Modified adjusted gross income, MAGI, is one of the most confusing figures in US tax because it is not one figure. It starts from adjusted gross income, the number on line 11 of Form 1040, and then adds back certain items. Which items get added back depends on the program using the number: Medicare premiums, premium tax credits, Roth IRA eligibility, and Social Security taxation each run their own version. Two people with identical AGIs can therefore have different MAGIs, and one person can have several MAGIs at once. This guide works through the starting point, the add-backs program by program, and a worked example you can check against the calculator.

The starting point: AGI, line 11

Adjusted gross income is total income minus specific adjustments, such as educator expenses, certain retirement contributions, and half of self-employment tax. It appears on line 11 of Form 1040, and it is the income tax figure every MAGI version starts from. If you do not have a return handy, the calculator lets you build AGI from its components, but the filed figure is the authority.

AGI is not taxable income. Taxable income comes later, after the standard or itemised deduction. MAGI work happens at the AGI stage, before deductions, which is why a large deduction does not lower your MAGI. People sometimes expect a big itemised deduction to rescue a Roth contribution or a Medicare premium tier; it cannot, because MAGI is computed before that step.

The add-backs, program by program

The table below is the heart of the subject. For Medicare IRMAA, the only add-back is tax-exempt interest from line 2a. For premium tax credits under the Affordable Care Act, add tax-exempt interest, non-taxable Social Security benefits in full, and excluded foreign earned income and housing. For Social Security benefit taxation, add half of total Social Security benefits. For the Roth IRA worksheet, the add-backs are narrower again, and the calculator asks only for the fields each purpose needs.

The reason for the differences is that each program measures something slightly different. Medicare wants a broad income picture for premium tiers, so it adds back the one big exclusion most retirees have. Premium credits measure household resources, so they add back the benefits a household actually lives on. Social Security taxation uses its own combined-income test, which is why only half the benefits count there. The acronym is shared; the recipe is not.

A worked example, checked against the calculator

Take the calculator prefilled ACA case: AGI $62,000, tax-exempt interest $800, and non-taxable Social Security benefits $12,400, with no excluded foreign income. ACA MAGI is $62,000 plus $800 plus $12,400, or $75,200. That is the household figure the premium credit works from.

The same person has a different Medicare MAGI: AGI plus tax-exempt interest only, or $62,800. The $12,400 of non-taxable Social Security does not count for IRMAA. And for Social Security taxation, the figure is AGI plus half of total benefits, a third number again. One return, three MAGIs, each correct for its own purpose, which is why the calculator asks which purpose you are computing for before it asks for numbers.

Roth IRA and retirement: where MAGI gates the door

For 2026, Roth IRA contributions phase out between $153,000 and $168,000 of MAGI for single and head-of-household filers, and between $242,000 and $252,000 for married couples filing jointly. Married filing separately while living with a spouse phases out between $0 and $10,000, which in practice means no direct contribution. These are the hub published 2026 figures, and they move with inflation each year, so always check the current year table.

Above the phase-out, the direct route closes but the backdoor route, a non-deductible traditional IRA contribution followed by a conversion, remains available, with its own pro-rata complications. A Roth conversion itself adds to AGI and therefore to MAGI in the conversion year, which can push a person over a threshold they were under. Timing a conversion in a low-income year is the standard planning response, and it is a MAGI decision as much as a tax decision.

Medicare IRMAA: the two-year lookback

Medicare premium tiers use MAGI from two years earlier: 2026 premiums rest on 2024 MAGI, and 2024 MAGI is already filed history by the time the tier is set. The standard Part B premium is $202.90 a month at the lowest tier, rising through bands to $689.90 at the top for married filing separately while living with a spouse. A one-off income spike, a Roth conversion or a large capital gain, can therefore raise premiums two years later, and the effect lasts a year.

Life-changing events allow an appeal: retirement, marriage, divorce, death of a spouse, or loss of income-producing property can justify a redetermination on more recent income. The appeal uses Form SSA-44 and needs documentation, but it exists precisely because the two-year lookback would otherwise punish people for income they no longer have.

Mistakes that move the wrong number

First, using taxable income instead of AGI as the starting point: deductions come after MAGI, not before. Second, adding back everything for every purpose: non-taxable Social Security counts for premium credits but not for IRMAA, and adding it to an IRMAA computation overstates the tier. Third, forgetting the year: IRMAA looks back two years, Roth limits use the contribution year, and premium credits use the coverage year estimate.

Fourth, treating MAGI as a single number to optimise. Lowering one MAGI can raise another: a Roth conversion lowers future required distributions but raises current-year MAGI. The calculator separates the purposes so each computation uses only its own add-backs, and the guide recommends computing the purpose you actually need rather than a generic MAGI.

Frequently Asked Questions

What is MAGI in simple terms?

MAGI is adjusted gross income, line 11 of Form 1040, with certain items added back. Which items are added back depends on the program: Medicare adds only tax-exempt interest, while premium tax credits also add non-taxable Social Security and excluded foreign income.

Is MAGI the same as AGI?

No. MAGI starts from AGI and adds items back, so it is usually equal to or higher than AGI. If you have none of the add-back items, the two numbers are the same for that purpose.

Does MAGI appear as a line on my tax return?

No. MAGI is computed on worksheets for specific purposes, such as the Roth IRA worksheet or the Medicare premium determination. It is not printed as a line on Form 1040.

What are the 2026 Roth IRA MAGI limits?

For 2026, direct Roth contributions phase out between $153,000 and $168,000 for single and head-of-household filers, and between $242,000 and $252,000 for married filing jointly. Married filing separately while living with a spouse phases out between $0 and $10,000.

Why is my MAGI different for Medicare and for premium credits?

Because the add-backs differ. Medicare IRMAA adds back only tax-exempt interest, while premium tax credits also add non-taxable Social Security benefits in full and excluded foreign earned income and housing.

Can I appeal a Medicare IRMAA tier?

Yes, after a life-changing event such as retirement, marriage, divorce, or loss of income-producing property. Form SSA-44 requests a redetermination based on more recent income, with documentation of the event.