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Fill the 22% bracket to the last dollar — but check the Medicare cliff hiding two years downstream.
The Roth conversion bracket-filler is the systematic version of conversion planning: each year, convert exactly enough traditional IRA to bring taxable income to the top of your chosen bracket, so no converted dollar ever gets taxed at a rate you didn't pick. A single filer with $45,000 of taxable income has $60,700 of room to the top of the 22% bracket in 2026 — converting exactly that costs $12,814 federal, an effective 21.1% because the first slice fills the 12% bracket. But the bracket math has a blind spot: Medicare IRMAA counts every conversion dollar in MAGI and bills it as a premium surcharge two years later, as a cliff — $1 over the line costs $1,148 a year per person. This calculator sizes the conversion, prices the tax, and checks the IRMAA line for anyone 63 or older.
Retirees in the gap years before RMDs use it each December: convert exactly enough to top out the 22% bracket — $60,700 on $45,000 of income costs $12,814 federal — and the 63+ IRMAA check stops the conversion from tripping a $1,148 Medicare cliff two years later.
Because brackets are the cheapest tax you'll ever buy back. Income below your current bracket's top is capacity you lose forever on December 31 — a low-income year (sabbatical, layoff, first retirement year before Social Security and RMDs) is a conversion sale. Filling to exactly the bracket top converts at a rate you chose, instead of letting RMDs at 73+ push the same dollars into a higher bracket later. The filler approach converts the most in the cheapest years and nothing in expensive ones.
It can override the bracket logic entirely once you're 63. IRMAA is a cliff, not a bracket: $1 of MAGI over the threshold ($109,000 single / $218,000 joint for 2026 premiums, based on 2024 income) triggers the full surcharge for the year — $1,148 per person at the first tier. A conversion that fills the 22% bracket but crosses the IRMAA line can be the wrong trade: the extra premium is a stealth tax on the last dollars converted. Income at 63 sets premiums at 65, so conversions before 63 carry no IRMAA cost at all — that window is precious.
Outside money, if you have it — the difference compounds for decades. Paying $12,814 of tax from a taxable account moves the full $60,700 into the Roth where it grows tax-free; paying from the conversion itself shrinks the Roth to $47,886, and if you're under 59½ the withheld piece counts as an early distribution with a 10% penalty on top. The other disciplines: conversions are irreversible since 2018 (no recharacterization), each conversion starts its own 5-year penalty clock if you're under 59½, and December conversions let you size the fill with near-complete knowledge of the year's income.