Make Sure You Do This Before 65: The 401(k) Conversion Move That Saves Six Figures in Taxes

A 60-year-old couple sitting on $1.5 million in a traditional 401(k) faces a question with a real number attached. Do you pay roughly $39,200 in voluntary tax in 2026 to avoid an estimated $100,000 in lifetime tax later? The answer for most readers in this band is yes, and the reason rests on bracket mechanics you can see today.

Here is the setup. You retire at 65, defer Social Security to 70, and let RMDs hit at 75. Between 60 and 65 sits a five-year window where you control your taxable income almost completely. That window is the most valuable tax-planning real estate most Americans will ever own, and it closes quietly.

Why filling the 22% bracket is the move

For 2026, the 22% bracket for married filing jointly runs from $100,800 to $211,400, and the standard deduction is $32,200. A retired couple with modest interest income and no wages can convert about $178,000 from a traditional 401(k) to a Roth IRA and stop precisely at the top of the 22% bracket. The federal tax bill on that conversion lands near $39,200, paid from a taxable brokerage account so the full conversion lands in the Roth.

That is the cost. The savings show up in three places that compound on each other.

The tax cascade you avoid

Leave that $178,000 inside the traditional 401(k) and it keeps growing. At a roughly 4.5% 10-year Treasury yield as a fixed-income anchor and a balanced blend on top, a 6% blended return over 15 years roughly doubles the balance before RMDs even begin at 75. Every dollar of that growth is taxable when it comes out.

That distribution does not arrive in a vacuum. It stacks on Social Security, pushing up to 85% of benefits into taxable income. It stacks on Medicare income testing, where crossing the first IRMAA threshold adds Part B and Part D surcharges that can run several thousand dollars per couple per year, with a two-year lookback that surprises retirees who treat one big withdrawal as a one-time event. A 22% marginal bracket plus Social Security taxation plus an IRMAA tier routinely produces an effective marginal rate near 40%.

Paying 22% today to avoid roughly 40% later on the same dollars is the entire trade. On a $178,000 base growing for 15 years, the spread between paying tax now versus paying tax on the larger, stacked withdrawal later comfortably clears $100,000 over a typical retirement horizon. That estimate ignores the Roth’s tax-free growth after conversion, which makes the gap wider.

What changed in 2026 that sharpens the math

If you are still working at 60, the contribution side now matters more. The 2026 elective deferral limit is $24,500, and the super catch-up for ages 60 to 63 is $11,250, for a total of $35,750. If you earned more than $150,000 in FICA wages in 2025, the SECURE 2.0 rule forces that entire catch-up into a Roth 401(k). Your catch-up dollars are already going Roth whether you planned for it or not, which strengthens the case for converting traditional balances on top of that rather than leaving the bias in the wrong direction.

The macro picture also helps. The Fed has held the upper bound at 3.75% since December 10, 2025, after cutting 0.75% over six months. A stable-to-easing rate environment gives you predictable cash yield to pay the conversion tax without selling growth assets at a bad moment.

Three moves before December 31

1. Run a bracket-fill projection for 2026. Add your wages, interest, dividends, and any pension to estimate taxable income. The space between that figure and $211,400 (MFJ) is your conversion headroom at 22%. Convert to fill it, not beyond it.
2. Pay the tax from outside the 401(k). Using the converted dollars to cover the bill defeats the strategy by shrinking the Roth and triggering withholding before 59½. Pull from taxable brokerage or cash, and adjust quarterly estimated payments so you do not get hit with an underpayment penalty.
3. Recheck the math each year through 65. The five-year window between retirement and Social Security is the cleanest conversion runway you will get. If your combined income will cross the first IRMAA tier in the year a conversion lands, model that surcharge against the long-term savings before pulling the trigger. The boomer cohort’s average 401(k) balance is only $267,900, so a $1 million-plus balance is exactly the profile where this planning pays for itself many times over.

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