A 66-Year-Old Couple With $1.8 Million Discovered Their 401(k) Was Running Their Retirement in Reverse
Their advisor said the retirement plan was working. Their CPA said it was slowly detonating. Both were correct, and the collision between a growing 401(k) and the tax code is what this couple never saw coming.
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A couple in a recent Bogleheads thread described a familiar setup: both 66, both retired last year, roughly $1.8 million in a traditional 401(k), living on taxable brokerage cash and delaying Social Security to 70. Their advisor told them the plan was working. Their CPA told them the plan was slowly detonating.
Both people were right. The portfolio is doing exactly what it was built to do. The tax code is doing exactly what it was built to do. The two are working against each other, and the couple is paying for the collision.
Why the Balance Keeps Growing but the Plan Keeps Shrinking
Here is the mechanic almost nobody sees at 66. Required minimum distributions start at age 73 under SECURE 2.0. That gives this couple seven years of untouched compounding inside the 401(k). At a 6% to 7% return, a $1.8 million balance grows into roughly $2.7 to $2.9 million before the first RMD hits. The first-year divisor under the Uniform Lifetime Table is about 26.5, which turns that first distribution into something in the neighborhood of $105,000 to $110,000, forced out as ordinary income, every year, indexed to a shrinking divisor as they age.
Stack Social Security on top. Delaying to 70 for both spouses easily produces combined benefits of $70,000 to $90,000 a year. Add the RMD and the couple is looking at gross taxable income around $180,000 to $200,000 in their mid-70s, in 2026 brackets that top the 22% band at $100,800 and push the 24% band through $211,400 for joint filers. That level of income makes 85% of Social Security taxable and drops the household into the first or second IRMAA tier, adding roughly $70 to $270 per person per month in Medicare Part B and Part D surcharges. The two-year lookback means a tax move at 71 shows up on the Medicare bill at 73.
That is the reverse gear. The bigger the 401(k) grows untouched, the higher the future forced withdrawal, the higher the future bracket, the higher the Social Security inclusion, and the higher the Medicare premium. Growth becomes a tax multiplier.
Seven Years to Rewrite the Tax Bill
The couple is sitting inside the most valuable tax window they will ever have. They have no wages, no Social Security yet, and no RMDs. The $32,200 standard deduction for joint filers in 2026 means they can recognize roughly $32,000 of income at 0%, fill the 10% and 12% brackets, and still stop the 22% bracket at $100,800 of taxable income. A Roth conversion of $120,000 to $130,000 a year, repeated for seven years, moves close to $900,000 out of the 401(k) at a 12% to 22% blended rate (we sized up this exact gap between the last paycheck and the first RMD in a free Roth Window guide). The same dollars distributed as RMDs at 75 come out at 24% plus IRMAA plus Social Security drag, an effective rate that lands closer to 40%.
The bond side matters here too. With the 10-year Treasury at 4.78% and the 2-year at 4.37%, the couple can hold conversion tax reserves in short Treasuries without giving up much yield, and a fed funds upper bound of 3.75% keeps money-market cash productive while they stage transfers.
What to Do Before Age 73
- Model the ceiling first, then size the conversion to fit. Work backward from the $100,800 top of the 22% bracket and the first IRMAA tier. Convert up to whichever ceiling is lower each year. That single decision usually beats any investment change available inside the plan.
- Watch the two-year Medicare shadow. Conversions at 66 land on Medicare premiums at 68. Build the IRMAA surcharge into the conversion math or you will overshoot and pay for it twice.
- Line up QCDs for age 70.5. Qualified charitable distributions can move up to $108,000 per spouse in 2026 straight from an IRA to charity, bypassing the RMD and the AGI entirely. Any charitable giving they already plan to do should be routed this way once eligible.
With the 2027 Social Security COLA tracking near 3.1% and average household spending at $78,535, this couple has plenty of money. What they lack is tax runway. Seven years is the whole game.
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Jake has been been working in financial media for almost 15 years. He focuses on all things personal finance for 24/7 Wall St. with high hopes to educate and entertain. Most recently, Jake spent 12 years working various roles at The Motley Fool. He started copy editing fool.com content, worked on premium and marketing campaigns, and helped launch The Ascent, a personal finance brand.
His work has been featured on platforms like MSN, Yahoo Finance, USA Today, and more. He's written about credit cards, social security, ETFs, savings accounts, and just about anything else you can imagine when thinking about money. Jake love to cook, play golf, and tell people he's never had a cavity. (It's true!)
