Roth conversions and Medicare premiums: the warning to remember.

A conversion can raise income

The taxable portion of a Roth conversion is generally included in income for the conversion year. That can be useful planning, but it can also create side effects.

Medicare is one of the side effects

Higher income can affect Medicare Part B and Part D premiums through income-related monthly adjustment amounts, known as IRMAA. The surcharge uses a two-year lookback, so income in the conversion year sets the premiums two years later.

The Roth conversion planner charges those surcharges in its household simulation: each year’s premium tier comes from income two years earlier, the surcharge is paid per person aged 65 or older, and it counts as a cost of the plan. One of its plan rules converts just enough each year to keep income under a tier you choose, and its heads-up list says when a plan puts you in a higher tier than converting nothing. Its cards, which say how much a year is worth converting, price the tax alone, so an amount they show as worth converting can still cross a threshold; the simulation below them counts it. Two other tools show the 2026 thresholds: the Real Marginal Rate Explorer draws them as cliffs on the marginal-rate curve, and the RMD & IRMAA Horizon shows which tier each year’s income lands in and how much could be converted before the first threshold.

For official premium context, start with Medicare.gov.

The useful warning

The plain version is still worth remembering: “This could increase your Medicare premiums.” It is not a full answer, but it is often the right interruption before someone treats a bracket-only estimate as a complete tax plan. Where the surcharge can be counted, it is better counted. Even then, the planner does not model the request you can file to lower the surcharge after a life-changing event, such as stopping work or the death of a spouse.