Your tax bracket is not your marginal rate. Between the Social Security inclusion formula and the Medicare surcharge steps, the rate on your next dollar can be double the bracket printed on the schedule — and it is not monotonic. This sheet draws the real curve and shows every step of the arithmetic behind it.
Adjusted annually for inflation by the IRS or CMS. These defaults are approximate for 2026 and will be wrong next year. Use the index control below to age them forward, or overwrite them directly.
10 / 12 / 22 / 24 / 32 / 35 / 37%402(g), catch-up, 415(c)Written into statute as fixed dollar amounts with no inflation indexing at all. They have not moved in four decades, so every year of inflation pulls more retirees into the torpedo. This is the single most under-appreciated fact on this sheet.
$25,000 single, $32,000 joint$34,000 single, $44,000 joint50% and 85%Drag the index control and watch the torpedo widen: the brackets move, these do not.
The most common tax misconception is that crossing into a higher bracket re-taxes all your income at the new rate. It does not. Only the dollars above the edge pay the higher rate. Earning one more dollar can never leave you worse off on brackets alone, and "contribute enough to drop a bracket" saves you the rate difference only on the sliver above the line — usually a trivial sum.
Each extra dollar of other income drags up to 85¢ of Social Security into taxable income alongside it. Your taxable income therefore rises by up to $1.85 per $1 earned, and your marginal rate is multiplied by the same factor:
The curve is a hump, not a staircase — the rate rises into the phase-in, peaks, then falls back once 85% of the benefit is already counted and there is nothing left to drag in. That falling section is genuinely counter-intuitive and it is why this curve cannot be read off a bracket table.
The Medicare surcharge is a step function, not a phase-in. One dollar over a threshold costs the full surcharge for the whole year — on the order of a thousand dollars per person, per tier. Two features make it hazardous: it is assessed per person (a couple pays twice), and it runs on a two-year lookback, so your income at 63 sets your premium at 65. Plan Roth conversions around that lookback, not around the year you turn 65.
The marginal rate is measured numerically rather than derived symbolically, because the cliffs have no derivative:
A finite increment means bracket edges appear very slightly rounded rather than perfectly square, and it means a true cliff shows as a tall finite spike instead of an infinity. IRMAA steps are drawn as separate markers rather than folded into the curve, since a one-off charge is not a rate.