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

A couple in their late sixties with $3.2 million in combined traditional 401(k) assets and a healthy spending plan can write a clean check to the IRS every April and still get hit with a surprise bill from Medicare. The bill arrives quietly, two years late, buried in the Part B and Part D premiums withheld from Social Security. For one common household profile, that hidden bill runs close to $13,000 a year.

The Couple, the Withdrawal, and the Tax Stack

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.

What Tier 3 Actually Costs

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 lag: today’s 401(k) withdrawal prices tomorrow’s premium, so cutting spending next year does not fix the surcharge already locked in.

Why the Standard Playbook Makes It Worse

The default advice for a couple this size is to defer everything until 73 and then let required minimum distributions do the work. On $3.2 million, the first RMD year alone produces a six-figure forced withdrawal that almost guarantees a Tier 3 or higher IRMAA bill for the rest of their lives. With the fed funds rate near 4% and the 10-year Treasury yielding 4.4%, fixed income inside the 401(k) keeps compounding the future RMD problem rather than solving it.

Four Moves That Change the Math

1. Recalibrate the annual withdrawal to land just under the next IRMAA tier. Trimming the draw from $360,000 to a figure that keeps MAGI below the Tier 3 ceiling can save the entire $12,975, and the gap can be funded from a brokerage account at long-term capital gains rates.
2. Bracket-fill Roth conversions in the pre-Medicare window (ages 60 to 64). Conversions before Medicare enrollment do not trigger IRMAA, and every dollar moved to Roth is a dollar that will not inflate MAGI later.
3. Use Qualified Charitable Distributions after age 70.5. A QCD from a traditional IRA satisfies the RMD without adding to MAGI. The QCD limit is approximately $108,000 per person in 2026 (verify against current IRS guidance).
4. Draw spending gaps from taxable accounts. Long-term capital gains at 15% beat ordinary income at 24% and do not stack into the IRMAA calculation the same way a 401(k) withdrawal does.

What to Do This Week

Pull the 2026 IRMAA tables directly from CMS.gov and find the MAGI ceiling for the tier below where the household currently sits. Then run a withdrawal-versus-conversion projection through age 75 that prices in the two-year lookback. With Social Security and Medicare transfer payments rising every quarter, Medicare transfers reached $1,301 billion in the first quarter of 2026, and IRMAA tiers indexed to inflation will keep moving. The household that watches the lookback wins back roughly $13,000 a year, every year, for the rest of retirement.