The 401(k) to Roth Bracket Filling Strategy That Saves a $300,000 Earner Couple $145,000 in Taxes Over 8 Years

Quick Read

• $400,000 Roth conversion over 8 years at 12% saves $145,000 vs. forced 24% RMDs plus IRMAA surcharges.

• Executing conversions between 65-73, before RMDs, forces income into higher brackets and triggers Medicare premium penalties.

• Are you ahead, or behind on retirement? SmartAsset’s free tool can match you with a financial advisor in minutes to help you answer that today. Each advisor has been carefully vetted, and must act in your best interests. Don’t waste another minute; learn more here.

A 60-year-old couple pulling in $300,000 a year with $1.8 million in a traditional 401(k) is sitting on a problem most high earners do not see until it is too late. Every dollar in that account is a future tax liability, and the IRS gets to pick the bracket. The good news: there is a 13-year window between now and the first required minimum distribution at 73 where the couple decides what bracket those dollars come out in.

The strategy is bracket filling: converting traditional 401(k) money to Roth in the years when marginal rates are lowest, intentionally pushing taxable income up to the top of a chosen bracket and stopping there. Done right over eight post-retirement years, this couple saves roughly $145,000 in lifetime taxes.

Why the 65-to-73 Window Is the Highest-Leverage Period in Retirement

While both spouses are still working, household income lands them squarely in the 24% federal bracket. Conversions today are possible but expensive. The math changes the moment the paychecks stop.

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After retiring at 65, wages drop to zero. If Social Security is delayed to 67 or 70, the couple has a stretch of years with almost no taxable income on autopilot. The 2026 MFJ 22% bracket runs from $96,950 to $206,700 of taxable income, and the 12% bracket sits below it. With no wages, the couple can deliberately generate income by converting traditional 401(k) dollars to Roth, filling the 12% bracket each year before letting the 22% bracket touch a single dollar.

The Year-by-Year Math

Convert $50,000 per year for eight years, ages 65 to 73. That moves $400,000 out of the traditional 401(k) and into a Roth, where it grows tax-free forever and never triggers an RMD.

Tax cost at the 12% marginal rate: roughly $48,000 across the eight years.

The counterfactual is ugly. Leave that $400,000 in the 401(k), let it compound, and the IRS forces it out as RMDs starting at 73. Combined household income from Social Security, pensions, dividends, and mandatory distributions plausibly lands in the 24% marginal bracket, costing about $96,000 in federal tax on the same dollars. Stack on Medicare IRMAA surcharges of roughly $30,000 over the affected years, and the do-nothing path costs about $145,000 more than the bracket-filling path.

With core PCE running near 0.7% month over month and the 10-year Treasury at 4.47%, the bracket creep risk on future RMDs is real.

Three Details That Make or Break the Strategy

1. Pay the conversion tax from a taxable account, not the IRA. Withholding tax from the conversion itself shrinks the amount that lands in the Roth and erodes the entire benefit. A separate brokerage or savings account should fund the IRS check so the full $50,000 makes it across.

2. Watch the IRMAA two-year lookback. Once Medicare starts at 65, MAGI from two years prior sets the premium surcharge. Keep MAGI below the $218,000 MFJ tier-one IRMAA threshold in any conversion year, or accept the surcharge knowingly. A $50,000 conversion stacked on Social Security and dividends can quietly cross a tier line and add hundreds per month in Part B and D premiums per spouse.

3. Check the state tax angle before converting. A conversion done while living in a 9% state income tax jurisdiction is a different deal than one done after relocating to Florida, Texas, or Tennessee. Some retirees explicitly time their conversions to the year they establish residency in a no-income-tax state, which can add tens of thousands to the savings.

What to Do Before the Next Tax Year Closes

Run a projection of taxable income for ages 65 through 72 assuming Social Security is claimed at 67 or 70. Identify the gap between projected taxable income and the top of the 12% bracket. That gap is the annual conversion target.

Pull the current 401(k) plan document and confirm in-plan Roth conversions are allowed, or plan a rollover to an IRA at retirement to gain conversion flexibility. And because conversion strategy interacts with Social Security claiming, IRMAA tiers, and state residency in ways no calculator fully captures, a fee-only CFP or CPA who quotes a flat project fee is worth the spend on a portfolio this size.

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