A Roth Conversion That Crosses a Medicare IRMAA Line: $1,148 for the First Dollar (2026 Table)

A Roth conversion is taxed as ordinary income in the year it happens, and most planning stops there. For someone on, or about to be on, Medicare there is a second bill that arrives two years later and does not grow smoothly with the amount converted: a surcharge on Medicare Part B and Part D that steps up at fixed income lines. This guide takes the 2026 table from the Centers for Medicare & Medicaid Services and shows what crossing each line costs, using the same engine as the Medicare IRMAA calculator.

What each line costs a year

These are the 2026 surcharges for Part B and Part D together, per person, for a year. A couple filing jointly pays each spouse's amount, so double it if both are on Medicare. The lines are the highest MAGI in the tier below: at exactly the line you are still in the lower tier.

The last tier starts at $500,000 (single) and $750,000 (joint) and is reached at or above the figure, not over it. Part B in the top tier totals $689.90 a month, against the $202.90 everyone else pays.

What a conversion costs, from $100,000

A single filer with MAGI of $100,000 before the conversion (2026 lines used as a stand-in for the premium year two years later, when they will be somewhat higher):

The cost does not rise with the size of the conversion: $10,000 and $20,000 both cost $1,148.40, because both land between the first and second lines. The first dollar over is the expensive one. Converting $5,000 costs nothing in Medicare terms; the next $5,000 costs $1,148.40. That is on top of income tax on the conversion, and the share column falls as the amount grows only until the next line is reached: $40,000 crosses the second line and costs $2,884.80.

What follows from the arithmetic is a way of thinking, not a recommendation: the dollars that cost the most per dollar are the few just over a line. Filling a tier to its top ($9,000 of room here) uses all of its room at no Medicare cost; spreading a larger conversion across several years can keep each year under a line. Whether that is worth the income tax and the delay is for your own situation.

Married filing separately turns the table around

The first four joint lines are double the single ones, which is why a couple usually pays less together. File separately while living together and CMS's table changes: nothing up to $109,000, then straight to the fifth tier above it. A couple with $120,000 of MAGI each ($240,000 together), both on Medicare, pays $2,296.80 a year for the pair on a joint return (just over the $218,000 line) and $12,710.40 filing separately: 5.5 times as much. Filing status is chosen for the whole tax result, and Medicare is only one part, but it is a part that is easy to miss.

The two-year delay, and when to ask for a different year

• Your first Medicare years look back at your working years. At 65 the return Social Security uses is from when you were likely still working full time. If you have since stopped working, cut hours, married, divorced or lost a spouse, or lost income-producing property to a disaster, an employer pension plan has ended or you received an employer settlement, you can ask them to use a more recent year with documentation (SSA lists these events; the statute is § 1839(i)(4)(C)).
• Premium year and income year differ by two. Income in 2026 sets 2028 premiums. The dollar lines are indexed to inflation and rounded to the nearest $1,000 each year, so the 2028 lines will be higher than the 2026 ones here; plan with a margin below the line you use, not a dollar below it.
• The tax return decides, not your bank balance. The figure is adjusted gross income plus tax-exempt interest. Qualified withdrawals from a Roth account are not in adjusted gross income, so they do not move you toward a line; the taxable amount of a conversion does.

What this guide leaves out

The income tax on a conversion, and how it interacts with the share of Social Security that becomes taxable (see the taxable Social Security calculator); the Part D plan's own premium and Medicare Advantage premiums, which are not set by this table; premium years other than 2026 (CMS publishes each year's lines in the autumn); the add-backs for foreign income and certain savings bond interest in the statute's MAGI; and anyone whose premium is paid by Medicaid. Not tax advice: confirm a conversion plan with a tax professional, and treat the letter Social Security sends as the authority on your own amount.