MAGI does not include your pre-tax 401(k) contributions, and that single fact quietly moves several tax cliffs.
Key Takeaways
- Pre-tax (traditional) 401(k) deferrals are excluded from Box 1 wages, so they lower adjusted gross income and, in turn, modified adjusted gross income (IRS Form W-2 instructions).
- Roth 401(k) deferrals are made after tax and do not reduce AGI or MAGI, even though the dollars still leave your paycheck (IRS Roth comparison chart).
- The 2026 elective deferral limit is $24,500, with an $8,000 catch-up at 50 or older and $11,250 for ages 60 to 63 (IRS retirement plan limits).
- Employer matching and profit-sharing dollars are never part of your taxable income, so they do not affect your MAGI at all (IRS 401(k) contribution limits).
- MAGI drives the 2026 Roth IRA phase-out ($153,000 to $168,000 single, $242,000 to $252,000 MFJ), the 3.8% net investment income tax ($200,000 single / $250,000 MFJ), and IRMAA (first tier $109,000 single / $218,000 MFJ) (IRS Roth limits; IRS NIIT; CMS 2026).
- Because MAGI starts from AGI, choosing pre-tax over Roth deferrals is one of the few levers a still-working saver has to stay under a threshold in a given year.
MAGI and 401(k) Numbers That Matter in 2026
Figures reflect 2026 IRS, CMS, and SSA guidance. Thresholds are per tax year and, for IRMAA, use MAGI from a return filed two years earlier.
Does MAGI Include 401(k) Contributions?
No, MAGI does not include your pre-tax 401(k) contributions. When you defer salary into a traditional 401(k), your employer leaves those dollars out of the taxable wages reported in Box 1 of your Form W-2, so they never reach your adjusted gross income in the first place.
Modified adjusted gross income is built on top of AGI. As the definition of modified adjusted gross income explains, you start with AGI and then add back a short list of specific items. Pre-tax retirement deferrals are not on that add-back list, so anything that lowered your AGI keeps your MAGI lower too.
The important exception is the Roth 401(k). Those contributions are made with money that has already been taxed, so they stay inside your Box 1 wages and do nothing to reduce AGI or MAGI.
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Why Does a 401(k) Deferral Lower AGI at All?
A pre-tax 401(k) deferral lowers AGI because the tax code treats it as salary you never constructively received. Your employer reports wages net of the deferral, so the reduction happens before you ever calculate AGI on Form 1040.
This is a different mechanism from an above-the-line deduction like a deductible IRA. There is no separate line to claim; the exclusion is baked into the W-2. The mechanics of pre-tax 401(k) contributions cover this wage-exclusion detail more fully.
Because AGI is the launch point for dozens of downstream calculations, that quiet exclusion ripples outward. It touches your adjusted gross income, then your MAGI, and finally the thresholds that hang off MAGI.
Pre-Tax vs Roth 401(k): The Effect on AGI and MAGI
The direct answer: only pre-tax deferrals move your income number down. Roth deferrals and employer money leave MAGI unchanged. The table below summarizes how each dollar type flows through.
| Contribution type | Taxed in the contribution year? | Included in Box 1 W-2 wages? | Effect on AGI | Effect on MAGI |
|---|---|---|---|---|
| Pre-tax (traditional) 401(k) deferral | No | No | Lowers it | Lowers it |
| Roth 401(k) deferral | Yes | Yes | No change | No change |
| Employer match or profit sharing | No (not your income) | No | No change | No change |
| After-tax (non-Roth) voluntary contribution | Yes | Yes | No change | No change |
The split matters because the two account types trade tax now for tax later. A saver weighing the Roth 401(k) versus traditional 401(k) question is also, whether they realize it or not, choosing whether to bend this year’s MAGI.
How Do Employer Contributions Fit In?
Employer contributions never touch your MAGI. Matching dollars and profit-sharing dollars are contributed on your behalf and are not part of your taxable compensation, so they sit outside both AGI and MAGI while they stay in the plan.
The combined employer-plus-employee limit under Section 415(c) is much higher than the elective deferral limit, reaching $72,000 for 2026 (plus catch-up) across defined contribution plans, per IRS contribution limit guidance. None of the employer share raises your reportable income.
Taxes on that money arrive later, when you take distributions in retirement. At that point the withdrawals are ordinary income and do count toward MAGI, which is why sequencing distributions matters just as much as sequencing contributions.
Why Does This Matter for the Roth IRA Phase-Out?
It matters because your ability to contribute directly to a Roth IRA is governed by MAGI, and a pre-tax 401(k) deferral is a legitimate way to pull that number down. For 2026 the Roth IRA contribution phase-out runs from $153,000 to $168,000 for single filers and $242,000 to $252,000 for married filing jointly, according to the IRS Roth contribution table.
A worker sitting just above the ceiling might drop back into the eligible range by increasing traditional 401(k) deferrals, since that lowers MAGI. Choosing Roth 401(k) deferrals instead would leave MAGI where it is. Savers who are already over the line often look at the levers that reduce MAGI before assuming a backdoor route is the only option.
How Does MAGI Interact With IRMAA in Retirement?
IRMAA, the income-related monthly adjustment amount, is a Medicare surcharge that keys off MAGI from a tax return filed two years earlier. For 2026, the first surcharge tier begins above $109,000 for single filers and $218,000 for married filing jointly, per the CMS 2026 premium fact sheet, with the standard Part B premium set at $202.90 per month.
While you are still working, pre-tax deferrals help hold MAGI down in the very years Medicare will later measure. The mechanics of the IRMAA Medicare surcharge reward that kind of two-year-ahead planning.
After you retire, the lever flips. Large pre-tax balances eventually produce required distributions and taxable withdrawals that push MAGI up, which is one reason many retirees examine partial Roth conversions in lower-income years.
What About ACA Subsidies and the NIIT?
Both of these also ride on MAGI, so the same contribution choice moves them. For the Affordable Care Act premium tax credit, MAGI determines subsidy size, and a household near an income cliff can change the outcome by deferring more pre-tax salary. The interaction between conversions and the ACA premium tax credit cliff shows how sensitive that math can be.
The net investment income tax is a flat 3.8% that applies once MAGI exceeds $200,000 for single filers or $250,000 for married filing jointly, per the IRS net investment income tax page. Because the surtax hits the lesser of net investment income or the MAGI excess, shaving MAGI with pre-tax deferrals can reduce or eliminate exposure. Our overview of the net investment income tax for 2026 walks through the calculation.
Where Do Roth Conversions Enter the Picture?
Roth conversions are the mirror image of pre-tax deferrals: they add to MAGI in the year you convert, because the converted amount is ordinary income. That is why the same thresholds discussed here (Roth IRA eligibility, IRMAA, ACA, and NIIT) are central to any conversion plan.
Retirees in a low-bracket year often consider whether a partial conversion fits under the next threshold, and a financial professional can model whether the added MAGI is worth the long-term tax positioning. Our guides on the tax impact of Roth conversions and Roth conversions for retirees explore that trade-off in more detail.
FAQ
Does MAGI include 401(k) contributions?
MAGI does not include pre-tax (traditional) 401(k) contributions, because those deferrals are excluded from your taxable Box 1 wages and therefore never enter AGI, the starting point for MAGI. Roth 401(k) contributions are different: they are made after tax and remain in your income, so they do not reduce MAGI.
Do Roth 401(k) contributions lower my MAGI?
No. Roth 401(k) contributions come out of already-taxed pay and stay inside your reported wages, so they leave AGI and MAGI unchanged. The tax benefit arrives later through qualified tax-free withdrawals, not through a reduction in current-year income.
How much can I contribute pre-tax to a 401(k) in 2026?
The 2026 elective deferral limit is $24,500. Savers age 50 or older can add an $8,000 catch-up, and those ages 60 to 63 can use an enhanced catch-up of $11,250, according to IRS guidance. Only the pre-tax portion of these deferrals reduces MAGI.
Do employer 401(k) contributions affect my MAGI?
No. Employer matching and profit-sharing contributions are not part of your taxable compensation, so they do not raise AGI or MAGI while they remain in the plan. They become taxable, and count toward MAGI, only when distributed in retirement.
Can increasing my 401(k) deferral keep me eligible for a Roth IRA?
It can. Because direct Roth IRA eligibility phases out based on MAGI ($153,000 to $168,000 single and $242,000 to $252,000 MFJ for 2026), raising pre-tax 401(k) deferrals lowers MAGI and may move a saver back into the eligible range. Roth 401(k) deferrals would not have that effect.
Does a 401(k) contribution reduce the income used for IRMAA?
A pre-tax 401(k) contribution reduces MAGI in the year it is made, and IRMAA uses MAGI from a return filed two years earlier. So deferrals made while working can help hold down the MAGI Medicare later measures. The 2026 first IRMAA tier starts above $109,000 single and $218,000 MFJ.
Does a 401(k) contribution help with the net investment income tax?
It can. The 3.8% NIIT applies once MAGI exceeds $200,000 single or $250,000 MFJ, on the lesser of net investment income or the MAGI excess. Lowering MAGI with pre-tax deferrals can shrink or remove the excess amount that the surtax applies to.
Are 401(k) withdrawals counted in MAGI?
Yes. Distributions from a traditional 401(k) are ordinary income, so they increase AGI and MAGI in the year received. Qualified Roth 401(k) withdrawals are generally tax-free and are not added to MAGI, which is part of why account-type mix matters in retirement.
Methodology
This article relies only on primary sources: IRS publications and pages (Form W-2 instructions, the Roth comparison chart, 2026 contribution limits, the 2026 Roth IRA phase-out table, and the net investment income tax page) and the CMS 2026 Medicare premium fact sheet. Figures are stated for the 2026 tax year. Because this is a Your Money or Your Life topic, anonymous forum anecdotes and secondary summaries were excluded in favor of official government guidance verified at the time of review.
This article is for educational purposes only and is not individualized investment, tax, or legal advice. Consult a qualified professional about your specific situation.
