Guide · Medicare
How much can you convert before triggering an IRMAA surcharge?
A Roth conversion doesn't just raise this year's tax bill — once you're near Medicare age, it can raise your Part B and D premiums two years from now. Here's how the IRMAA cliff actually works, and how to convert without stepping on it by accident.
The two-year lookback is the trap
Medicare sets your Part B and D premiums each year from your MAGI two years earlier — your 2026 premium is set by your 2024 tax return, not your current one. That delay is what makes IRMAA easy to miss: the conversion that feels harmless this year can raise your premium in a year you're no longer even thinking about it. IRMAA becomes relevant once either spouse is within about two years of turning 65, or is already enrolled in Medicare.
It's a cliff, not a slope
Unlike an ordinary tax bracket, IRMAA doesn't phase in. Cross a tier's MAGI threshold by even a single dollar and you owe the full surcharge for that tier — for the whole year, per enrolled person. A married couple who both cross a tier pay the surcharge twice. That makes the last dollar of a conversion, the one that tips you into the next tier, far more expensive than the dollars before it.
The 2026 numbers
Standard Part B premium: $202.90/month. MFJ surcharge tiers begin above $218,000 MAGI. Part B surcharges step up through $81.20, $202.90, $324.60, $446.30, and $487.00 per month as MAGI climbs, with a parallel Part D surcharge on top (up to roughly $91/month at the highest tier). Single-filer thresholds are half the MFJ figures for the lower tiers. The top tier is fixed by statute and isn't indexed for inflation, unlike the tiers below it.
A worked example
Say a married couple's 2026 MAGI, before any conversion, is $205,000 — comfortably under the $218,000 first tier. A $20,000 Roth conversion pushes MAGI to $225,000, crossing into the first IRMAA tier. Because IRMAA is a cliff, that isn't a small marginal cost — both spouses now owe an extra $81.20/month on Part B ($974/year each, roughly $1,950/year combined), plus the Part D surcharge, and it lands on their 2028 premiums, not their 2026 tax bill. A conversion of $13,000 instead — staying at $218,000 MAGI — avoids the tier entirely.
How Harbor handles it
Harbor tracks the two-year lookback explicitly: it seeds each year's projected IRMAA from the correct no-conversion baseline, so a conversion is never blamed for a surcharge it can't legally affect yet, and it shows you the exact tier and dollar cost before you cross it. You can set IRMAA as a hard wall (never cross a tier) or a soft one (show the cost and let you decide) — because sometimes accepting one tier of higher premiums for a few years is worth a meaningfully smaller RMD later. A life event like retirement or the death of a spouse can also support an IRMAA appeal via Form SSA-44, which is a mitigation lever worth knowing about even though Harbor doesn't model it directly.
See your own IRMAA exposure
Enter your numbers and Harbor shows the exact tier ceiling for every year of your plan.
Start planning — it's free →Common questions
How much can I convert to a Roth before triggering an IRMAA surcharge?
It depends on your current MAGI and which IRMAA tier you're closest to — for a 2026 MFJ household the first tier starts at $218,000 MAGI. Harbor finds the largest conversion each year that stays under your next tier, and shows the surcharge if you choose to cross it anyway.
Does IRMAA apply to me if I'm not on Medicare yet?
It becomes relevant once either spouse is within about two years of turning 65, because of the two-year MAGI lookback — a conversion today can affect a premium you won't pay for two more years.