A 63-Year-Old’s $600,000 401(k) Roth Conversion Plan Saves Tens of Thousands in Taxes Before RMDs Hit

A 63-year-old couple with $1.5 million in a traditional 401(k) and no earned income has just entered the most valuable tax planning window of their lives. From now until age 73, when required minimum distributions begin, they control exactly how much taxable income they show each year. Most people fill that window with a handful of small IRA withdrawals and a delayed Social Security claim, then move on. That choice quietly costs tens of thousands of dollars in taxes that were never inevitable.

The core strategy here: convert roughly $75,000 a year from the traditional 401(k) to a Roth IRA over eight years, moving $600,000 in total. Done with precision, the tax bill on that $600,000 lands well below what the IRS will eventually extract once RMDs stack on top of two Social Security checks after age 73.

Filling the 12% and 22% Brackets on Purpose

For a married couple filing jointly in 2026, the standard deduction is $32,200. The 12% bracket runs to $100,800 of taxable income, and the 22% bracket extends to $211,400. A couple with no other income can convert about $133,000 and stay entirely inside the 12% bracket, or stretch to $243,600 and remain inside 22%.

Leave those brackets unused and let RMDs pile on top of two Social Security checks after age 73, and the picture changes sharply. The same couple often gets pushed into the 24% bracket, which kicks in at $211,400 for joint filers and runs to $403,550. Two percentage points sounds modest, but the surcharges that attach at that income level are more damaging than the rate difference alone suggests.

The IRMAA Trap Kills Aggressive Conversions

Medicare prices premiums using a two-year lookback on modified adjusted gross income. Convert too aggressively at 63 and the surcharge arrives at 65. The standard 2026 Part B premium is $202.90 per month for joint filers with MAGI at or below $218,000. Cross that line, and the first surcharge tier raises the monthly bill to $284.10. The tiers climb across five levels, with the top Part B premium reaching $689.90 per month — roughly $8,300 per person per year — at the highest income threshold.

This is precisely why spreading $600,000 across eight annual $75,000 conversions is superior to a single $300,000 move. Concentrate the income and IRMAA transforms a smart strategy into a five-figure annual penalty. Spread it carefully, and the surcharge either never triggers or hits at the lowest tier for just one year. Because IRMAA tiers operate as hard cliffs, being $10 over a boundary costs exactly the same as being $10,000 over.

Social Security Timing Sharpens the Math

Every year a benefit is delayed past full retirement age adds roughly 8% to the monthly check, up to age 70. Holding off on Social Security keeps taxable income low during the conversion years, creating more headroom under the bracket ceilings. It also locks in a permanently larger benefit, indexed each year by the COLA adjustment — 2.8% for 2026, confirmed by the Social Security Administration.

Once benefits start, up to 85% of Social Security becomes taxable when combined income crosses the relevant thresholds. Roth withdrawals, by contrast, do not count toward combined income. Every dollar sitting in a Roth at age 73 is a dollar that stays outside the provisional income formula that drags Social Security into taxation, a compounding advantage that grows larger with each passing year.

Why the Current Rate Environment Matters

The Federal Reserve has held its target range for the federal funds rate at 3.5% to 3.75% since December 2025, and the 10-year Treasury yield has climbed toward 4.8% — levels not seen since late 2023. Bond-heavy allocations inside a traditional 401(k) now throw off meaningful taxable interest at those rates, which compounds the future RMD problem. Shifting that bond sleeve into a Roth while it is generating higher income locks in tax-free compounding for the life of the account.

Inflation continues to nudge bracket ceilings higher each year. That annual adjustment gives each successive conversion slightly more room at the same marginal rate, making a multi-year sequence more efficient than a one-time lump conversion.

Three Actions Before Year-End

1. Set the ceiling, then convert to it. Add up projected pension income, part-time earnings, taxable dividends, and interest. Subtract that total from $243,600, which represents the top of the 22% bracket plus the joint standard deduction. The remainder is the maximum conversion this year without crossing into the 24% bracket.
2. Verify the IRMAA line before submitting the transfer. If MAGI will exceed $218,000 for joint filers, stop one dollar below the tier boundary. Two years from now, that discipline is worth roughly $1,000 per spouse in Part B and Part D surcharges avoided.
3. Convert in late fall rather than early in the year. By November, the year’s dividends, mutual fund capital gains distributions, and any wages are known quantities. Guessing in January routinely produces a five-figure overshoot into the next tax or IRMAA tier, an error that current law does not permit anyone to undo.

Editor’s note: This article was updated to correct the top-tier IRMAA annual cost per person, revised from “more than $6,900” to roughly $8,300, reflecting the confirmed 2026 top Part B monthly premium of $689.90. The 10-year Treasury yield was also updated to approximately 4.8%, reflecting current market levels as of early September 2026, and post-publication context on the Federal Reserve holding its rate target at 3.5% to 3.75% through mid-2026 was added.

Contact [email protected] for any questions or corrections.