A single retiree turns 73 this year sitting on $1.6 million in a traditional 401(k) and a $3,000 monthly Social Security check. The first required minimum distribution is no longer a future problem. It lands in 2026, and the size of it reshapes everything that follows: the tax bracket, the Medicare premium two years out, and the room left for any Roth conversion still on the table.
The arithmetic is the easy part. The trap sits one threshold away.
The first RMD, in plain dollars
The IRS Uniform Lifetime Table assigns a divisor of 26.5 at age 73. Dividing the prior year-end balance of $1,600,000 by 26.5 produces a first RMD of $60,377, call it $60,400. That distribution is ordinary income, stacked directly on top of the Social Security check.
With provisional income this high, 85% of the $36,000 Social Security benefit becomes taxable, adding $30,600 to the pile. Gross taxable income before deductions comes to roughly $91,000. Subtract the 2026 single standard deduction of $16,100 and the $2,050 age-65 add-on, and taxable income lands near $72,850. That sits inside the 22% single bracket, which runs from $50,400 to $105,700 in 2026.
So far, predictable. The reader knew the RMD was coming.
Where the Medicare trap opens
Medicare IRMAA is calculated on modified adjusted gross income from two years earlier. The 2026 RMD shows up on the 2028 Part B premium. The first IRMAA threshold for a single filer is $109,000 in MAGI, and below it, the standard Part B premium is $202.90.
A MAGI near $91,000 clears the line with room to spare. But the cushion is roughly $18,000, and a single decision can vaporize it. A $25,000 Roth conversion in the same year. A large capital gain from rebalancing. A larger 401(k) balance next year as the divisor shrinks. Cross $109,000 and the first surcharge tier hits: an extra $81.20 per month, taking Part B to $284.10. A Part D surcharge of $14.50 rides along. None of that bill arrives until 2028, which is why most retirees never see it coming.
Two tiers up, at MAGI above $137,000, the Part B premium climbs to $405.80. The IRMAA cliff is the steepest tax in the code: one dollar over the threshold triggers the entire surcharge.
What actually moves the outcome
The 401(k) balance grows with the market, and at a 10-year Treasury yield near 4.5%, the bond sleeve alone produces meaningful reinvestment income. The RMD divisor falls every year. By age 80 the divisor is around 20, which on a $1.6 million-ish balance pushes the RMD past $80,000 and the MAGI past $109,000 without any help from conversions or gains.
Three levers matter:
- Qualified charitable distributions. The QCD allows a direct transfer from the IRA to a qualified charity, counts toward the RMD, and is excluded from taxable income. Suze Orman pegs the 2025 cap at roughly $105,000 per person, indexed up for 2026. For a charitably inclined retiree, a $20,000 QCD shaves the same amount off MAGI and creates real distance from the $109,000 line.
- Partial Roth conversions, but only inside the 22% bracket and only with the IRMAA cliff modeled. A conversion done at 70 or 71 before RMDs start is a different calculation than one done at 73 when the RMD has already filled most of the bracket. After 73, every dollar converted is a dollar of additional MAGI that can flip the Medicare surcharge.
- Timing of discretionary income. Capital gains, large Roth conversions, and one-off withdrawals belong in years where the combined MAGI stays clearly below $109,000, or in years where the surcharge is already triggered and the marginal cost of one more dollar is zero.
Action items before December 31
Pull the December 31, 2025 statement and divide by 26.5 to lock in the exact RMD figure. Project full-year MAGI including the taxable Social Security portion and any planned conversions, then measure the gap to $109,000. If the gap is under $15,000, route part of the RMD through a QCD before taking any personal distribution. The QCD only counts if it leaves the IRA before the cash does.
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