Roth conversions and IRMAA without the surcharge
Every Roth conversion dollar raises MAGI, and IRMAA brackets are cliffs. Cross by $1 and the surcharge applies to every Medicare premium for the year — charged two years later, with no way to undo it. Get the sizing right and conversions still work. Here is the timing math, the cliffs, the pre-63 window, and how to size conversions once Medicare starts.
Where conversions and IRMAA collide
A Roth conversion is fully taxable as ordinary income in the year it happens, which means every converted dollar flows straight into MAGI — the same figure Social Security uses to set Medicare Part B and Part D premiums under IRMAA.
Federal tax brackets are smooth ramps: cross from 12% into 22% and only the dollars above the line pay the higher rate. IRMAA brackets are cliffs. Cross by $1 and the full surcharge applies to every premium for the entire year. A conversion sized perfectly for the federal bracket map can land $300 over an IRMAA threshold and cost a couple roughly $2,300 in extra premiums they had no plan to spend.
The 2-year lookback timing math
Social Security uses the MAGI from your tax return two years earlier to set your current-year Medicare premium.
A conversion done in 2026 appears on the 2026 return filed in early 2027. Social Security pulls that return mid-2027 and uses it to set 2028 premiums. Today's conversion has zero effect on this year's premium and zero effect on next year's. The bill arrives in year three.
There is no IRMAA equivalent of a recharacterization: once December 31 passes, your premiums two years out are set. And because the lookback reaches back two years from age 65, a conversion at age 64 still bills you at age 66 — which is why age 63 is the real pre-Medicare cutoff, not age 65.
Bracket cliffs and the $1 overage
IRMAA brackets are step functions. Going $1 over a threshold triggers the full surcharge for that tier on every Medicare premium for the entire premium year.
In 2026, the first IRMAA threshold sits at $109,000 MAGI for single filers and $218,000 for married filing jointly. Cross the joint threshold by $1 and each spouse on Medicare pays roughly $974 more per year in Part B and $174 more in Part D — about $2,296 for a couple. The second threshold ($272,000 joint) roughly triples that, and each tier above is another cliff of similar magnitude.
The implication: a few-thousand-dollar buffer below a threshold is almost always rational. Stopping at $215,000 MAGI instead of $218,000 forgoes $660 in federal tax savings (the last $3,000 at the 22% bracket). Crossing the cliff by accident costs $2,300. The buffer pays for itself the first time a year-end dividend distribution moves MAGI by more than $660.
The pre-63 golden window
Conversions done before age 63 cannot trigger IRMAA. Medicare begins at 65, the lookback reaches back two years, so the window closes January 1 of the year you turn 63.
For a couple who retires at 60 and delays Social Security to 70, the window is wide. No wages, no RMDs until 73, no Social Security yet. The federal brackets are empty up to the standard deduction plus the top of the 12% bracket — in 2026, that is $133,000 ($32,200 + $100,800) of gross conversion room for a joint filer, all taxed at 12%.
Convert $133,000 a year for three years and you have moved $399,000 to Roth at a 12% cost with zero Medicare consequence. The same $399,000 converted later — after RMDs and Social Security fill the lower brackets — is partly taxed at 22% and can trigger several IRMAA tiers. The lifetime tax delta between converting inside and outside the window is frequently larger than the headline tax savings of the conversion itself.
Claiming Social Security at 62 does not change the IRMAA-free status of the window, but it fills the 12% federal bracket with benefit income — pushing any age-62 conversion straight into the 22% bracket and making the conversion expensive on the federal side. Delaying Social Security to 67 or 70 keeps the 12% bracket empty through the full window. See the Roth conversion strategy guide for how this fits the overall plan.
Sizing conversions after Medicare starts
After age 63, every conversion dollar must be sized against IRMAA thresholds. The strategy shifts from "fill the federal bracket" to "fill the bracket minus a buffer below the next IRMAA tier."
Start with the next IRMAA threshold for your filing status. Subtract everything already in MAGI — pension, 85% of Social Security, dividends, interest, capital gains, RMDs, prior IRA withdrawals. Subtract a $3,000–$5,000 buffer for year-end estimate error. The remainder is your conversion headroom.
| JOINT FILER, AGE 65, 2026 MAGI BUILD | AMOUNT |
|---|---|
| 2026 IRMAA Tier 1 threshold (MFJ) | $218,000 |
| − Pension | −$48,000 |
| − 85% of Social Security ($60,000 gross) | −$51,000 |
| − Taxable dividends and interest | −$8,000 |
| − Capital gains realized | −$5,000 |
| − Buffer for estimate error | −$3,000 |
| = Conversion headroom for the year | $103,000 |
Converting $103,000 keeps MAGI safely under $218,000 and captures the full Tier 1 headroom. The buffer is what makes it safe — December mutual fund distributions can easily move MAGI by $2,000 without warning.
If your projection shows the lifetime federal tax savings from a larger conversion exceed the surcharge cost of crossing the next tier, crossing can be rational. The point is to make that call deliberately, not to discover the surcharge after the fact.
The table above uses the harder case — a retiree already on Medicare with multiple income sources active. The pre-loaded scenario at the bottom of this page goes the other direction: a couple still in the pre-63 window with no pension and Social Security delayed to 70, where the constraint is the federal 12% bracket rather than IRMAA.
The widow's trap
Single IRMAA brackets are exactly half the joint brackets. When one spouse dies, household income rarely halves — but the cliffs do.
A couple at $190,000 MAGI is comfortably below the $218,000 joint Tier 1. After the first death, the survivor inherits the IRAs and rolls into survivor Social Security. MAGI may settle around $140,000 — lower than household income, but well past the single Tier 1 threshold of $109,000. The survivor lands in Tier 2, paying $2,500+ per year in surcharges they never paid as a couple.
This is the strongest argument for aggressive Roth conversions during the joint-filing years. Every dollar moved to Roth before the first death is a dollar that does not flow into the survivor's MAGI when the brackets collapse — and the Roth escapes survivor RMDs at 75, where the trap deepens.
Model your conversion
The single-year arithmetic above is workable on paper. The lifetime question — how much to convert at each age, given federal brackets, IRMAA cliffs, RMD pressure, and the widow's trap — needs year-by-year projection.
Drawdown Arc projects MAGI, federal tax, and account balances through age 95 using 2026 tax law. Pro adds IRMAA surcharges so you can size conversions against the actual cliffs.
The pre-loaded scenario assumes a couple retiring at 60 with $1.2M in tax-deferred and delaying Social Security to 70 — the textbook setup for a wide pre-63 window. Pro users can compare a Roth conversion strategy against a no-conversion baseline on the Roth Analysis tab, with IRMAA surcharges included in the lifetime tax math.
For broader context, see Roth conversion strategy and Roth vs. traditional IRA.
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