Pre-tax 401(k) contributions are not included in MAGI for the tax provisions that use it. Traditional 401(k) deferrals come out of your wages before your W-2 is issued, so they never enter your adjusted gross income, and none of the common Modified Adjusted Gross Income formulas add them back. Every dollar you defer into a traditional 401(k) lowers the income figure the IRS uses to test your eligibility for Roth IRAs, the traditional IRA deduction, ACA premium credits, the Net Investment Income Tax, and Medicare’s IRMAA surcharges. For 2026, the employee deferral cap is $24,500, which sets the ceiling on how much you can shave off MAGI through this single move.
How Pre-Tax 401(k) Contributions Stay Out of MAGI
A traditional 401(k) contribution is not a deduction you claim on Form 1040. It is an exclusion at the source. Your employer subtracts the deferral from your gross pay before printing Box 1 of your W-2, and that lower wage figure is what flows onto your return. Earn $100,000, defer $24,500, and Box 1 reads $75,500.
The distinction matters because MAGI formulas work by adding certain items back to AGI. A 401(k) contribution can’t be added back to something it was never part of. Contrast that with the student loan interest deduction or the foreign earned income exclusion, both of which reduced AGI and both of which several MAGI formulas restore.1Internal Revenue Service. Modified Adjusted Gross Income (MAGI)
Roth 401(k) Contributions Are Different
Roth 401(k) contributions are made with after-tax dollars. They stay in your W-2 wages and do not reduce AGI or MAGI at all.2Internal Revenue Service. Roth Comparison Chart If lowering MAGI is the point, the traditional (pre-tax) 401(k) is the only version that does the work.
What MAGI Actually Is
Modified Adjusted Gross Income starts with AGI and adds back a specific list of items. That list changes depending on which tax provision you are looking at, so there is no single MAGI number. Common add-backs across the various formulas include tax-exempt interest, foreign earned income exclusions, the student loan interest deduction, savings bond interest, and employer-provided adoption benefits.1Internal Revenue Service. Modified Adjusted Gross Income (MAGI) Traditional 401(k) deferrals appear on none of them.
Roth IRA Contribution Eligibility
Direct Roth IRA contributions phase out based on MAGI. For 2026, the phase-out runs from $153,000 to $168,000 for single filers and $242,000 to $252,000 for married couples filing jointly.3Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 Above the top of the range, direct contributions are barred entirely.
Because the Roth IRA MAGI calculation does not add back 401(k) deferrals, maxing out a traditional 401(k) can pull a taxpayer back under the threshold. A single filer earning $175,000 who contributes $24,500 to a traditional 401(k) drops MAGI to roughly $150,500, below the phase-out entirely.
Traditional IRA Deduction
If you are covered by a workplace retirement plan, your ability to deduct a traditional IRA contribution phases out by MAGI. For 2026, a single filer covered by a workplace plan loses the deduction above $91,000, and a married couple filing jointly loses it above $149,000. If your spouse has the workplace plan and you do not, the joint range is $242,000 to $252,000.3Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
The MAGI formula here does not add back 401(k) contributions.1Internal Revenue Service. Modified Adjusted Gross Income (MAGI) So a large 401(k) deferral can lower your MAGI enough to unlock the IRA deduction, letting you claim both the 401(k) deferral and a deductible IRA contribution in the same year.
ACA Premium Tax Credits
Marketplace premium tax credits use a household income figure built on AGI plus tax-exempt interest and foreign earned income. Pre-tax 401(k) contributions are not added back, so they reduce the income used to size your subsidy. For self-employed people and early retirees who manage their income deliberately, a 401(k) deferral is one of the more effective tools for staying under the credit’s income ceiling.
Net Investment Income Tax
The 3.8% Net Investment Income Tax applies when MAGI exceeds $200,000 for single filers or $250,000 for married couples filing jointly.4Internal Revenue Service. Topic No. 559, Net Investment Income Tax The tax hits the lesser of net investment income or the amount by which MAGI exceeds the threshold. These thresholds are set by statute and are not indexed to inflation.
For NIIT, MAGI is AGI plus the foreign earned income exclusion, and nothing else.5Internal Revenue Service. Questions and Answers on the Net Investment Income Tax A traditional 401(k) deferral reduces AGI and carries straight through to a lower NIIT MAGI. In some cases the deferral is enough to move a household below the threshold and eliminate the tax entirely.
Medicare IRMAA Surcharges
Medicare’s Income-Related Monthly Adjustment Amounts add surcharges to Part B and Part D premiums when income crosses set thresholds. The IRMAA MAGI is AGI plus tax-exempt interest.6Social Security Administration. HI 01101.010 – Modified Adjusted Gross Income (MAGI) Pre-tax 401(k) contributions are not added back. IRMAA uses the tax return from two years earlier, so your 2026 premium is based on your 2024 return.
For 2026, individuals with MAGI at or below $109,000, or joint filers at or below $218,000, pay no surcharge. Above those thresholds the surcharges escalate by bracket:7CMS. 2026 Medicare Parts A and B Premiums and Deductibles
- $109,001–$137,000 single / $218,001–$274,000 joint: $81.20 added to Part B, $14.50 to Part D each month
- $137,001–$171,000 single / $274,001–$342,000 joint: $202.90 Part B, $37.50 Part D
- $171,001–$205,000 single / $342,001–$410,000 joint: $324.60 Part B, $60.40 Part D
- $205,001–$499,999 single / $410,001–$749,999 joint: $446.30 Part B, $83.30 Part D
- $500,000+ single / $750,000+ joint: $487.00 Part B, $91.00 Part D
At the top bracket, the combined surcharge runs $578 per month, close to $7,000 a year per person. A worker approaching Medicare age can use 401(k) deferrals in their last working years to hold MAGI under a threshold that would otherwise trigger a surcharge two years later.
Social Security Benefit Taxability
Whether Social Security benefits are taxable depends on “combined income,” which is AGI (excluding Social Security) plus any tax-exempt interest plus half of your benefits.8Internal Revenue Service. Publication 915 (2025), Social Security and Equivalent Railroad Retirement Benefits Once combined income exceeds $25,000 (single) or $32,000 (joint), a portion of benefits becomes taxable. Above $34,000 (single) or $44,000 (joint), up to 85% of benefits can be taxed. These base amounts have not changed since 1984.
A pre-tax 401(k) contribution reduces the AGI component of combined income. For someone still working part-time while drawing Social Security, deferring into that employer’s 401(k) can keep more benefits out of tax.
2026 Contribution Limits
The more you can defer, the more MAGI drops. For 2026:3Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
- Under age 50: $24,500
- Age 50 and older: $32,500 ($24,500 plus an $8,000 catch-up)
- Ages 60 through 63: $35,750 ($24,500 plus an $11,250 enhanced SECURE 2.0 catch-up)
These figures are the employee’s elective deferral only. Employer matching contributions do not count against the cap and do not reduce your AGI. They sit in a separate bucket.
Miscalculating MAGI and Roth Excess Contributions
If you contribute to a Roth IRA and your MAGI turns out to be too high, the excess contribution triggers a 6% excise tax for every year it stays in the account.9Internal Revenue Service. Retirement Topics – IRA Contribution Limits You can avoid the penalty by withdrawing the excess and any earnings on it before your tax filing deadline, including extensions.10Internal Revenue Service. IRA Year-End Reminders The earnings are taxable in the year of the original contribution.
The trap usually springs when someone contributes early in the year based on an income estimate, then gets a bonus or capital gain that pushes MAGI over the phase-out. Leaving a buffer between projected MAGI and the Roth threshold, or waiting to make the Roth contribution until the return is filed, keeps the problem from happening.