Roth Conversions Between 62 and 70: The $600,000 Tax Hack That Could Save You Tens of Thousands

A 64-year-old retires in 2026 with $1.4 million in a traditional 401(k), a paid-off house, and Social Security he plans to claim at 70. He posted the question on a retirement forum that gets asked dozens of times a month: should he leave the money alone and let it compound, or start pulling some out before he has to? The answer almost always points to the same eight-year stretch.

The window between 62 and 70 is the only stretch of life when most retirees control their taxable income with precision. Wages have stopped. Social Security has not started. Required minimum distributions from the 401(k) do not begin until age 73 under SECURE 2.0. Every dollar of income in those years is voluntary, making it the single best opportunity to reshape a six- or seven-figure 401(k) before the IRS forces the issue.

The Bracket You Get to Fill on Purpose

For a married couple filing jointly in 2026, the 12% federal bracket runs up to $100,800 of taxable income, and the 22% bracket runs to $211,400. A couple both over 65 with no wages claims a standard deduction that, with the new senior bonus, can reach roughly $46,700. That means the first $147,500 of gross income before Social Security falls inside the 12% bracket.

On a $1.4 million traditional balance, converting $100,000 a year to a Roth from 64 to 70 moves $600,000 out of the pre-tax pile at a blended federal cost in the low teens. The same dollars pulled after 73, on top of Social Security and a full RMD, frequently land in the 22% or 24% bracket. The difference on $600,000 of lifetime conversions is real money: ten percentage points of tax on each dollar moved at the right time.

The IRMAA Cliff That Punishes Sloppy Conversions

The trap in this strategy is Medicare. IRMAA uses a two-year lookback, so 2026 income drives 2028 premiums. For a couple, MAGI above $218,000 triggers Tier 1, adding $2,297 a year in combined Part B and Part D surcharges. Crossing $274,000 jumps the surcharge to $5,772, and $342,000 pushes it to $9,240. These tiers are hard cliffs: one extra dollar of conversion can cost thousands.

The planning move is to size each conversion to land just under a bracket line. A 65-year-old couple already on Medicare with $40,000 of pension and dividend income has roughly $178,000 of headroom before the first IRMAA cliff. Convert $175,000, not $225,000. Bracket creep is a real risk if conversions get layered on top of inflation-adjusted Social Security in later years.

Why Delaying Social Security Magnifies the Window

Every year a retiree delays Social Security past full retirement age adds roughly 8% to the lifetime benefit, fully inflation-indexed. Claiming at 70 instead of 67 raises a $3,200 monthly benefit to about $3,968. That higher base is permanent and shifts more lifetime income into the tax-favored 85%-taxable-Social-Security structure rather than fully taxable IRA withdrawals.

The delay also keeps provisional income low during the conversion years. With Social Security off the books, a 67-year-old can run a clean $150,000 Roth conversion without dragging benefits into taxation. Start Social Security at 62 and that same conversion causes up to 85% of benefits to become taxable, which can push the effective marginal rate near 40% once IRMAA layers on.

The current environment helps. Ten-year Treasuries yield almost 5% and 30-year yields sit near 5%, which makes a partially de-risked Roth account productive while it grows tax-free. The Fed funds rate has eased from 4.5% a year ago to 3.75%, supporting equity valuations inside the converted balance.

Three Moves Before December 31

1. Pull a draft 2026 tax return now and calculate exactly how much room sits between projected MAGI and the $218,000 IRMAA cliff for joint filers. That figure is your conversion ceiling for the year.
2. If you are 60 to 63 and still earning, fund the SECURE 2.0 super catch-up of $11,250 on top of the $24,500 base, for a $35,750 total. Anyone who earned more than $150,000 in 2025 must route catch-ups to a Roth 401(k), which doubles as a conversion substitute.
3. File Form SSA-44 the year you retire if a one-time conversion pushes you over an IRMAA tier. The Social Security Administration will recalculate premiums based on actual lower income, sparing you the surcharge.

The 62-to-70 window closes once. Income that stays in a traditional 401(k) past 73 is no longer voluntary, and the tax cost compounds alongside the balance.