The 8-Year Window That Turns a $1.6 Million 401(k) Into a $310,000 Roth and a Bigger Social Security Check at 70

The decision appears constantly on retirement forums: a single filer in their early sixties, recently retired, with a seven-figure traditional 401(k), trying to figure out what to do during the gap between the last paycheck and the first Social Security check. The standard answer is “convert some to Roth.” The better answer is to treat the gap as a one-time engineering problem with a precise dollar size.

Take a 62-year-old who just retired with $1.6 million in a traditional 401(k), $250,000 in a taxable brokerage, and a plan to claim Social Security at 70. Wages are zero. Social Security is eight years away. Required minimum distributions are eleven years away. Nothing else is pushing taxable income up. That is the most valuable Roth conversion window a single filer will ever see, and it closes the moment Social Security turns on.

Fill the 22% Bracket, Stop Exactly There

The 2026 single-filer brackets, as adjusted under the One, Big, Beautiful Bill, run 12% on income over $12,400, 22% over $50,400, and 24% over $105,700. The standard deduction for a single filer under 65 is $16,100. That last number matters because every dollar of conversion below it is sheltered outright.

The mechanic: convert $105,700 from the 401(k) to a Roth IRA each year, which fills the 22% bracket to the penny and stops one dollar short of 24%. Run the bracket math on a single year and the federal tax comes to roughly $14,400. Repeat for eight years. Total gross converted: $845,600. Total federal tax: about $115,000, or an effective federal rate near 14% on the conversions.

Pay every dollar of that tax from the $250,000 brokerage account, not from the converted amount. The brokerage covers it comfortably and the full $105,700 lands in the Roth each year, compounding tax-free. At a 7% growth assumption, eight annual contributions of $105,700 are worth roughly $1.3 million by age 73, when RMDs begin. None of that pile generates a 1099-R for the rest of the retiree’s life.

The Two Side Effects Most People Miss

The first is the RMD shrink-ray. The original 401(k) goes from $1.6 million down to roughly $1.2 million after subtracting the conversions and adding growth. That is a permanently smaller base when the IRS Uniform Lifetime Table starts demanding withdrawals at 73, which means smaller RMDs, smaller ordinary income, and less pressure on the brackets in the seventies and eighties.

The second is IRMAA, the Medicare premium surcharge that uses a two-year lookback. Modified adjusted gross income at age 63 determines Part B and Part D premiums at 65. Heavy conversions in years one and two of the window are fine. Heavy conversions in year three (age 64) feed directly into the 65 enrollment year. The cleanest sequence: front-load bigger conversions at 62 and 63, then taper or pause from 64 through 66 to keep IRMAA tiers quiet during the first two Medicare years, then resume up to the 22% cap from 67 to 69.

The Social Security Side of the Same Trade

Claiming Social Security at 70 instead of full retirement age at 67 raises the benefit by roughly 24%, the result of delayed retirement credits. On these numbers, that is $42,000 a year at 70 versus $34,000 at 67, an extra $8,000 every year for life, indexed for COLA. The 401(k) and Social Security decisions are tightly linked. The conversion window only works because there is no W-2 and no Social Security competing for bracket space. Claiming at 67 would compress the last three conversion years into a much higher marginal rate and undo a meaningful slice of the strategy.

Three Things to Do Before Year-End

1. Run the conversion in December, not January. Waiting until the last month confirms the full year’s taxable income (interest, dividends, capital gains in the brokerage) and lets the bracket-fill number be exact. A January conversion guesses.
2. Pre-fund the tax outside the IRA. Keep at least $20,000 of the brokerage in something liquid (a short Treasury ladder or money market currently yielding north of the 3.75% fed funds upper bound) so the April tax bill never forces a withholding from the conversion itself.
3. Map the IRMAA calendar before the 64th birthday. The 2026 IRS brackets are public; the CMS IRMAA tiers are public; if the planned conversion at 63 pushes MAGI past the first surcharge tier, shrink that year’s conversion and recover the room at 67 or 68. The two-year lookback is the part that quietly costs people thousands in Medicare premiums they never see coming.