Required minimum distributions (RMDs) at age 75 combine with Social Security to temporarily push earnings into areas where up to 85% of benefits are taxable, and Medicare’s IRMAA surcharges (ranging from $90 to $500+ monthly) are triggered by modified adjusted gross income above $109,000 in the two-year lookback, compressing the net after-tax value of Social Security by 20% to 30%.
Making a Roth conversion during retirement years (age 63) and initiating RMDs (ages 73-75) at the lower marginal tax rate can significantly reduce the traditional balance and reduce future RMDs, Social Security taxes, and IRMAA risk, although the two-year IRMAA lookout creates an effective cap at less than $109,000 in modified adjusted gross income per year.
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A traditional 401(k) balance of $800,000 looks like a retirement success story, and at age 75, with Social Security factored in and the investment portfolio intact, the numbers seem manageable. The IRS and Medicare have teamed up to tax a large portion of it, including income the owner hasn’t touched.
The mechanism starts with a required minimum distribution. A retiree with $800,000 in a traditional 401(k) who receives an RMD 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 comes to about $49,000, which is within the 85% Social Security taxable zone.
That means up to 85% of Social Security benefits count as ordinary taxable income. Above $28,000 in benefits, about $23,800 will be taxable. Combined with the RMD 35,000, the retiree reported nearly $59,000 in taxable income before accounting for other sources. Social Security benefits have not been reduced in the account, but their net after-tax value has been silently compressed.
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The second layer comes from Medicare. IRMAA surcharges are triggered above $109,000 in modified adjusted gross income, and lookback rules make them especially difficult to avoid: Medicare uses income from the previous two years when setting premiums. A retiree who crosses that threshold in 2026 will face surcharges in 2028, regardless of income then. The combined impact of taxes and premiums could reduce the real value of Social Security by 20% to 30%.
The difference between a retiree who makes a Roth conversion during their retirement years and one who does not is significant. Consider two retirees, both starting at age 68 with $800,000 in traditional 401(k)s and identical Social Security benefits of $28,000 annually.
Retiree A does nothing. By 75, the account had grown and RMDs were required. The $35,000 annual distribution is stacked on top of Social Security, pushing benefits into the 85% taxable range and leaving the entire balance exposed to larger RMDs each year as a pooled account.
Retiree B makes a Roth conversion between retirement and the start of RMDs, which for most readers is between ages 63 and 73, when the conversion can be done at a lower marginal rate. Over five years, transferring $50,000 per year into a Roth will significantly reduce the traditional balance. At age 75, base RMDs are smaller, annual distributions are lower, earnings temporarily fall into a lower Social Security tax bracket, and Medicare’s IRMAA surcharges may not be triggered.
The retiree who converted would pay taxes on those $50,000 annual conversions in the gap years, possibly at the 22% federal tax rate. Retirees who don’t convert will have to pay taxes on the larger RMDs at the same or higher rate of 75, plus the hidden tax of having most Social Security benefits rolled into ordinary income, plus potential IRMAA surcharges that can range from about $90 to nearly $500 per month per person, depending on tier.
SECURE 2.0 pushed the RMD starting age to 73, and increased it further to 75 for those born in 1960 or later. That creates a potential transition period of about a decade for someone retiring at age 63. The drawback is that IRMAA looks back two years: the transition is large enough to push MAGI upward 109,000 USD would trigger Medicare surcharges two years later. The actual ceiling for most single filers remains just below that threshold each year, which limits how sharply traditional balances decline.
With 10-year Treasuries yielding around 4%, the opportunity cost of moving money from a tax-deferred account to a Roth is real. But that yield also means traditional balances are growing faster, pushing future RMDs higher. Larger balances generate larger RMDs.
The IRS Uniform Life Table factor for a given age, when divided by the traditional 401(k) balance, will yield the correct RMD. Adding that number to expected Social Security benefits and comparing it to $34,000 (for single filers) or $44,000 (for joint filers) shows whether the 85% Social Security tax would apply. If the combined sum passes those thresholds, the conversion operation becomes appropriate.
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 the conversion below that line will prevent surcharges from being triggered.
If your combined income already exceeds the first IRMAA threshold of $109,000, the interaction between RMDs, Social Security taxes, and the IRMAA surcharge becomes specific enough to individual balances and benefit amounts that a fee-only advisor’s one-time analysis can offset the costs through tax and premium reductions.
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