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. Divide the prior year-end balance of $1,600,000 by 26.5 and the first RMD comes to $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 annual Social Security benefit becomes taxable, adding $30,600 to the pile. Gross income before deductions comes to roughly $91,000. Subtract the 2026 single standard deduction of $16,100 and the $2,050 additional deduction available to single filers age 65 and older, and taxable income lands near $72,850. That sits squarely inside the 22% single bracket, which runs from $50,400 to $105,700 in 2026.
One other wrinkle from recent tax law is worth noting. The One Big Beautiful Bill Act, signed into law on July 4, 2025, created a new $6,000 deduction for taxpayers aged 65 and older, available for tax years 2025 through 2028. But it phases out at a 6% rate for single filers with MAGI above $75,000 and disappears entirely above $175,000. With MAGI near $91,000, this retiree clears the $75,000 floor by roughly $16,000, which erodes much of the benefit before it can be claimed. The remaining partial deduction may shave a few hundred dollars from the tax bill, but it will not move the needle on the IRMAA calculation below.
So far, predictable. The RMD was coming either way.
Where the Medicare trap opens
Medicare IRMAA is calculated on modified adjusted gross income from two years earlier. The 2026 RMD lands on the 2026 tax return, which means it shows up as the income base for the 2028 Part B premium. The first IRMAA threshold for a single filer is $109,000 in MAGI. Below it, the standard Part B premium is $202.90 per month.
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 portfolio rebalancing, or a larger 401(k) balance the following year as the divisor continues to shrink. Cross $109,000 and the first surcharge tier activates, adding an extra $81.20 per month to Part B and bringing the total to $284.10. A Part D surcharge of $14.50 per month rides along. None of that bill arrives until 2028, which is precisely why most retirees never see it coming.
Two tiers up, at MAGI above $137,000, the Part B premium climbs to $405.80 per month. The IRMAA cliff is among the steepest effective tax rates in the tax code: one dollar over any threshold triggers the entire surcharge for that tier.
What actually moves the outcome
The 401(k) balance grows with the market, and the RMD divisor falls every year. By age 80, the divisor is around 20, which on a balance in this range pushes the RMD past $80,000 and the MAGI past $109,000 without any conversions or capital gains required. The clock is running.
Three levers matter:
- Qualified charitable distributions. The QCD allows a direct transfer from an IRA to a qualified charity. It counts toward the RMD and is excluded from taxable income entirely, with no need to itemize. The 2026 QCD limit is $111,000 per individual. For a charitably inclined retiree, a $20,000 QCD shaves the same amount off MAGI and creates real distance from the $109,000 IRMAA line. One critical detail: QCDs must come from an IRA, not a 401(k). A retiree holding assets in a 401(k) needs to roll the account into a traditional IRA first before the QCD strategy is available.
- Partial Roth conversions, inside the 22% bracket only and with the IRMAA cliff modeled first. A conversion done at 70 or 71 before RMDs begin 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 push the Medicare surcharge into effect two years later.
- 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 has already been 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, consider rolling the 401(k) into a traditional IRA and routing part of the RMD through a QCD before taking any personal distribution. The QCD only counts if the funds leave the IRA as a direct transfer to the charity before any cash reaches the account holder.
Editor’s note: This article was updated to reflect the confirmed 2026 QCD annual limit of $111,000 per individual, up from $108,000 in 2025, and to note that QCDs require an IRA rather than a 401(k), meaning a rollover is a prerequisite for this strategy. New context was also added on the One Big Beautiful Bill Act’s $6,000 senior deduction, which phases out for single filers with MAGI above $75,000 and is therefore largely unavailable to the retiree described in this scenario.
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