The IRMAA Two-Year Lookback, Explained: Which Tax Year Counts When

Why the lag exists at all

The Social Security Administration sets your Medicare premiums each fall for the coming calendar year. To do that, it needs your income data. But your tax return for the current year does not exist yet; you will not file it until the following spring. So the SSA uses the most recently available complete return, which is the one from two years prior. There is no way around this; it is baked into the system.

The practical consequence is a permanent two-year echo. Your 2024 federal tax return determined your 2026 premiums. Your 2025 return will determine your 2027 premiums. The income decisions you are making right now, Roth conversions, property sales, capital gains harvesting, are setting your Medicare prices for 2028.

The one-time spike that echoes for a year

This is the scenario that burns people. Imagine you did a large Roth conversion in 2024, say $120,000, while still working. Your 2024 MAGI came in at $260,000 as a single filer, landing you in a high IRMAA tier for 2026. In 2025 you retired completely. Your 2025 income is a fraction of what it was. Your 2026 premiums, however, are priced off 2024. The context of that income, a final working year plus a one-time conversion, is invisible to the system. Medicare just sees the number.

Here is the part people get wrong: the spike does not haunt you for two years. It affects premiums for one full calendar year, the year two years after the event. Once your lower 2025 return enters the lookback window, your 2027 premiums drop back down. The pain is sharp but temporary, which is exactly why it pays to think of IRMAA costs in per-year terms rather than as a permanent sentence.

Thresholds from CMS "2026 Medicare Parts A & B Premiums and Deductibles" fact sheet, November 14, 2025.

What counts as income in the lookback year

The number the SSA uses is your Modified Adjusted Gross Income: your AGI (line 11 of Form 1040) plus tax-exempt interest (line 2a). That definition is what makes the lookback so dangerous, because it captures far more than wages. Capital gains, Social Security (the taxable portion), pension income, traditional IRA and 401(k) distributions, rental income, and yes, municipal bond interest, which is exempt from income tax but added back for IRMAA, all land on the same return and all count.

The common triggers I see: a large Roth conversion done without cliff planning, a rental property sale, exercising stock options, a lump-sum pension distribution, or an inherited IRA distribution. Any one of these can blow through several tiers in a single tax year. And because IRMAA tiers are cliffs, not gradual phase-ins, one dollar into Tier 4 costs the same as $40,000 into Tier 4. If you want the playbook for keeping these events under the thresholds, I covered it in How Retirees Actually Lower MAGI to Avoid IRMAA.

The age 63 planning window

The lookback has a direct implication for pre-Medicare planning. If you turn 65 and enroll in Medicare in 2028, the SSA will price your premiums off your 2026 return, the year you turn 63. That is why planners tell clients to start managing MAGI around age 63: the income from your 63rd year sets your first Medicare prices.

The flip side is the golden window before it. Roth conversions done before 63 never enter the lookback at all, which is why ages 60 to 62 are prime conversion years for early retirees. After 63, every conversion has to be sized against the IRMAA cliffs, because the income will show up in your premiums two years later. This is also the right lens for thinking about the 2027 brackets: my 2027 projections are built around what 2025 income, the income year you are living right now, will buy you.

The one escape hatch: Form SSA-44

If your income has dropped since the lookback year because of a qualifying life-changing event, you do not have to accept the two-year-old number. Form SSA-44 lets you ask Social Security to use your current income instead. Qualifying events include retirement (work stoppage), divorce, death of a spouse, work reduction, loss of income-producing property, and loss of pension income.

The catch: lower income alone does not qualify. You cannot file SSA-44 because your income happened to be lower this year; it has to be tied to one of the qualifying events. A big investment gain in the lookback year, with no qualifying event attached, is not appealable. The full walkthrough, the eight qualifying events, the paperwork, and the refund rule, is in How to Appeal IRMAA With Form SSA-44.

Put the lookback on your calendar

Here is the routine the lookback demands:

1. Every October: project this year's MAGI, knowing it will set premiums two years out. Leave a $3,000 to $5,000 buffer under the nearest threshold.
2. Every December: execute final adjustments: the QCD, the Roth conversion amount, the loss harvest. After December 31, the lookback-year number is final.
3. Every fall: when the IRMAA determination notice arrives, check which tax year it used and whether a qualifying event makes an SSA-44 appeal worthwhile.

The notices arrive in the fall and reflect the surcharge tier for the coming calendar year. If something looks wrong, that is your window to act. Check your tier and cliff distance