This year's picture
One tax year. Change anything; the curve redraws instantly.
Your real marginal rate curve
Each point: the federal cost of the next dollar from the traditional IRA, after it drags Social Security into taxable income and shoves realized gains up the capital-gains bands. Amber verticals are Medicare IRMAA cliffs — not taxes, but real dollars, two years later.
The zones, spelled out
Where each rate starts and stops. The shaded rows are rates above the highest printed bracket you touch — money the bracket table never warned you about.
How the math works
Why the curve doesn't look like the bracket table
Three mechanisms distort the printed brackets for retirees. First, the Social Security torpedo: while your provisional income crosses the §86 bands, each extra IRA dollar makes 50–85 cents of your benefit taxable too, so a dollar "in the 12% bracket" can cost 18.5% or 22.2% — and a dollar "in the 22% bracket" can cost 40.7%. The torpedo ends once 85% of the benefit is taxable; the curve visibly falls back to the plain bracket. Second, gains pushing: capital gains stack on top of ordinary income, so an IRA dollar that shoves a gain dollar across the 0%→15% boundary costs its own rate plus 15 points — the famous 27% zone (12% + 15%). Third, bracket edges themselves.
IRMAA cliffs are different
The amber lines aren't tax — they're next-next-year's Medicare premiums. Cross a threshold by one dollar and the full surcharge applies for that premium year: the first single-filer cliff (MAGI $109,000 in 2026) costs about $1,148 per person per year in Part B and D surcharges; joint filers pay it per spouse on Medicare. There's a two-year lookback — 2026 income sets 2028 premiums — and the thresholds shift with inflation each year, so treat the markers as "you are near a cliff," not to-the-dollar cartography. MAGI here is AGI plus tax-exempt interest (not modeled); it includes only the taxable share of Social Security.
Withdrawals and conversions are the same dollar
For this year's tax, a traditional-to-Roth conversion and a plain withdrawal are identical: ordinary income. The difference is what happens afterward — the conversion keeps compounding tax-free. This tool prices the toll booth; the Roth conversion tool models whether the trip is worth it.
What this tool is not
2026 federal law only, one tax year at a time. Not modeled: state tax, NIIT (3.8% above $200k/$250k MAGI — real for large conversions), the temporary 2025–2028 senior bonus deduction and its own phase-out zone (which adds roughly 6 points of hidden marginal rate between $75k–$175k single / $150k–$250k joint MAGI while it lasts), ACA subsidy cliffs for the pre-65, QCDs, and dependents' credits. Married statuses assume both spouses are 65+ where the age deduction applies. The curve is a map of mechanisms, not a filing document.