The Roth conversion window that closes faster than most 401(k) savers expect

Reddit's r/Fire is full of savers in their early 60s with seven-figure traditional 401(k) balances asking whether they should aggressively convert to Roth before required minimum distributions start. The answer, after running the numbers, is yes, and the window closes faster than most expect.

Consider this 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 your balance keeps compounding while 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 if you never contribute another dollar. 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, with the required percentage climbing every year after.

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

The Medicare Surcharge Most People Do Not See Coming

Medicare uses a two-year lookback. The income you report at 71 determines what you pay at 73. In 2026, the first IRMAA tier hits at $109,000 modified adjusted gross income for singles and $218,000 for married couples filing jointly, with the standard Part B premium around $203 per person per month before surcharges. Cross that first threshold and Part B and Part D surcharges together add roughly $1,150 per person per year. The top tier adds close to $7,000 per person. 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 rate, plus the Social Security taxation hit, plus IRMAA. A retiree nominally in the 22% bracket can face an effective marginal rate near 40% on the last dollar of an RMD.

Why 62 to 72 Is the Conversion Sweet Spot

Between retirement and RMD start, you control your taxable income almost completely. If you delay Social Security to 70, the years from 62 to 69 are especially clean. 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 matters.

Converting $1 million to $1.5 million over eight years costs 22% in tax. Doing nothing lets the balance grow into a forced 24% to 32% withdrawal at 73, plus IRMAA, plus Social Security taxation. The math almost always favors paying tax voluntarily now at a rate you choose.

Two factors sharpen the case right now. Core PCE sits at the 90th percentile of its 12-month range, and the 10-year Treasury is around 4.6%, near the top of its 12-month band. Sticky inflation and elevated rates suggest today's brackets, historically low by post-1980 standards, are unlikely to become more generous.

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. If you are charitably inclined, QCDs after 70.5 satisfy RMDs without adding to MAGI, complementing conversions rather than replacing them.

The reader who leaves a $1.5 million traditional balance untouched from 62 to 73 is locking in the highest lifetime tax bill the IRS allows. With the 2026 Social Security COLA at 2.8% quietly pushing future benefits higher and bracket thresholds barely keeping pace, the conversion decision made at 62 is worth far more than the same decision made at 68.

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