The 7-Year Window That Turns a $1.8 Million 401(k) Into a $400,000 Roth and a $1,200 Bigger Social Security Check
A 63-year-old couple can convert roughly $400,000 of a $1.8 million 401(k) into a Roth at blended 12% rates before RMDs force higher taxes at 73.
Delaying Social Security to 70 adds $1,200 per month permanently and keeps conversion income clean by avoiding 85% benefit taxation.
Spreading conversions across ages 63 to 70 sidesteps Medicare's IRMAA surcharge entirely, given that Roth withdrawals never count toward the two-year income lookback.
A 63-year-old couple with $1.8 million in a traditional 401(k) is about to walk into the most valuable seven years of their financial life. Between the day they stop working and the day required minimum distributions begin at 73, they control their taxable income almost to the dollar. Skip the window, and RMDs, Social Security, and Medicare surcharges collide in the mid-70s. Use it, and roughly $400,000 of that balance ends up in a Roth while the monthly Social Security check grows by a four-figure amount.
The setup is mechanical: no wages, no Social Security claimed yet, and no RMDs until 73. Taxable income collapses to whatever interest and dividends spill out of the brokerage account. That gap is the runway for a Roth conversion ladder.
The 2025 married-filing-jointly brackets tax the first $23,850 at 10%, income up to $96,950 at 12%, and income up to $206,700 at 22%. The top of the 22% bracket is the target. A couple with modest brokerage income can convert into a Roth each year at a blended rate in the low teens without breaching that ceiling, or push closer to $180,000 per year if they are willing to pay the top marginal rate on the top slice.
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Seven conversions of about $57,000 move roughly $400,000 into a Roth. The tax bill lands mostly in the 12% bracket during a stretch when the couple has no earned income and no benefit check stacking on top. Those dollars then grow tax-free forever, with no future RMDs on the Roth side.
Every year the ladder is skipped is a year the 401(k) compounds toward a larger forced distribution at 73, taxed at whatever rate Congress writes then. Those quiet years between the last paycheck and the first RMD may be the lowest tax rate this couple ever sees again, which is the whole subject of our free Roth window guide.
Under current rules, full retirement age is 67, and each year of early claiming reduces benefits by about 6.7%. Waiting past 67 works in reverse through delayed retirement credits, which in this scenario translates to an illustrative $1,200 per month permanent increase at 70, indexed to future cost-of-living adjustments.
The two levers compound each other. Claiming Social Security at 63 would pull benefits into taxable income and shrink the conversion runway, because provisional income at those levels makes up to 85% of the benefit taxable. Waiting keeps the window clean for conversions and lets the base benefit grow. The 2027 COLA is tracking toward 3.1%, so the delayed benefit is escalating off a higher base each year the couple holds off.
With the 10-year Treasury near 5%, a slice of the 401(k) can sit in a short T-bill ladder to fund living expenses during the conversion years. That prevents forced equity sales into a downturn and keeps ordinary income predictable enough to control the bracket ceiling.
Medicare's income-related monthly adjustment amount uses a two-year lookback. A retiree who postpones all conversions until 71 or 72 walks straight into IRMAA surcharges the moment Medicare starts pricing off that income. Spread across the 63-to-70 window, the same conversions happen well before Medicare enrollment matters, and Roth withdrawals afterward do not count toward IRMAA at all.
Core PCE at 130.27 in June, near the period high, is a reminder that waiting exposes the same dollars to whatever future rates Washington sets. Locking in today's brackets is the cheaper bet.
Model the conversion ceiling. Pull last year's return, subtract wages, and calculate exactly how much room sits between projected retirement income and the top of the 22% bracket at $206,700. That number is the annual conversion budget for each of the seven years.
Pull Social Security statements for both spouses. Confirm in writing the projected benefit at full retirement age of 67 and at 70 before committing to the delay. The gap between those two numbers is what the seven-year withdrawal load has to justify.
Set a hard IRMAA guardrail. If a conversion would push modified adjusted gross income above the first IRMAA threshold two years before Medicare enrollment, shrink the conversion. Paying 22% federal is acceptable; adding a Medicare surcharge on both spouses on top of that crosses the line.
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