The premium arrives long after the income that caused it
Medicare charges higher earners more for Part B and Part D. The surcharge is called IRMAA, and the income it is based on is not this year’s. It is the modified adjusted gross income on the return you filed two years ago.
That is not an oversight. Premiums for a year have to be set before the year begins, and the most recent return the IRS has finished processing by then is the one from two years earlier. The rule is in the statute rather than left to administration: 42 U.S.C. §1395r(i)(4)(B) fixes the income year as “the second calendar year preceding the year involved”.
So this year’s premium is decided by the return covering the year before last, which you filed last year. Our IRMAA bracket calculator names that year explicitly rather than asking for current income, because entering this year’s figure produces a confident answer to a question nobody asked.
Which income, exactly
Modified adjusted gross income here means adjusted gross income plus tax-exempt interest. Both parts matter. Municipal bond interest is exempt from income tax and still counts toward IRMAA, which catches people who hold munis precisely because they are tax-free.
It is AGI, not taxable income, so it is measured before the standard or itemized deduction. Reading the wrong line off a return understates it by the deduction amount, which is enough to move a tier on its own.
The gap is the problem
The two-year lag means the surcharge lands when your circumstances may have changed completely. The most common case is retirement: a final year of full earnings, sometimes inflated by a payout of unused leave or a vesting event, sets the premium for a year in which you are no longer working.
Nothing corrects this automatically. Social Security applies the figure it has, and a lower current income does not change it on its own.
When the drop was a life-changing event
There is a route, and it is not an appeal against the arithmetic. Form SSA-44 asks for the determination to be made on more recent income when the fall was caused by one of a listed set of events: marriage, divorce or annulment, the death of a spouse, work stoppage, work reduction, loss of income-producing property, loss of pension income, or an employer settlement payment.
Retirement counts as work stoppage, which is the most common path by a wide margin. A fall in income with no qualifying event behind it does not qualify, however large it is.
What this means in practice
If you are approaching Medicare age, the income you record two years before enrolling is the one that sets your first premium. That is the year to look at when timing a Roth conversion, a property sale, or the realisation of a large gain. Our IRMAA calculator shows which tier a given figure lands in and how much headroom is left before the next one.