IRMAA — the Income-Related Monthly Adjustment Amount — is the surcharge Medicare adds to Part B and Part D premiums when your modified adjusted gross income crosses one of five tier lines. The lines are cliffs, not slopes: one dollar of MAGI above the line and your Medicare premium goes up for the full year, per spouse enrolled.
For a married couple in 2026, the first line is $212,000. Cross it by a single dollar and you owe an extra ~$1,050 per spouse for the year. The math is the same shape at $266K, $334K, $400K, and $750K — five cliffs, each one a few thousand dollars apart.
Why this matters for Roth conversions
A $150,000 Roth conversion adds $150,000 to your MAGI for the year you do it. If your base income is already near a tier line, that conversion shoves you into the next bracket and triggers a Medicare surcharge that hits two years later (Medicare uses your MAGI from two years prior to set the current year’s premium).
The optimal Roth conversion year fills the federal-tax bracket you want but stays just under the next IRMAA line. Sometimes the right conversion size is determined by the IRMAA cliff, not the tax bracket. We’ve seen subscribers leave $50,000 of conversion capacity on the table because the next dollar would have crossed a $4,000 IRMAA hit. That’s a free $50,000 of tax-locked Roth conversion you should be doing.
The 2-year lookback (the part most people get wrong)
Medicare sets your current-year premium using your MAGI from two years prior. A conversion you do in 2026 affects your 2028 Medicare premium — not 2026 and not 2027. That gives you a planning window: you can choose to take the surcharge in a year when you’ll be on Medicare anyway, or thread the needle if you’re close to the line.
What this calculator does
Enter your current MAGI (last year’s AGI plus tax-exempt interest is a close enough proxy), the planned increase (Roth conversion size, capital gain, or any one-time MAGI bump), your filing status, and the number of spouses on Medicare. The tool will tell you:
- Which tier you’re in today and which one you’ll cross into
- The annual household surcharge that’ll hit in two years
- Whether you’re sitting on a “cliff” — within $5,000 of the next line, where shrinking the conversion slightly might save you thousands
A word of honesty: this is single-year math. A coordinated multi-year Roth conversion ladder optimizes against IRMAA, Social Security taxation, and the federal bracket simultaneously. That’s the kind of planning we do at T&T Capital Management for households with $500K+ in investable assets — needs more inputs than a single web form can hold, but always starts with a clear-eyed look at numbers like the ones below.
Run yours.