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 can look like a retirement finish line. At age 75, with Social Security arriving monthly and the portfolio still intact, the numbers feel comfortable. What rarely shows up in that picture is a parallel system in which the IRS and Medicare are quietly taxing a large share of that balance, 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 a formula the IRS calls provisional income. That figure equals adjusted gross income (excluding Social Security) plus tax-exempt interest plus half of Social Security benefits. Add Social Security of $28,000 to the example, and provisional income reaches approximately $49,000, 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 any other income sources enter the picture. The Social Security check itself has not been reduced by one dollar, but its real purchasing power shrinks substantially once taxes are applied. The thresholds that trigger this outcome ($34,000 for single filers, $44,000 for joint filers) have been frozen since Congress set the upper 85% tier in 1993. Because they are not indexed for inflation, a growing share of retirees get pulled into the highest taxability band each year, even with modest benefit amounts.

One recent development offers limited relief. The One Big Beautiful Bill Act, signed into law on July 4, 2025, created a temporary above-the-line deduction of $6,000 per qualifying individual age 65 or older, or $12,000 for a couple where both spouses qualify, covering tax years 2025 through 2028. Worth noting: President Trump had campaigned on eliminating Social Security income taxes entirely, but that provision did not survive Senate reconciliation rules in the final law. The deduction that did pass is available whether the filer itemizes or takes the standard deduction, and it reduces taxable income after the taxable benefit amount is calculated, limiting the federal tax bite for many middle-income retirees. The benefit phases out beginning at $75,000 in modified adjusted gross income for single filers ($150,000 for joint filers) and disappears entirely at $175,000 for single filers and $250,000 for joint filers, providing little relief to retirees already deep in IRMAA territory.

The second layer of exposure arrives from Medicare. IRMAA surcharges kick in above $109,000 in modified adjusted gross income for single filers in 2026, and the system functions as a cliff rather than a phase-in. Cross a tier by even a single dollar and the full surcharge applies for the entire year. The two-year lookback makes this especially difficult to manage: Medicare uses income from two years prior to set premiums, so a retiree who crosses the threshold in 2026 will face 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, the total monthly Part B premium ranges from $284.10 to $689.90, with Part D surcharges adding a further $14.50 to $91.00 per month on top. Taken together, the 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 completed Roth conversions during the gap years and one who took no action is substantial. Consider two retirees, both entering retirement at 68 with $800,000 in a traditional 401(k) and identical Social Security benefits of $28,000 per year.

Retiree A takes no action. By 75, the account has grown and RMDs are mandatory. The $35,000 annual distribution stacks directly on top of Social Security, pushing benefits into the 85% taxability range and leaving the full remaining balance exposed to even larger RMDs each year as the account continues to compound. The feedback loop works against the retiree: larger balances produce larger RMDs, which raise provisional income, which triggers greater Social Security taxation, which further erodes purchasing power.

Retiree B executes Roth conversions during the window between retirement and the RMD start date, roughly ages 63 to 73 for those born between 1951 and 1959, when conversions can be completed at lower marginal rates. Over five years, shifting $50,000 annually into a Roth meaningfully shrinks the traditional balance. By 75, the RMD base is smaller, the annual distribution is lower, provisional income lands in a less costly Social Security taxability band, and IRMAA surcharges may never 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 faces tax on larger RMDs at 75 at the same or a higher rate, absorbs the implicit cost of having most Social Security benefits treated as ordinary income, and carries the risk of IRMAA surcharges ranging from $81.20 to $487.00 per month for Part B alone in 2026, before any Part D exposure is added.

The Window Is Narrower Than It Looks

SECURE 2.0 pushed the RMD start age to 73 for those born between 1951 and 1959, with a further increase to 75 for those born in 1960 or later, effective January 1, 2033. That structure creates a conversion window of roughly a decade for someone retiring at 63. The binding constraint, however, remains the IRMAA two-year lookback: conversions large enough to push MAGI above $109,000 will trigger Medicare surcharges two years later. For most single filers, the practical ceiling is staying just under that threshold each year, which limits how aggressively a traditional balance can be drawn down.

With the 10-year Treasury yielding approximately 4.8% to 5% as of mid-September 2026, the opportunity cost of moving money from a tax-deferred account into a Roth is real and rising. That same yield means traditional balances are compounding faster, pushing future RMDs higher still. 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 over time.

Editor’s note: This pass updated the 10-year Treasury yield reference from approximately 4.7% to approximately 4.8% to 5%, reflecting data as of mid-September 2026, and added context that President Trump’s proposal to eliminate Social Security income taxes entirely was not included in the final One Big Beautiful Bill Act.

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