Retirees With Over $800,000 in a Traditional 401(k) Are Being Warned About This Social Security Clawback

A traditional 401(k) balance of $800,000 looks like a retirement success story, and at age 75, with Social Security coming in and the portfolio still intact, the numbers look manageable. In practice, the IRS and Medicare are already coordinating to tax a large share of it, including income the holder has not yet touched.

How the Provisional Income Trap Closes

The mechanism starts with required minimum distributions. A retiree with $800,000 in a traditional 401(k) taking RMDs of $35,000 per year at age 75 must fold that withdrawal into the calculation the IRS calls provisional income. Provisional income is not simply total income; it equals adjusted gross income (excluding Social Security) plus tax-exempt interest plus half of Social Security benefits. Add Social Security of $28,000, and provisional income reaches approximately $49,000, which is well past the threshold where 85% of benefits become taxable for single filers.

On $28,000 in benefits, roughly $23,800 becomes ordinary taxable income. Stack that on top of the $35,000 RMD and the retiree is reporting close to $59,000 in taxable income before accounting for any other sources. The Social Security payment itself has not been cut, but its real after-tax purchasing power has been quietly compressed. Because those taxability thresholds ($34,000 for single filers, $44,000 for joint filers) have not been inflation-adjusted since 1984, a growing share of retirees are pulled into the 85% band every year, even with modest benefit amounts.

The second layer arrives from Medicare. IRMAA surcharges are triggered above $109,000 in modified adjusted gross income for single filers in 2026, and the two-year lookback rule makes them especially hard to anticipate. Medicare uses income from two years prior to set premiums, so a retiree who crosses the threshold in 2026 faces surcharges in 2028 regardless of what income looks like at that point. The 2026 standard Part B premium is $202.90 per month. For those subject to IRMAA, total monthly Part B premiums range from $284.10 all the way to $689.90, with Part D surcharges adding a further $14.50 to $91.00 per month on top. The combined tax and premium impact can reduce the real value of Social Security by 20% to 30%.

Roth Conversions at 68 vs. Doing Nothing: What the Gap Years Cost

The difference between a retiree who ran Roth conversions during the gap years and one who did not is substantial. Consider two retirees, both starting at 68 with $800,000 in a traditional 401(k) and identical Social Security benefits of $28,000 annually.

Retiree A does nothing. By 75, the account has grown, and RMDs are mandatory. The $35,000 annual distribution stacks on top of Social Security, pushing benefits into the 85% taxability range and leaving the full balance exposed to even larger RMDs each year as the account compounds. The problem is self-reinforcing: larger balances generate larger RMDs, which in turn generate higher provisional income.

Retiree B executes Roth conversions during the window between retirement and RMD start, roughly ages 63 to 73 for most people today, when conversions can be executed at lower marginal rates. Over five years, shifting $50,000 per year into a Roth meaningfully shrinks the traditional balance. At 75, the RMD base is smaller, the annual distribution is lower, provisional income falls into a less costly Social Security taxability band, and IRMAA surcharges may not trigger at all.

Retiree B pays tax on those $50,000 annual conversions during the gap years, likely at the 22% federal rate. Retiree A pays tax on larger RMDs at 75 at the same or higher rate, absorbs the implicit cost of having most Social Security benefits pulled into ordinary income, and faces potential IRMAA surcharges that range from $81 to $487 per month for Part B alone in 2026, before adding any Part D exposure.

The Window Is Narrower Than It Looks

SECURE 2.0 pushed the RMD start age to 73, with a further increase to 75 for those born in 1960 or later. That creates a potential conversion window of roughly a decade for someone retiring at 63. The binding constraint is the IRMAA two-year lookback: conversions large enough to push MAGI above $109,000 will trigger Medicare surcharges two years later. The practical ceiling for most single filers is staying just under that threshold each year, which limits how aggressively the traditional balance can be drawn down.

With the 10-year Treasury yielding approximately 4.5% in mid-2026, the opportunity cost of moving money from a tax-deferred account into a Roth is real. That yield also means traditional balances are compounding faster, which pushes future RMDs higher. The math cuts both ways, but for those with $800,000 or more in a traditional account, the RMD pressure tends to dominate over time.

How to Size Conversions Around the IRMAA and Social Security Thresholds

1. The IRS Uniform Lifetime Table factor for a given age, when applied to the traditional 401(k) balance, yields the exact RMD. Adding that figure to projected Social Security benefits and comparing it with $34,000 (for single filers) or $44,000 (for joint filers) shows whether the 85% Social Security taxability zone applies. If the combined total crosses those thresholds, the conversion math becomes relevant.
2. MAGI above the 2026 IRMAA threshold of $109,000 will be reported to Medicare in 2028, meaning a large Roth conversion this year could affect premiums two years later. Sizing conversions to stay below that line prevents the surcharge from being triggered.
3. If combined income already exceeds the first IRMAA threshold of $109,000, the interaction between RMDs, Social Security taxation, and IRMAA surcharges becomes specific enough to individual balance and benefit amounts that a fee-only advisor’s one-time analysis can offset its cost through reduced taxes and premiums.

Editor’s note: This article was updated to reflect the current 10-year Treasury yield of approximately 4.5% (revised from the earlier reference of “around 4%”), the precise 2026 IRMAA Part B surcharge range of $81 to $487 per month, the 2026 standard Part B premium of $202.90 per month, and the fact that Social Security taxability thresholds have not been inflation-adjusted since 1984.

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