Roth conversions and IRMAA: convert up to the line, not over it

Example 1: a couple converting $50,000

Married, filing jointly, both on Medicare. MAGI before the conversion: $180,000. With the 2026 brackets:

• Room before the first threshold: $38,000 (up to $218,000 included).
• Converting $50,000 brings MAGI to $230,000: tier 1. Surcharge $1,148.40 per person, $2,296.80 for the two of them over the year.
• Converting $38,000 instead keeps them at no IRMAA; the remaining $12,000 can wait for another year.

Example 2: already in tier 1

Single, MAGI $130,000 (tier 1, $1,148.40 a year). Room before tier 2: $7,000. A $40,000 conversion reaches $170,000, tier 2: $2,884.80 a year, $1,736.40 more than now.

Example 3: the one-dollar cliff

Single with MAGI of exactly $109,000: no IRMAA, because the threshold belongs to the lower tier. At $109,001: $1,148.40 for the year. Give yourself a margin: a year-end dividend, a mutual fund capital-gain distribution or a bit more interest than expected can push you over after the conversion is done.

A method that works

1. Estimate this year’s MAGI without the conversion: pensions, wages, IRA withdrawals and RMDs, the taxable part of Social Security, interest, dividends, capital gains, plus tax-exempt interest.
2. Pick the threshold you want to stay under, using the latest projected brackets (they apply two years later).
3. Convert the difference minus a safety margin. Many people convert late in the year, when the other income is known.

What the brackets do not tell you: the income tax cost or saving of the conversion itself. That depends on your tax brackets now and later, and is a question for a tax professional.