Pre-tax traditional 401k contributions are not included in your modified adjusted gross income. They come out of your wages before federal income tax applies, so they never enter your adjusted gross income in the first place, and no MAGI add-back rule pulls them back in. Roth 401k contributions are different: because they’re made with after-tax dollars, they stay in your taxable wages and therefore in your MAGI.
Why Pre-Tax 401k Contributions Drop Out of MAGI
Your employer withholds traditional 401k contributions from your paycheck before applying federal income tax.1Internal Revenue Service. 401(k) Plan Overview The deferred amount shows up in Box 12 of your W-2 with Code D, and Box 1, your taxable wages, already reflects the reduction.2Internal Revenue Service. Common Errors on Form W-2 Codes for Retirement Plans That lower Box 1 figure is what feeds the AGI line on your Form 1040.
MAGI starts from AGI and adds back specific items the tax code names for each benefit you’re testing eligibility for.3Internal Revenue Service. Modified Adjusted Gross Income There’s no single MAGI formula. But across every version — Roth IRA eligibility, the IRA deduction, the Premium Tax Credit, the Net Investment Income Tax, IRMAA — pre-tax 401k deferrals are never listed as an add-back. What comes off your AGI stays off your MAGI.
The dollars involved can be significant. For 2026, the elective deferral limit is $24,500. Workers aged 50 and older can add an $8,000 catch-up, and workers aged 60 through 63 get an enhanced catch-up of $11,250, bringing their maximum deferral to $35,750.4Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 Every pre-tax dollar there reduces both AGI and MAGI by the same amount.
Why Roth 401k Contributions Stay in Your MAGI
Roth 401k contributions run through a different path. Federal law treats designated Roth contributions as elective deferrals but does not exclude them from gross income.5Office of the Law Revision Counsel. 26 USC 402A – Optional Treatment of Elective Deferrals as Roth Contributions You pay income tax on the money before it lands in the account. Your Box 1 wages are not reduced, your AGI is not reduced, and your MAGI is not reduced.
Choosing a Roth 401k over a traditional 401k can leave you with a noticeably higher MAGI, which can push you across phase-out ranges for other tax benefits.
SECURE 2.0 also lets employers designate matching contributions as Roth. If your employer offers this and you elect it, those matched dollars count as gross income for the year they’re allocated, even though no tax is withheld at the time.6Internal Revenue Service. SECURE 2.0 Act Changes Affect How Businesses Complete Forms W-2 The added income raises both AGI and MAGI.
The 2026 Roth Catch-Up Rule for High Earners
Starting in 2026, some workers lose the option to make pre-tax catch-up contributions. If your FICA wages from the plan sponsor exceeded $145,000 (indexed for inflation) in the prior calendar year, all catch-up contributions must be designated as Roth.7Internal Revenue Service. Internal Revenue Bulletin 2025-40
The MAGI consequence is direct. A worker who could previously have shielded $7,500 of catch-up money from AGI now sees the full $8,000 catch-up (or $11,250 for ages 60 through 63) remain in taxable income.4Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 Workers whose prior-year wages fall below the threshold can still make pre-tax catch-ups if their plan allows it.
Why the Traditional-Versus-Roth Distinction Matters for MAGI
MAGI decides eligibility or premium levels for several tax provisions, and every one of them is sensitive to whether your 401k money is pre-tax or Roth.
Roth IRA Contributions
For 2026, direct Roth IRA contributions phase out between $153,000 and $168,000 of MAGI for single filers, and $242,000 to $252,000 for married couples filing jointly.4Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 Maxing out pre-tax 401k deferrals can keep MAGI under the ceiling.
Traditional IRA Deduction
If you’re covered by a workplace plan, the 2026 IRA deduction phases out between $81,000 and $91,000 (single) and $129,000 to $149,000 (married filing jointly, contributor covered). If only your spouse is covered, the range is $242,000 to $252,000.4Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
Premium Tax Credit
The MAGI used for the Premium Tax Credit is broader than the IRA version. It adds tax-exempt interest and non-taxable Social Security benefits on top of the standard add-backs.8CMS. Job Aid – Income Eligibility Using MAGI Rules Traditional 401k deferrals still don’t appear on the add-back list, so they lower this MAGI too.
Net Investment Income Tax
A 3.8% surtax applies to the lesser of your net investment income or the amount your MAGI exceeds $200,000 (single) or $250,000 (married filing jointly).9Office of the Law Revision Counsel. 26 USC 1411 – Imposition of Tax These thresholds are not indexed. Pre-tax 401k contributions that pull MAGI below the line reduce or eliminate the surtax.
Medicare Part B and Part D Premiums
Medicare’s income-related monthly adjustment amount uses AGI plus tax-exempt interest. For 2026, higher premiums begin at $109,000 (single) and $218,000 (joint), with surcharges climbing across five tiers up to $500,000 individual and $750,000 joint.10CMS. 2026 Medicare Parts A and B Premiums and Deductibles Because IRMAA uses the return from two years prior, a 401k contribution made this year affects a premium bill two years out.