Roth Conversion Sizing Calculator: How Much Should You Convert Each Year?

The Roth conversion question most retirees get wrong isn't whether to convert — it's how much. Convert too little and you leave cheap bracket capacity untouched. Convert too much and you cross into a higher bracket, trigger IRMAA Medicare surcharges, or push through the OBBBA senior deduction phaseout zone. This calculator finds the optimal annual conversion amount based on your income, deductions, and 2026 tax thresholds — and shows what it does to your projected RMDs.

How bracket-filling conversions work

Your tax brackets set a ceiling on how much income you can earn at each rate. If your taxable income is $44,500 (MFJ), you're in the 12% bracket — and the 22% bracket doesn't start until $100,800. That $56,300 gap is "bracket capacity" you can fill with a Roth conversion at 12 cents on the dollar. The strategy: identify the gap between your current taxable income and the top of your target bracket, then convert that amount each year during the pre-RMD window.

A concrete example: a couple with $80,000 in pension and Social Security income. After the $35,500 standard deduction for a couple both age 65+,¹ their taxable income is roughly $44,500. The top of the MFJ 22% bracket is $211,400. They could theoretically convert up to $166,900 before crossing into 24% — though IRMAA often limits this sooner.

The IRMAA constraint that caps most conversions

IRMAA (Income-Related Monthly Adjustment Amount) surcharges add to your Medicare Part B and Part D premiums when MAGI exceeds $109,000 (single) or $218,000 (MFJ) in 2026.² Roth conversions count as ordinary income and push your MAGI up. The critical detail: IRMAA uses a two-year lookback — your 2026 income affects 2028 Medicare premiums.

For many retirees, the IRMAA threshold is the binding constraint — not the bracket ceiling. If your current income is $170,000 MFJ, you have $48,000 of IRMAA headroom even though your bracket has much more room. Crossing $218,000 MFJ triggers about $2,000/year in extra premiums per couple, and the cliff nature of IRMAA tiers makes staying just below the threshold a high-priority target.

The OBBBA senior deduction: additional headroom not shown above

Starting 2025 and permanent under OBBBA, taxpayers age 65 and older may claim a new $6,000 senior deduction per qualifying individual — separate from the standard deduction and also available to itemizers.³ For a couple both 65+, that's $12,000 of additional deductions that further reduces taxable income.

The catch: this deduction phases out above $150,000 MAGI for joint filers. Conversions that push your MAGI past $150,000 begin eroding this deduction, creating a marginal-rate "hump" in that range. The calculator above does not model this phaseout — if your pre-conversion income is below $150,000, your effective bracket headroom is somewhat wider than shown. Work with a tax professional to capture this precisely.

Pre-RMD years: the primary conversion window

The window between retirement and your RMD start age — for most retirees, the early-to-mid 60s through age 72 — is typically your lowest-income period since your 30s. Earned income has stopped, RMDs haven't begun, and if you delay Social Security to 70, even that income is absent for a few years. This window creates the most bracket capacity to convert at controlled rates.

The math compounds: every $100,000 you convert to Roth reduces your RMD base at 73 by $100,000 (plus forgone growth on that amount). With an IRS Uniform Lifetime Table divisor of 26.5 at age 73, each $100,000 removed from the IRA base reduces the annual RMD by $3,774. That reduction persists and grows as divisors shrink each year. A 5-year conversion program at $100,000/year removes roughly $500,000–$600,000 in RMDs you'd otherwise have taken at higher brackets.

Post-RMD years: conversions still work, RMD comes first

After RMD age, you must satisfy your annual required distribution before converting anything — the RMD itself cannot be rolled into a Roth. But once the RMD is taken, any remaining balance above the RMD is eligible for conversion. Bracket-filling conversions remain valid in post-RMD years, especially in income-light years (markets down, no large capital gains, medical expense deductions that offset other income).

Sources

1. IRS Revenue Procedure 2025-32 — 2026 Tax Inflation Adjustments. 2026 federal income tax brackets (10/12/22/24/32/35/37%); MFJ brackets: 22% top $211,400, 24% top $403,550; single: 22% top $105,700, 24% top $201,750. Standard deduction: $32,200 MFJ / $16,100 single. Additional standard deduction for age 65+: $1,650/spouse MFJ, $2,050 single.
2. CMS 2026 Medicare Parts B Premiums and Deductibles Fact Sheet. 2026 IRMAA thresholds (based on 2024 MAGI): Tier 1 triggers above $109,000 single / $218,000 MFJ. Part B base premium $185.00/month; Tier 1 surcharge $70.30/month/person. Two-year lookback: 2026 income drives 2028 premiums.
3. IRS — 2026 Tax Adjustments Including One Big Beautiful Bill Amendments. Senior deduction: $6,000 per qualifying taxpayer age 65+, available to both standard-deduction filers and itemizers. Phases out above $150,000 MAGI for joint filers. Permanent provision under OBBBA.
4. IRS Publication 590-B — Distributions from Individual Retirement Arrangements. IRS Uniform Lifetime Table divisors for RMD projection (post-2022 update per T.D. 9930): age 73 = 26.5, age 74 = 25.5, age 75 = 24.6. SECURE 2.0 RMD start ages: 73 (born 1951–1959), 75 (born 1960 or later).

Tax brackets and standard deduction verified against IRS Rev. Proc. 2025-32. IRMAA thresholds verified against CMS 2026 fact sheet. Calculator uses simplified assumptions — does not model Social Security provisional income formula, OBBBA senior deduction phaseout, state taxes, or alternative minimum tax. Confirm values at irs.gov before making conversion decisions.