Retirees With $1.2 Million in a 401(k) Face a Hidden Tax Bomb at 73: Here’s How to Avoid It

A married couple turning 73 this year with $1.2 million in a traditional 401(k) faces a first required minimum distribution that looks manageable on paper. The dollar amount looks manageable. What that withdrawal does to the rest of the tax return is where retirees get caught.

The pattern shows up regularly on r/retirement and the Bogleheads forum: a saver completes a Roth conversion or first RMD, then learns months later that Medicare premiums will jump because the IRS and Social Security Administration look backward two years to set them. Anyone with seven figures in a pre-tax account faces this rule.

The first RMD is small. The cascade behind it is enormous

The IRS Uniform Lifetime Table uses a divisor of 26.5 at age 73. A $1.2 million balance at the prior year-end produces a first RMD near $45,283. For a couple already collecting Social Security, that figure feels modest. It changes the math on two other numbers that matter far more than the withdrawal itself.

The first is how much Social Security becomes taxable. Once a joint filer’s provisional income clears $44,000, up to 85% of benefits get pulled into ordinary income. A couple drawing $60,000 a year in combined benefits can see roughly $51,000 of that flow onto the 1040 because the RMD pushed them over the line.

The second is the Income-Related Monthly Adjustment Amount, the Medicare surcharge keyed to modified adjusted gross income from two years prior. For 2026, the standard Part B premium is $202.90 a month, and the surcharge tiers start where many RMD-takers naturally land.

What the IRMAA tiers actually cost

The 2026 thresholds and per-person annual surcharges (Part B and Part D combined) work like a staircase. Cross a tier by a single dollar and the full surcharge applies for the entire year:

1. Tier 1: MAGI above $109,000 single or $218,000 joint adds $1,148 per person per year. A couple just over the line pays roughly $2,297 in extra Medicare premiums.
2. Tier 2: above $137,000 single or $274,000 joint adds $2,886 per person, or about $5,772 for a couple.
3. Tier 3: above $171,000 single or $342,000 joint adds $4,620 per person, lifting couples close to $9,240.
4. Tier 4 and the top tier push surcharges past $7,700 and $8,200 per person, territory that pension income plus RMDs can reach faster than expected.

Stack the 22% federal bracket on the RMD, the newly taxable Social Security, and the IRMAA add-on, and the effective marginal cost on the next dollar withdrawn can sit near 40 cents. That is the tax bomb. It surfaces on the Medicare premium statement two years later, long after the 1099-R is filed.

How the napkin math holds up at today’s yields

With the 10-year Treasury near 4.5% and the 30-year around 5%, retirees who do not need the RMD for spending can park the after-tax proceeds in fixed income and roughly offset the withdrawal rate the table forces. The portfolio survives. The tax cost on the way out cuts the legacy.

That comfort can mislead. Core PCE has continued grinding higher into April 2026, and headline CPI has risen roughly 3.9% over the past year. RMDs scale with the prior year-end balance, so a strong market year leaves a larger forced withdrawal even when real spending power shrinks.

Three moves that bend the math

1. Use a Qualified Charitable Distribution to satisfy part or all of the RMD. The 2026 QCD limit is $111,000 per person. Dollars sent directly from the IRA to a qualified charity never hit MAGI, which means they cannot trigger Social Security taxation or push the household into a higher IRMAA tier.
2. Project MAGI to the dollar each fall before the December 31 deadline. The IRMAA tiers behave like cliffs. Pulling a $2,000 capital gain forward into the current year, or deferring it, can be worth several thousand dollars in 2028 premiums for a couple sitting near $218,000, $274,000, or $342,000 of joint MAGI.
3. For retirees still in the gap between retirement and age 73, fill the 22% and 24% brackets with Roth conversions before RMDs start. Each $10,000 converted at 22% today removes a future RMD that could otherwise arrive at an effective 40% rate after the Social Security and IRMAA cascade.

If household MAGI is within $10,000 of any IRMAA threshold, the cost of a fee-only tax planner runs well below what a single missed tier will cost over a two-year premium cycle.