Guide · updated 2026-08-03
What MAGI Means for Medicare, Line by Line
Medicare's income surcharge is set by a figure called MAGI, and it is not the same MAGI used for Roth contributions or for the health-insurance marketplace. For IRMAA it is two lines from Form 1040 added together. Here is exactly which two, and what does not belong in the sum.
It is two lines from Form 1040, and nothing else
For the Medicare income-related surcharge, modified adjusted gross income is adjusted gross income — line 11 of Form 1040 — plus tax-exempt interest, which is line 2a. That is the whole definition. It is not a percentage of anything, it does not subtract the standard deduction, and it does not exclude Social Security that was already taxed. If you have a copy of the return in question, you can produce the exact figure Social Security used in about ten seconds.
Tax-exempt interest counts, which surprises people who bought municipal bonds
Municipal bond interest is exempt from federal income tax and is not exempt from this calculation. It goes on line 2a of the return precisely so that programs which need a fuller picture of income can add it back, and Medicare is one of those programs. A portfolio deliberately shifted toward tax-exempt income to reduce a tax bill can therefore leave the Medicare surcharge exactly where it was, which is not an obvious result and is worth knowing before the shift rather than after.
Three other things called MAGI that are not this one
The term is reused across the tax code with different definitions each time, and confusing them is common. The MAGI that limits a Roth IRA contribution adds back different items. The MAGI used for premium tax credits on the health-insurance marketplace has its own definition again, and includes the untaxed portion of Social Security. The MAGI for the net investment income tax is another. A figure calculated for one of those is not the figure Medicare is using, and substituting one for another can move the answer by a tier.
The year it comes from is two years back
The surcharge for a given year is set from the return filed two years earlier, because that is the most recent return the IRS can certify to Social Security by the time premiums are set. So a 2026 premium rests on the 2024 return. This is why a surcharge so often appears with no visible cause in the current year, and why the income figure worth watching is the one accumulating right now, for a bill that will not arrive for two Januarys.
What to do if the figure they used is wrong
Two different situations, with two different routes. If Social Security used the wrong number — an amended return, or an IRS transcription error — that is a reconsideration, and the evidence is the corrected return. If the figure was right at the time but your income has since fallen because of one of eight life-changing events, that is Form SSA-44, which asks them to use a more recent year instead. Neither costs anything to file, and only Social Security can make either determination.
Official sources for this guide
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