Roth Conversion Strategy for $1M–$5M Investors: 2026 Bracket Optimization Guide

The backdoor Roth gets new money in. Roth conversions deal with the $300K–$2M already sitting in pre-tax IRAs and 401(k)s you've been accumulating for 20 years. Done strategically across a decade, annual conversions can eliminate six figures of forced RMD tax exposure — or create IRMAA surcharges if timed carelessly.

The core logic: pay 22% now or 32% later

A Roth conversion is a voluntary taxable event. You pull money from a traditional IRA or 401(k), pay income tax on it now at your current marginal rate, and it grows tax-free forever. The math works when your current marginal rate is lower than your expected future rate at distribution time.

For $1M–$5M investors in their late 50s to early 70s, this window typically exists. You may be in the 22%–24% bracket today. But once Required Minimum Distributions begin at 73 or 75 (SECURE 2.0), plus Social Security income (85% of which is includible above the base threshold), your taxable income in retirement can push you into the 32%–35% bracket — even without a salary. Converting at 22%–24% now to avoid forced distributions at 32%–35% later is the trade.

5 factors that determine your conversion window

1. Current vs. future marginal rate. The key rate is the marginal rate at the margin — what rate applies to the last dollar converted. In 2026, the 22% bracket tops out at $211,400 of taxable income for married filers ($105,700 for single).¹ If you're in the 22% bracket today and expect distributions to push you into 32%, every dollar converted now saves 10 cents on the dollar. Spread conversions annually to fill the 22%–24% bracket each year.
2. Years before RMDs begin. SECURE 2.0 (§ 107) sets RMD age at 73 for those born 1951–1959, and 75 for those born 1960 or later.² More years before RMDs means more time to convert at controlled rates before distributions become mandatory. An investor who retires at 60 with a $1M IRA has 13–15 years of conversion runway. A 70-year-old has 3–5.
3. IRMAA exposure. Large conversions inflate MAGI, which can trigger Medicare Part B and Part D surcharges. In 2026, the first IRMAA tier activates above $109,000 MAGI for single filers ($218,000 for married).³ That first-tier surcharge adds $81.20/month per person — $974/year — on top of the $202.90 base Part B premium. A large conversion year that crosses multiple tiers can cost $3,000–$6,000 extra per person.
4. ACA premium cliff (pre-Medicare). If you've retired before 65 and purchase health insurance on the Marketplace, your premium tax credit phases out above 400% of the federal poverty level (~$60K for a single person in 2026). A large Roth conversion in a year you're relying on ACA credits can eliminate $8,000–$15,000 in subsidies — more than the bracket savings. These years require careful planning.
5. Estate planning benefit. Roth IRAs pass to heirs without a lifetime RMD requirement on the account owner, and without ordinary-income tax during the beneficiary's 10-year distribution window. Under IRS T.D. 10001 (July 2024), inherited traditional IRAs require annual RMDs in years 1–9 if the decedent had passed their Required Beginning Date, plus full liquidation by year 10.⁴ Heirs liquidating a $500K inherited traditional IRA over 10 years will owe ordinary income tax on each distribution at their own marginal rate. A $500K inherited Roth IRA grows tax-free during the same window.

Roth Conversion Sweet Spot Finder — 2026

Enter your filing status, annual income (before any conversion), and IRA balance to see how much you can convert this year before crossing into the next tax bracket.

Uses 2026 federal tax brackets (IRS Rev. Proc. 2025-32) and 2026 standard deduction ($32,200 MFJ / $16,100 single). Assumes standard deduction applies. Does not account for state income tax, itemized deductions, or AMT. IRMAA is Medicare-specific and applies from age 65.

Who should convert most aggressively

• Gap years: retired but not yet drawing Social Security. This is the best conversion window. No W-2 income, no RMDs yet, no Social Security MAGI. A married couple with $0 in other income can convert up to $98,900 of taxable income in the 12% bracket for roughly $8,944 in federal tax — locking in a 12% effective rate on amounts that would otherwise be taxed at 22%–32% when RMDs begin.
• Ages 60–72 with large traditional balances. 10–13 years of systematic annual conversions can dramatically shrink the balance subject to RMDs. A $1.2M traditional IRA that grows at 7% will generate approximately $75K/year in RMDs starting at age 75 (Uniform Lifetime Table divisor ~26.5 at 75). On top of Social Security, that's a 32%–35% bracket problem. Annual conversions in the 22%–24% bracket over 13 years can eliminate most of that exposure.
• Surviving spouses facing the "widow penalty." A married couple filing jointly in the 22% bracket becomes a single filer when the first spouse dies — same income level, dramatically narrower bracket. In 2026, the single-filer 22% bracket tops out at $105,700 vs. $211,400 for MFJ. Systematic conversions before that event equalize the situation at a lower rate.
• Business owners in a transition year. Selling a business segment, a sabbatical, or an unusually large charitable contribution (DAF or direct gift offsetting income) may create a temporary bracket dip. A one-time large conversion at a low effective rate while the window exists can compress years of future conversion work into a single tax year.

The IRMAA trap: one careless conversion year costs $3,900+

Medicare Part B and Part D surcharges are based on your MAGI from two years prior. A $250,000 MAGI in 2026 affects your 2028 Medicare premiums. The surcharge jumps in steps — not gradually — so crossing a threshold by $1 costs the same as crossing it by $50,000. The table below shows 2026 Part B monthly premiums. Annual added cost is per person (a married couple doubles it).

† Intermediate 2026 IRMAA thresholds are approximate (inflation-adjusted from 2025 CMS data). Confirm exact thresholds at Medicare.gov before planning a conversion. Source for base premium and surcharge range: CMS 2026 Medicare Parts A & B Premiums and Deductibles.³

Practical rule: target conversions to stop just below each IRMAA tier threshold. If you're at $190,000 MAGI (married) and considering a $50,000 conversion, the full amount stays below the $218,000 tier — clean. If you're at $200,000 and considering the same $50,000, you cross the tier at $218,000, and $32,000 of the conversion triggers the surcharge. The tax savings on that $32,000 at 22% is $7,040; the two-year Medicare hit at tier 1 is $1,948 for a couple. Still worth it — but barely, and only if it stays at tier 1.

How to execute a Roth conversion in 5 steps

1. Project your full-year MAGI. Add all expected income: W-2, self-employment, interest, dividends, rental income, pension, and 85% of Social Security if applicable (includible above the $44,000 combined-income threshold for MFJ). This is your baseline before conversion.
2. Determine your target conversion amount. Use the calculator above to find how much fills your current bracket without crossing into the next bracket or an IRMAA tier threshold. Choose the more conservative limit if they conflict.
3. Request a partial Roth conversion from your IRA custodian. Vanguard, Fidelity, and Schwab all support online partial conversions. Specify the dollar amount. The custodian will ask about withholding — decline it.
4. Pay the tax from outside funds — not the IRA. Using IRA funds to pay the tax bill is itself a taxable distribution and reduces the amount you converted to Roth. Write the check to the IRS from your taxable account or bank account.
5. File IRS Form 8606. Required for any IRA conversion. Tracks basis and documents the transaction. Do not skip this — missing Form 8606 in a prior year creates a double-taxation problem on future withdrawals.⁵

When NOT to convert

• You're still in your peak earning years. Converting at 32%–35% to avoid a hypothetical 24% future rate is backward. The math only works going from lower to higher expected rates. Wait for a lower bracket year.
• You need the cash. Roth conversions are irreversible (the recharacterization option was eliminated by TCJA in 2018). If the market drops 30% after you convert, you still owe the tax on the pre-drop value. Only convert balances you won't need for 5+ years.
• Your heirs are in a low bracket. If beneficiaries earn $40K/year and would take inherited distributions at 12%–22%, the tax on their inherited traditional IRA may be lower than what you'd pay at 24%+ to convert. Model who pays less — you at conversion or them at distribution.
• State taxes change the math. In California (13.3% top rate) or New York (10.9%), large conversion income can push the combined federal+state marginal rate above 35% — potentially higher than your expected future federal-only rate in a lower-tax state in retirement. Factor state taxes before acting.

Sources

1. IRS Rev. Proc. 2025-32 — 2026 federal income tax bracket thresholds and standard deduction amounts: irs.gov/pub/irs-drop/rp-25-32.pdf. Standard deduction 2026: $32,200 MFJ / $16,100 single. 22% bracket top: $211,400 MFJ / $105,700 single. 24% bracket top: $403,550 MFJ / $201,775 single.
2. SECURE 2.0 Act of 2022, § 107 (Public Law 117-328) — RMD age 73 for those born 1951–1959; age 75 for those born 1960 or later. Eliminates Roth 401(k) lifetime RMDs starting 2024 (§ 325): Congress.gov.
3. CMS 2026 Medicare Parts A & B Premiums and Deductibles Fact Sheet — Part B base premium $202.90/month; IRMAA Part B surcharge range $81.20–$487.00/month; first-tier threshold $109,000 single / $218,000 MFJ (MAGI-based, two-year lookback): cms.gov.
4. T.D. 10001 (July 2024) — IRS final regulations on inherited IRA annual RMD requirements when the original account owner died on or after their Required Beginning Date; codifies the "at least as rapidly" rule and enforces annual distributions in years 1–9 with full liquidation by year 10: irs.gov.
5. IRS Form 8606 — Nondeductible IRAs (required for any traditional IRA contribution made without a deduction, and for all Roth conversions): irs.gov/forms-pubs/about-form-8606.

Tax bracket values verified against 2026 rules via IRS Rev. Proc. 2025-32. IRMAA first-tier threshold per CMS 2026 fact sheet. Intermediate IRMAA tiers are inflation-adjusted estimates from 2025 data — confirm at Medicare.gov before executing large conversions. Values current as of April 2026.