Not a rate, a cliff
Most of the tax system phases in. Income tax brackets apply a higher rate to the dollars above a threshold, so crossing one costs a few cents. IRMAA does not work that way.
Cross an IRMAA threshold by a single dollar and the entire year moves to the next tier. At the first threshold that costs $81.20 a month on Part B and $14.50 on Part D, which is $1,148.40 over a year, for one dollar of income. If both spouses are enrolled in Medicare, the household pays it twice.
Our IRMAA bracket calculator shows how far you are from the next threshold and what crossing it would cost, which is the figure that makes the rest of this worth doing.
Why the top of the table behaves differently
The lower thresholds include their upper bound: exactly $205,000 is still in the fourth tier. The top of the table reverses, and the highest tier starts at its threshold rather than above it, so exactly $500,000 is in the top tier rather than the one below. It is the kind of detail that is invisible until it costs $40.70 a month.
What actually moves the number
Because the income is from two years back, the useful window is the year itself, not the year the premium arrives. Within that year, the levers are the ones that change adjusted gross income:
- Deductible contributions. A deductible IRA or HSA contribution reduces AGI directly, and near a threshold the saving is worth far more than the marginal tax rate suggests.
- The timing of a Roth conversion. A conversion adds to AGI in the year it happens. Splitting one across two years can keep both below a threshold where a single conversion would cross it. The pro-rata rule decides how much of a conversion is taxable in the first place.
- Realised gains. Selling an asset is usually discretionary in its timing, and a large gain is the most common single cause of a one-off IRMAA year.
- Tax-exempt interest. It does not reduce the figure. Municipal bond interest is added back, so moving into munis to manage income tax does not help here.
What does not help
The standard deduction and itemized deductions come after AGI, so they do not change the IRMAA figure at all. Neither does filing status on its own: there is no head-of-household column in the IRMAA table, and single, head of household and qualifying surviving spouse all read the same thresholds.
Filing separately is worth a warning rather than a recommendation. Married couples who file separately and lived together at any point in the year read a three-tier table that jumps straight to a $446.30 monthly surcharge just above the base threshold, which is usually far worse than filing jointly.
A one-year problem, not a permanent one
IRMAA is redetermined every year against a new return. A single high year raises the premium for one year and then falls away, which makes it easier to live with than it first appears, and worth planning around only in the years where a threshold is genuinely close.