The $3.2 Million 401(k) Tax Bomb That Early Retirees Can Dodge With Strategic Conversions

A 58-year-old husband with $1.9 million in his traditional 401(k) and his 59-year-old wife with $1.3 million in hers retired this spring. Add $850,000 in a taxable brokerage and $250,000 in cash, and the household has $4.3 million sitting across three tax buckets. They plan to claim Social Security at 70. The question is what to do with the 11 years between retirement and the first required minimum distribution.

The bracket-fill conversion ladder

The escape route is a 14-year Roth conversion ladder sized to the tax brackets. Left untouched, the $3.2 million traditional balance compounding for 14 years roughly doubles, and RMDs at 73 on a $6 million balance start above $225,000 a year, pushing the couple into the 24% to 32% federal brackets, making 85% of Social Security taxable, and triggering IRMAA surcharges of $70 to $400+ per person per month.

The 2026 standard deduction for a married couple filing jointly is $30,000. The 12% bracket runs to roughly $96,950 of taxable income, and the 22% bracket runs to roughly $206,700. With no wages, no Social Security yet, and brokerage income managed carefully, every dollar converted lands at the bottom of the stack.

The plan converts about $200,000 per year. The first roughly $67,000 fills the 12% bracket above the standard deduction. The next roughly $110,000 fills the 22% bracket. A small slice spills into 24%. The blended federal rate lands near 17%. Across 14 years, $2.8 million moves from traditional to Roth, and the IRS collects roughly $476,000 in conversion tax.

By 73, the traditional 401(k)s hold around $400,000. RMDs on that residual run well under the standard deduction once Social Security is layered in. Federal tax after 73 is effectively zero.

The IRMAA cliff that starts at 63

Medicare premiums in any given year are set from the tax return filed two years earlier. Conversions done at 63 and 64 set the IRMAA bill at 65 and 66, which is why heavy conversion years should finish before age 63.

For 2026, the first IRMAA tier hits MFJ modified adjusted gross income above roughly $212,000, with surcharges climbing in steps to over $5,000 per couple per year at the top tier. A $200,000 conversion plus brokerage interest can clear that threshold quietly.

The fix is to front-load. Heavy conversions at 59 through 62, when IRMAA does not apply yet, then dial back from 63 to 73 to keep MAGI under the first surcharge tier. The total dollars converted stay the same. The Medicare surcharge stays at zero.

Paying the tax without bleeding the Roth

The conversion tax has to come from somewhere outside the IRA, or the math collapses. The $850,000 brokerage is the funding source.

The 0% long-term capital gains bracket for MFJ in 2026 tops out around $96,700 of taxable income. That ceiling is already consumed by the conversion itself, so brokerage sales used to pay the tax will realize gains at 15%. The workaround: harvest the lowest-basis lots in the lightest conversion years, hold high-basis lots for the heavier years, and lean on the $250,000 cash reserve to bridge.

At today’s 4.4% on the 10-year Treasury and a Fed funds upper bound of almost 4%, the cash reserve earns enough to cover roughly two years of conversion tax without touching the brokerage at all. With core PCE sitting in the 90.9th percentile of its 12-month range, leaving that cash idle in checking is the more expensive mistake.

Three moves to make this quarter

1. Pull the IRS Uniform Lifetime Table and model RMDs at 73 on the do-nothing path against the $400,000 residual the ladder leaves behind. The gap between those two numbers is the case for converting.
2. Map every conversion year against the 2026 IRMAA tiers, not just the federal brackets. The first surcharge tier becomes the binding constraint from age 63 onward, and missing it by $1,000 of MAGI costs the same as missing it by $20,000.
3. Inventory the brokerage lot by lot before the first conversion clears. Pair high-basis sales with the heaviest conversion years to keep realized gains inside the 15% LTCG band, and reserve low-basis lots for years when total income drops.