Do 401(k) Contributions Reduce MAGI: Traditional vs Roth

Traditional, pre-tax 401(k) contributions reduce your Modified Adjusted Gross Income dollar-for-dollar, because that money is excluded from your taxable wages before your income is ever calculated. Roth 401(k) contributions do not reduce MAGI, because they come out of your pay after tax. For 2026, that means a pre-tax elective deferral can shave up to $24,500 off your MAGI, with larger amounts available if you’re 50 or older.¹ Because every version of MAGI starts with AGI, anything that reduces AGI reduces MAGI by the same amount for every test in the tax code.

How a Traditional 401(k) Contribution Actually Lowers MAGI

When your employer withholds a traditional 401(k) contribution from your paycheck, that money is excluded from taxable wages before AGI is calculated. It doesn’t appear in Box 1 of your W-2, the figure that flows into your tax return. It shows up separately in Box 12 with Code D, confirming the amount set aside pre-tax.

The statutory basis is straightforward. Elective deferrals to a qualified plan like a 401(k) are excluded from gross income up to the annual limit.² Amounts above the limit get added back to your gross income.³ Stay within the limit and the exclusion is automatic: those dollars never enter gross income, never reach AGI, and never reach MAGI.

For 2026, the elective deferral limits are:

$24,500 standard deferral limit for all eligible employees
An additional $8,000 catch-up if you’re 50 or older, for a total of $32,500
An additional $11,250 super catch-up if you’re 60 through 63, for a total of $35,750

The higher catch-up for employees aged 60 to 63 comes from SECURE 2.0.¹

One boundary to keep in mind: employer matching and profit-sharing contributions don’t reduce your MAGI. Those amounts never pass through your paycheck and aren’t in Box 1 to begin with. Only the portion you elect to defer from your own pay generates the reduction.

Roth 401(k) Contributions Do Not Reduce MAGI

Roth 401(k) contributions are made with after-tax dollars. The full amount stays in your taxable wages reported in Box 1 of your W-2 and is tracked in Box 12 with Code AA. Because the money was already included in your gross income, a Roth 401(k) contribution does nothing to your AGI or MAGI for the current year. The tradeoff is on the back end: qualified withdrawals in retirement, including all growth, come out tax-free.

If you’re well below any problematic MAGI threshold, Roth may still be the better long-term choice. If you’re trying to get your MAGI under a specific line this year, it won’t help.

One newer wrinkle: SECURE 2.0 lets employers designate matching contributions as Roth. If your employer offers this and you elect it, the match is reported as taxable income on a Form 1099-R for the year it’s allocated to your account.⁴ Most employers still default to pre-tax matching, but it’s worth checking.

The 2026 Mandatory Roth Catch-Up Rule

For taxable years beginning after December 31, 2025, a SECURE 2.0 provision changes catch-up contributions for higher earners. If you earned more than $145,000 in FICA wages from your plan’s sponsoring employer in the prior calendar year (indexed for inflation, expected to be about $150,000 for 2026), your catch-up contributions must go into a designated Roth account.⁵

For MAGI planning, that’s a real limit. A high earner who used to get a pre-tax reduction from the full catch-up now only gets the MAGI benefit from the $24,500 base deferral. The catch-up portion provides no current-year income reduction. The test uses FICA wages (generally W-2, Box 3) from the single sponsoring employer, not total household income. Employees who earned $145,000 or less from that employer in the prior year are unaffected and can keep making pre-tax catch-up contributions.

Where the MAGI Reduction Actually Buys You Something

Lowering MAGI isn’t abstract. Specific dollar thresholds throughout the tax code decide whether you qualify for credits, face surcharges, or can use certain accounts. These are the places a traditional 401(k) contribution can move you across a line.

Roth IRA Eligibility

Your ability to contribute directly to a Roth IRA phases out once MAGI passes a limit. For 2026, the range is $153,000 to $168,000 for single and head-of-household filers, and $242,000 to $252,000 for married couples filing jointly.¹ Someone earning $175,000 who defers $24,500 pulls their MAGI down to roughly $150,500, back inside the Roth IRA window.

Traditional IRA Deduction

If you’re covered by a workplace plan, your traditional IRA deduction phases out based on MAGI. For 2026, the phase-out runs from $81,000 to $91,000 for single active participants, and from $129,000 to $149,000 for joint filers where the contributing spouse participates in a plan.⁶ A separate, higher range applies if only your spouse has a plan. Pre-tax 401(k) deferrals can preserve a deduction you’d otherwise lose.

Net Investment Income Tax

A 3.8% surtax applies to the lesser of your net investment income or the amount MAGI exceeds a fixed threshold: $200,000 for single filers, $250,000 for joint filers.⁷ These thresholds are not indexed for inflation. A joint filer with MAGI of $270,000 could owe the surtax on up to $20,000 of investment income; maxing out a traditional 401(k) at $24,500 could eliminate the tax entirely by pulling MAGI below $250,000.

Medicare IRMAA Surcharges

Medicare bases Part B and Part D premium surcharges on your MAGI from two years earlier. For 2026, individuals with MAGI above $109,000 (or $218,000 filing jointly) pay an Income-Related Monthly Adjustment Amount, climbing to an extra $487 per month at the top bracket for those with MAGI of $500,000 or more.⁸ Because of the two-year lookback, contributions you make now affect your Medicare costs later. Workers in their final earning years often have the most to gain from maximizing pre-tax deferrals.

Student Loan Interest Deduction

The student loan interest deduction is worth up to $2,500 a year but phases out based on MAGI.⁹ For 2026, the single-filer phase-out runs from $85,000 to $100,000. For joint filers, it’s $175,000 to $205,000. A traditional 401(k) contribution can preserve at least part of the deduction.

ACA Premium Tax Credits

Eligibility for marketplace premium subsidies depends on household income as a percentage of the federal poverty line.¹⁰

Medical Expense Deduction

Unreimbursed medical and dental expenses are deductible only above 7.5% of AGI.¹² This is an AGI test rather than a MAGI test, but pre-tax deferrals lower AGI and therefore lower the floor. If your AGI is $100,000, the floor is $7,500. Drop AGI to $75,500 by deferring $24,500, and the floor falls to $5,663, freeing up roughly $1,840 more in deductible expenses.

You Have Until December 31, Not April

Unlike IRA contributions, which you can make up until the tax filing deadline in April, 401(k) elective deferrals must happen through payroll within the calendar year. You can’t write a check in March and apply it to the prior year. For a $24,500 MAGI reduction for 2026, every dollar has to come out of paychecks dated on or before December 31, 2026.

If you realize in October that your MAGI is going to be too high, you may only have a few pay periods left. Most payroll systems let you change your deferral election at any time, but the math tightens quickly when you try to squeeze the full limit into a handful of paychecks. Setting the target early in the year is easier on cash flow.

What Happens if You Contribute Too Much

If your total elective deferrals across all employers exceed the annual limit, the excess is included in your gross income for the year it was contributed, so it doesn’t reduce MAGI as intended.³ To fix it, notify the plan and have the excess plus any earnings distributed back to you by April 15 of the following year.

Correct it by that deadline and the damage is contained: the excess is taxed in the year deferred, the earnings are taxed in the year distributed, and no early withdrawal penalty applies.¹³ Miss April 15 and the outcome is much worse. The excess gets taxed in the year contributed and again when eventually distributed, plus the distribution may trigger the 10% early withdrawal penalty and mandatory 20% withholding. This is where people who work multiple jobs or switch employers mid-year run into trouble, since each employer’s payroll only tracks its own plan.

1
Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
2
Office of the Law Revision Counsel. 26 USC 402 – Taxability of Beneficiary of Employees Trust
3
Internal Revenue Service. Consequences to a Participant Who Makes Excess Deferrals to a 401(k) Plan
4
Internal Revenue Service. SECURE 2.0 Act Impacts How Businesses Complete Forms W-2
5
Internal Revenue Service. Notice 2023-62 – Guidance on Section 603 of the SECURE 2.0 Act
6
Internal Revenue Service. Notice 25-67 – 2026 Amounts Relating to Retirement Plans and IRAs
7
Internal Revenue Service. Net Investment Income Tax
8
Centers for Medicare & Medicaid Services. 2026 Medicare Parts A and B Premiums and Deductibles
9
Internal Revenue Service. Topic No. 456, Student Loan Interest Deduction
10
Internal Revenue Service. IRS Reminds Taxpayers Their Social Security Benefits May Be Taxable
12
Internal Revenue Service. Publication 502 (2025), Medical and Dental Expenses
13
Internal Revenue Service. 401(k) Plan Fix-It Guide – Elective Deferrals Werent Limited to the Amounts Under IRC Section 402(g)