Updated July 28, 2026. Quick answer: Three different definitions share one acronym, and conflating them produces real planning errors. IRMAA MAGI is adjusted gross income plus tax-exempt interest, with a few narrow exclusions. ACA MAGI also adds back the untaxed portion of Social Security benefits. Roth-eligibility MAGI excludes conversion income under IRC §408A(c)(3)(B) — the opposite direction.
The three, side by side
| Used for | Starts from | Notable adjustment |
|---|---|---|
| IRMAA | AGI | Plus tax-exempt interest |
| ACA premium tax credit | AGI | Plus tax-exempt interest and untaxed Social Security |
| Roth contribution eligibility | AGI | Minus conversion income |
The Roth one is the trap most worth knowing. A conversion does not count against your eligibility to contribute to a Roth, because §408A(c)(3)(B) takes it out. It absolutely does count for IRMAA. Someone reasoning “the conversion does not affect my MAGI” is right about one provision and badly wrong about the other.
Tax-exempt interest is not exempt here
Municipal bond interest is excluded from taxable income and then added straight back for IRMAA. A retiree holding munis specifically to keep taxable income down gets no IRMAA benefit from them at all — which is not how they are usually sold.
Because the brackets are cliffs, getting the definition wrong by even a small amount can cost a full bracket.
No dollar thresholds appear here. Most IRMAA bracket boundaries are CPI-indexed and change annually, and the top boundary is on a different schedule from the rest. Take the current year’s thresholds from CMS or the Social Security Administration directly.
Sources
42 U.S.C. §1395r(i) (Social Security Act §1839(i)), including §1395r(i)(4) (the year used and the modified adjusted gross income definition) and §1395r(i)(5) (inflation adjustment and its exclusions); 20 C.F.R. §418.1205 and §418.1210 (life-changing events and the exclusivity of that list); SSA Program Operations Manual System HI 01120.005 and HI 01140.005. All read July 2026.
This states what the cited authority says. It is not tax advice, and retirement-plan design turns on facts about your business and your other entities that no page can see. Every dollar limit referenced here is indexed and changes annually.