What you convert at 63 sets your Medicare premiums at 65. It is a cliff, not a phase-in.
Figures on this page are for the 2026 tax year. Rules that are fixed in law rather than indexed — the Social Security thresholds, for instance — are noted where they come up.
IRMAA is the Income-Related Monthly Adjustment Amount: a surcharge on Medicare Part B and Part D premiums for people whose income is above a threshold. Two features make it genuinely dangerous for anyone planning a Roth conversion.
It's a cliff, not a phase-in. One dollar over the line and you pay the full surcharge for the entire year. There is no taper, no partial step.
It looks back two years. Your 2026 income determines your 2028 premiums. What you convert at 63 sets what you pay at 65 — before you're even on Medicare, and long before the bill turns up.
That combination catches people who are otherwise being careful. You can do everything right in the year you convert, feel fine about it, and then get a letter from Social Security two years later.
The thresholds are based on modified adjusted gross income:
| MAGI (single) | MAGI (married filing jointly) | Tier |
|---|---|---|
| up to $109,000 | up to $218,000 | Standard — no surcharge |
| $109,001–$137,000 | $218,001–$274,000 | Tier 1 |
| $137,001–$171,000 | $274,001–$342,000 | Tier 2 |
The standard Part B premium for 2026 is $202.90 a month. Tier 1 adds $81.20 a month to Part B and $14.50 to Part D.
That's $95.70 a month per person. For a couple, both on Medicare, crossing the first threshold by a single dollar costs:
$95.70 × 12 × 2 = $2,297 for the year
For one person, about $1,148. Nothing about the conversion has changed. You crossed a line.
The number IRMAA uses is your adjusted gross income plus tax-exempt interest. Two consequences people miss:
Deductions don't help. IRMAA looks at AGI, which is calculated before the standard or itemized deduction. Bracket planning and IRMAA planning use different numbers, and the IRMAA one is bigger.
Municipal bond interest counts. It's added straight back in. Muni interest is invisible for income tax and fully visible for IRMAA — the same trick it plays with Social Security taxability.
Consider someone converting aggressively at 63 to drain a traditional balance before RMDs start. Perfectly reasonable strategy. But 63 + 2 = 65, which is exactly when Medicare begins.
So the conversions done in the last low-income years before Medicare land as surcharges in the first years of Medicare. It's the most common way people get hit by this: the strategy is sound, the timing is unlucky, and the feedback arrives too late to change anything.
There's a related problem. The thresholds that will apply to your 2026 income haven't been published yet. The 2028 figures come out in late 2027. Anyone planning today — including this calculator — is projecting them forward from the 2026 numbers. They'll be close, but "close" is not what you want when the consequence is a cliff.
Which is the argument for margin. A few thousand dollars of headroom below a threshold costs you very little in unused bracket space. Crossing costs $2,297.
IRMAA is a cost, not a prohibition. Sometimes you should cross it deliberately.
If you're converting a large balance over several years, one year of surcharge may be a good trade for getting a lot of money converted at a favourable rate. $2,297 against, say, $60,000 of extra conversion at 22% instead of 24% later is arithmetic worth doing rather than a rule to obey.
What you should never do is cross it accidentally, or by a small amount. Landing $500 over a threshold means paying the entire $2,297 for $500 of extra conversion — an effective rate on that last slice of over 400%.
If your income has dropped because of a life-changing event — retirement being the most common — you can ask Social Security to use current income rather than the two-year-old figure. That's form SSA-44, and the qualifying events include work stoppage, work reduction, marriage, divorce and the death of a spouse.
Worth knowing this exists. It will not help you with a Roth conversion, though: a conversion is a voluntary choice, not a life-changing event, and it won't be accepted as grounds.
Know which threshold you're near before you convert, not after. Your MAGI including the conversion is the number that matters, and it includes muni interest.
Decide deliberately whether to cross. Look at the surcharge as a dollar figure against the benefit of the extra conversion.
Leave room for surprises. A December capital gain distribution from a mutual fund you don't control can push you over after you've already converted. Converting late in the year, when your income is nearly settled, is the standard defence.
Remember it repeats. Each year's income sets a surcharge two years later. A multi-year conversion plan needs checking against IRMAA every year, not once.
The calculator caps its recommendation below the next IRMAA threshold by default, shows the surcharge as a dollar amount if you turn that guard off, and marks the cliffs on the marginal cost curve. It detects them by watching for a tier change rather than a dollar amount, which is what stops a coarse sweep from stepping straight over one.