a $100,000 salary with $20,000 deferred, income tax applies to $80,000 while the 6.2% Social Security tax and 1.45% Medicare tax still hit the full $100,000. Those higher Social Security wages can raise your eventual benefit.

Employer matching contributions are a separate matter. The match doesn’t count toward the money that reduced your Box 1 wages because it was never in your pay in the first place; it’s not reported as current income, and it doesn’t affect your AGI in the year it lands.¹ You’ll owe tax on employer money when you take distributions later.

Why a Lower AGI Matters Beyond This Year’s Tax

The income tax savings are the obvious payoff. The quieter one is that AGI (and Modified AGI, which usually matches for W-2 employees) is the gatekeeper for a long list of tax benefits. Trimming your AGI by a few thousand dollars can drop you below a threshold you’d otherwise exceed.

Medical Expense Deduction

You can only deduct unreimbursed medical expenses above 7.5% of AGI.⁷ Lower AGI, lower floor. Drop your AGI from $80,000 to $60,000 and the floor falls from $6,000 to $4,500, moving an extra $1,500 of medical spending into deductible territory.

Child Tax Credit

The full Child Tax Credit starts phasing out at a Modified AGI of $200,000 for single filers and $400,000 for married couples filing jointly.⁸ Taxpayers just over those lines can slide back under by raising traditional deferrals.

Net Investment Income Tax

A 3.8% surtax on certain investment income kicks in when Modified AGI passes $200,000 for single filers or $250,000 for joint filers.⁹ Deferrals that bring MAGI under the threshold eliminate the tax; partial reductions still help, since the surtax applies to the lesser of net investment income or the amount MAGI exceeds the line.

Traditional IRA Deductibility

If you’re covered by a workplace plan, your traditional IRA deduction phases out at set AGI ranges. For 2026, a covered single filer loses the deduction between $81,000 and $91,000 of AGI. For married couples filing jointly where the contributing spouse is covered, the range is $129,000 to $149,000.⁴ Keeping AGI inside those windows through 401(k) deferrals can preserve the IRA deduction on top.

Saver’s Credit

The Retirement Savings Contributions Credit pays back 10%, 20%, or 50% of retirement contributions (up to $2,000 per person), with the tier set entirely by AGI. The 2026 ceilings:¹⁰

50% credit at AGI up to $48,500 married filing jointly, $36,375 head of household, or $24,250 single.
20% credit at AGI up to $52,500, $39,375, or $26,250 respectively.
10% credit at AGI up to $80,500, $60,375, or $40,250 respectively.

Above the ceilings, the credit is gone. Because traditional 401(k) deferrals reduce AGI, the same contribution that earns the credit can also qualify you for it.

If You’re Self-Employed

The W-2 exclusion doesn’t apply if you don’t have a W-2. Solo 401(k) contributions instead reduce AGI through a deduction on Schedule 1 of Form 1040, line 16.¹¹ It doesn’t go on Schedule C, which means it reduces your income tax but not your self-employment tax.

You can contribute in two roles: as the employee (up to $24,500 for 2026) and as the employer (up to 25% of net self-employment income), with the combined figure capped at $72,000, or $80,000 with the standard age-50 catch-up.¹² The AGI reduction is the same in effect; the mechanics catch people off guard at tax time.

The One Way This Backfires

Contributing more than the annual limit creates an excess deferral. The excess is included in your taxable income for the year you contributed it, and if left uncorrected, it gets taxed a second time when it eventually comes out of the plan.¹³

To fix it, you must withdraw the excess and any earnings on it by April 15 of the following year. A filing extension does not extend this deadline.¹³ The trap catches mid-year job changers most often, because your new employer has no visibility into what you already deferred at the old one. Tracking the combined total is on you.

1
Internal Revenue Service. 401(k) Plan Overview
2
Internal Revenue Service. Roth Comparison Chart
3
Internal Revenue Service. 2026 General Instructions for Forms W-2 and W-3
4
Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
5
Internal Revenue Service. Retirement Plans FAQs on Designated Roth Accounts
6
Internal Revenue Service. Retirement Plan FAQs Regarding Contributions
7
Internal Revenue Service. Publication 502 (2025), Medical and Dental Expenses
8
Internal Revenue Service. Child Tax Credit
9
Internal Revenue Service. Questions and Answers on the Net Investment Income Tax
10
Internal Revenue Service. 2026 Amounts Relating to Retirement Plans and IRAs, as Adjusted
11
Internal Revenue Service. Calculating Your Own Retirement Plan Contribution and Deduction
12
Internal Revenue Service. Retirement Topics – 401(k) and Profit-Sharing Plan Contribution Limits
13
Internal Revenue Service. Consequences to a Participant Who Makes Excess Deferrals to a 401(k) Plan