The Roth Conversion Window That Closes Faster Than Most 401(k) Savers Expect

Reddit’s r/FIRE draws a steady stream of savers in their early 60s, seven-figure traditional 401(k) balances in hand, all asking the same question: should they convert aggressively to Roth before required minimum distributions begin? After running the numbers, the answer is almost always yes. The harder part is accepting how quickly the window closes.

Consider the scenario: You are 62, semi-retired, with roughly $1.5 million in a traditional 401(k). RMDs do not begin until age 73 under SECURE 2.0. That runway is shorter than it looks, because the balance keeps compounding the entire time you wait.

What the Traditional 401(k) Looks Like at 73

At a modest 6% annual return, $1.5 million at age 62 grows to roughly $2.85 million by age 73, even without another dollar contributed. The first RMD divisor on the IRS Uniform Lifetime Table at age 73 is 26.5, producing a mandatory withdrawal of about $107,000 in year one. That required percentage climbs every subsequent year, compounding the tax exposure.

Layer that on top of two Social Security checks and any pension or dividend income, and a married couple filing jointly lands comfortably inside the 24% federal bracket, which begins at $211,400 of taxable income in 2026. That same RMD also triggers taxation on up to 85% of Social Security benefits and can push modified adjusted gross income well past the IRMAA cliffs.

One piece of good news: the One Big Beautiful Bill Act, signed in July 2025, made the TCJA tax brackets permanent and added a $6,000 senior deduction per qualifying person (phasing out above $150,000 MAGI). For couples in the conversion sweet spot, that deduction widens the low-bracket window slightly in the early retirement years.

The Medicare Surcharge Most People Do Not See Coming

Medicare uses a two-year lookback. The income reported at 71 determines what you pay at 73. In 2026, the first IRMAA tier triggers at $109,000 MAGI for singles and $218,000 for married couples filing jointly. The standard Part B premium is $202.90 per person per month before any surcharge. Cross the first threshold and combined Part B and Part D surcharges add roughly $1,150 per person per year. The top tier adds close to $7,000 per person annually. For a couple, a single bad-planning year can cost $14,000 in Medicare premiums two years later.

The combination is what advisors call the tax cascade: ordinary income tax, plus the Social Security taxation hit, plus IRMAA surcharges arriving as a cliff, not a gradual slope. A retiree nominally in the 22% bracket can face an effective marginal rate approaching 40% on the last dollar of an RMD. The IRMAA cliff structure means exceeding a threshold by even $1 triggers the full surcharge for that tier.

Why 62 to 72 Is the Conversion Sweet Spot

Between retirement and the RMD start date, taxable income is almost entirely under your control. Delaying Social Security to 70 makes the window even cleaner: the years from 62 to 69 carry little mandatory income. A married couple with the 2026 standard deduction of $32,200 can realize about $129,000 of Roth conversion income and stay inside the 12% bracket. Push to the top of the 22% bracket and you can convert closer to $239,000 per year, while remaining below the first IRMAA threshold in the years Medicare premiums are being set.

Converting $1 million to $1.5 million over eight years costs roughly 22% in tax. Leaving that balance untouched lets it grow into a forced 24% to 32% withdrawal at 73, with IRMAA and Social Security taxation piled on top. The math almost always favors paying voluntarily now at a rate you control.

Two factors sharpen the case in 2026. The 10-year Treasury yield has climbed to around 5%, the highest level since 2007, well above the “around 4.6%” range cited earlier in the year. Elevated long-term rates reflect persistent inflation expectations and growing fiscal pressure, both forces that make today’s historically low post-1980 brackets unlikely to become more generous. With the TCJA brackets now made permanent by the OBBBA, the rate structure is locked in for the foreseeable future, but the brackets themselves do not prevent Congress from adding new levies or phasing out deductions.

Three Moves to Make This Year

1. Model the 11-year window, not one year. Build a spreadsheet from age 62 to 72 showing projected balance, planned conversion amount, resulting taxable income, and the IRMAA MAGI two years forward. The goal is filling the 22% bracket without breaching $218,000 MAGI once Medicare enrollment starts at 65.
2. Pay the conversion tax from a taxable brokerage or cash reserve. Using pre-tax dollars to cover the tax bill defeats the strategy. Keep a cash or brokerage reserve equal to at least two years of expected conversion taxes before you start.
3. Plan the QCD handoff at 70.5. The 2026 qualified charitable distribution limit is $111,000 per person. For charitably inclined retirees, QCDs after 70.5 satisfy RMDs without adding to MAGI, complementing Roth conversions rather than replacing them.

The saver who leaves a $1.5 million traditional balance untouched from 62 to 73 is effectively choosing the highest lifetime tax bill the code allows. With the 2026 Social Security COLA confirmed at 2.8%, future benefits will keep rising while bracket thresholds can barely keep pace with inflation. The conversion decision made at 62 is worth substantially more than the identical decision deferred to 68.

Editor’s note: This article has been updated to reflect the 10-year Treasury yield rising to approximately 5% in September 2026, the highest level since 2007, and to include context on the One Big Beautiful Bill Act’s permanent extension of TCJA tax brackets and its new $6,000 senior deduction. The 2026 Medicare Part B standard premium has also been corrected to the exact figure of $202.90 per month.

Contact [email protected] for any questions or corrections.