The $3.2 Million 401(k) Tax Surprise Retirees Miss Until It's Too Late

"Picture a married couple, both age 67, filing jointly. They collect $48,000 in combined Social Security and pull $360,000 a year from their traditional 401(k)s to fund travel, the lake house, and the grandkids' 529s. Because the withdrawal pushes their provisional income well past the upper threshold, 85% of their Social Security ($40,800) becomes taxable, and their modified adjusted gross income lands at roughly $400,800."

"That MAGI is the number Medicare cares about. The Income-Related Monthly Adjustment Amount (IRMAA) uses a two-year lookback, so the tax return filed this spring sets the premium two years out. At roughly $400,800 of MAGI, a married-filing-jointly couple lands in IRMAA Tier 3 (approximately the $326,000 to $408,000 window for 2026). One more dollar of withdrawal pushes them into Tier 4, where the surcharge jumps again with no phase-in."

"The surcharge is per person, not per household, and it stacks Part B on top of Part D. At Tier 3, the math runs like this: Part B IRMAA: roughly $474.20 per month above the standard premium, per spouse, or about $11,381 per year combined. Part D IRMAA: roughly $66.40 per month each, or about $1,594 per year combined. Total Medicare surcharge: approximately $12,975 per year on top of standard premiums."

"That figure works like a hidden tax that bypasses Form 1040 entirely. It is deducted from Social Security checks for the next 12 months and recalculated annually from a tax return the couple already filed and forgot about. The trap is the l"

A married couple in their late sixties can pay federal taxes on retirement withdrawals yet still face a delayed Medicare premium bill. Medicare IRMAA uses a two-year lookback based on modified adjusted gross income, which is influenced by taxable Social Security and traditional 401(k) withdrawals. When withdrawals push provisional income above the upper threshold, a large portion of Social Security becomes taxable, raising MAGI to around $400,800. That MAGI places the couple into IRMAA Tier 3, with a higher surcharge that applies per person and stacks Part B and Part D. The surcharge is withheld from Social Security for the following 12 months and recalculated annually, creating a hidden cost that can approach $12,975 per year.

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