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. 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 add that withdrawal to other income when calculating what the IRS calls provisional income. Add Social Security income of $28,000, and provisional income reaches approximately $49,000, which is well into the 85% Social Security taxability zone.

That means up to 85% of Social Security benefits are counted as ordinary taxable income. On $28,000 in benefits, roughly $23,800 becomes taxable. Combined with a $35,000 RMD, the retiree reports taxable income of close to $59,000 before accounting for other sources. The Social Security benefit has not been reduced in the account, but its real after-tax value has been quietly compressed.

The second layer arrives from Medicare. IRMAA surcharges are triggered above $109,000 in modified adjusted gross income, and the lookback rule makes them especially hard to avoid: Medicare uses income from two years prior to set premiums. A retiree who crosses that threshold in 2026 faces surcharges in 2028, regardless of what income looks like then. 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 larger RMDs each year as the account compounds.

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

The retiree who converted pays tax on those $50,000 annual conversions during the gap years, likely at the 22% federal rate. The retiree who did not convert pays tax on larger RMDs at 75 at the same or higher rate, plus the implicit tax of having most Social Security benefits pulled into ordinary income, plus potential IRMAA surcharges that can range from roughly $90 to nearly $500 per month per person, depending on the tier.

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 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 around 4%, the opportunity cost of moving money from a tax‑deferred account into a Roth is real. But that yield also means traditional balances are growing faster, which pushes future RMDs higher. Larger balances produce larger RMDs.

How to Size Conversions Around the IRMAA and Social Security Thresholds

1. The IRS Uniform Lifetime Table factor for a given age, when divided by 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 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 your 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.