Updated July 28, 2026. Quick answer: It is a cliff, not a phase-in. The statute assigns a single flat percentage to each closed range of modified adjusted gross income, with no interpolation between them. Going one dollar over a boundary does not raise your premium slightly — it moves you into the next bracket for the whole year, for both Part B and Part D.
Why the structure matters more than the amounts
Because a cliff makes the last dollar of income enormously more expensive than the ones before it. A conversion, a capital gain or a required distribution that lands just over a boundary carries an effective marginal cost far above the headline tax rate on that dollar.
And a married couple can pay the cliff twice. Each spouse’s surcharge is measured against the joint return’s income. So two people both enrolled in Medicare are each charged the bracket that the couple’s combined income reaches — the same single dollar of excess income triggers two surcharges.
What this makes worth doing
| Action | Why the cliff makes it valuable |
|---|---|
| Knowing the boundary before a conversion | The whole cost is in the last dollar |
| Splitting a conversion across years | Staying under a boundary twice beats crossing once |
| Timing a large gain | The premium consequence arrives two years later |
| Using a QCD instead of a deduction | A QCD never enters income at all, so it cannot push you over |
Most brackets are CPI-indexed against a rebased comparison year, but the top boundary is expressly excluded from inflation adjustment until a later statutory date — so it catches more people every year purely by drift.
No dollar thresholds appear here. Most IRMAA bracket boundaries are CPI-indexed and change annually, and the top boundary is on a different schedule from the rest. Take the current year’s thresholds from CMS or the Social Security Administration directly.
Sources
42 U.S.C. §1395r(i) (Social Security Act §1839(i)), including §1395r(i)(4) (the year used and the modified adjusted gross income definition) and §1395r(i)(5) (inflation adjustment and its exclusions); 20 C.F.R. §418.1205 and §418.1210 (life-changing events and the exclusivity of that list); SSA Program Operations Manual System HI 01120.005 and HI 01140.005. All read July 2026.
This states what the cited authority says. It is not tax advice, and retirement-plan design turns on facts about your business and your other entities that no page can see. Every dollar limit referenced here is indexed and changes annually.