A $2.2 Million 401(k) Could Cost You $30,000 in Medicare Surcharges Without This Five-Year Roth Conversion Window

"Assume the 401(k) earns 6% annually and grows to approximately $4.7 million by age 73. The first RMD, calculated against the IRS Uniform Lifetime Table, comes in at roughly $177,400. Add 85% of Social Security (about $35,700) and Modified Adjusted Gross Income lands near $213,000. That MAGI parks the retiree in IRMAA Tier 3 or Tier 4, the Medicare income brackets that surcharge Part B and Part D premiums. Combined Part B and Part D IRMAA at that level runs $4,200 to $5,300 per year."

"Across ages 73 to 85, that is $50,000 to $65,000 in cumulative surcharges, paid on top of regular Medicare premiums and ordinary income tax on every RMD dollar. The IRMAA calculation uses a two-year lookback, so income earned at 71 sets the premium at 73. Inflation makes this worse. Core PCE has climbed from 125.79 in May 2025 to 129.28 in March 2026, and CPI has risen from 320.8 in April 2025 to 333.0 in April 2026. Medicare premiums track that pressure, and IRMAA thresholds adjust on a lag."

"The fix is to drain the 401(k) deliberately during the years when IRMAA does not yet apply. Before 65, Medicare is not in the picture, so income can be lifted without triggering a p... The five-year conversion window is the most valuable real estate on the retirement calendar, and most retirees waste them. The decision to convert, or not convert, traditional dollars to a Roth between 60 and 64 will quietly determine whether Medicare costs an extra $30,000 or more across retirement."

A retiree with a traditional 401(k) and no Roth balance has a limited window before Medicare enrollment changes tax outcomes. If the 401(k) grows at 6% to about $4.7 million by age 73, the first required minimum distribution is estimated near $177,400. Adding 85% of Social Security yields a modified adjusted gross income around $213,000, which can place the retiree in higher IRMAA tiers. Higher IRMAA tiers increase Part B and Part D premiums by roughly $4,200 to $5,300 per year, totaling about $50,000 to $65,000 from ages 73 to 85. Because IRMAA uses a two-year lookback, income at 71 affects premiums at 73. Converting during ages 60 to 64 can reduce future taxable withdrawals and Medicare surcharges. Inflation and rising premium pressures can further increase the cost if thresholds are exceeded.

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