Here is the sequence nobody warns you about. You convert $150,000 from a traditional IRA to a Roth in 2026 — a sound decision on the income-tax math. Your 2026 return shows the conversion as ordinary income. In late 2027, Social Security mails a letter: your 2028 Medicare premium has been recalculated using that 2026 return. The bill for a decision made two years ago arrives after two more premium cycles have already been planned around the old number — and unlike almost everything else in the tax code, there is no do-over.
That is the two-year lookback: Medicare’s income-related surcharge (IRMAA) for any year is set by your modified adjusted gross income — AGI plus tax-exempt interest — from two years prior. The machinery is indifferent to why the income appeared. A salary, a capital gain, and a Roth conversion all look identical to it.
Cliffs, not ramps — and doubled for couples
What makes the lookback expensive rather than merely annoying is the shape of the brackets. IRMAA is a cliff: exceed a threshold by one dollar and the entire year’s surcharge for that tier applies — and it applies per beneficiary, so a married couple pays it twice. In 2026, one dollar over the first joint threshold of $218,000 costs a couple $2,296.80 for the year. The complete tier table, verified against the CMS notice, is on our IRMAA bracket page.
A conversion that lands $30,000 into a higher tier does not cost you a little more. It costs you the whole tier, twice, for a full year, two years from now.
Why there is no appeal
Medicare does allow surcharge appeals — Form SSA-44 — but only for a fixed list of life-changing events: retirement, death of a spouse, divorce, and a handful of others. A Roth conversion is not on the list and never has been. It is income you chose, and the SSA treats chosen income as fully intentional. We wrote up how the SSA-44 works and who actually qualifies — the short version for converters: the appeal door is closed before you knock. The bracket you land in is the bracket you keep.
The window arithmetic
The lookback also reshapes when conversions are cheap. Because income at 63 sets the premium at 65 — the first Medicare year — the conversion clock effectively starts two years before enrollment, not at enrollment. The genuinely unconstrained years are before age 63; from 63 on, every conversion dollar is also a future premium input.
After retirement, the classic conversion window — the years between the last paycheck and the first required minimum distribution — is still real. Income is low, brackets are open, and converting then can shrink the RMDs that would otherwise force income (and IRMAA tiers) on you permanently in your eighties. The point is not that IRMAA argues against converting. Often the opposite: a few deliberately surcharged years in your late sixties can buy two decades of lower mandatory income later, and for married couples the survivor scenario often decides it — the same distributions on a single filer’s thresholds, which sit at roughly half the joint levels, land two or three tiers higher. Converting while both spouses are alive is frequently the single largest lever the household has.
Working the brackets deliberately
In practice the discipline looks like this: pick the tier you are willing to occupy two years from now, and convert to just under its ceiling — with a margin. The margin matters because MAGI is not fully known until the year closes: mutual fund distributions arrive in December, interest and dividends drift, and a threshold missed by $500 costs the same as one missed by $50,000. Filling a bracket to the last dollar is precision the inputs do not support.
Remember also that the thresholds are inflation-indexed — the 2028 lines that will judge your 2026 conversion have not been published yet. Planning against them means estimating conservatively, another reason for the margin. This is exactly the modeling our Roth conversion & IRMAA optimizer runs: the two-year offset, the cliffs, RMDs, and the survivor scenario, over the whole horizon rather than a single tax year.
Where this fits in the larger picture
The lookback is one constraint inside a larger optimization: conversion sizing against federal brackets, gain timing, charitable stacking, and — for households relocating to Florida — the state-tax window that often does double duty in the same years. None of this is a one-time decision; it is a December discipline, every year, with the brackets chosen rather than suffered.