One Dollar Over the Line

Your 401(k) Isn't All Yours showed how a large traditional 401(k) quietly taxes the Social Security you spent a career earning. The fix: convert money into a Roth in the low-income stretch of your early 60s, shrinking the balance before the clock at 73 can turn it into forced, taxed income.

So you started converting. It works. A smaller traditional balance means smaller required minimum distributions (RMDs), less of your Social Security taxed away, and a benefit that finally comes through mostly whole.

Unfortunately, there is a side effect. Every large conversion counts as income the year you make it — a spike of ordinary income, taxed at the same rate as a paycheck — and Medicare bills you for it, two years later.

The Lookback

Medicare is the federal health insurance nearly everyone enrolls in at 65. It comes in parts. Part B covers doctor visits and outpatient care. Part D covers prescription drugs. Both are the everyday coverage you actually use, and both charge a monthly premium for the rest of your life.

Those premiums aren't flat. Above an income line, each one gets a surcharge stapled on: the income-related monthly adjustment amount, or IRMAA. Earn under the line and you pay the standard premium. Cross it and the premium on both parts climbs, every month, all year.

The cruel part is the timing. The income Medicare reads isn't this year's. It's the income from your tax return two years ago.

So a conversion you run at 65 doesn't touch your premium at 65. It surfaces at 67, long after you've stopped thinking about it. By the time the bill lands, there's nothing left to undo.

The Cliff

IRMAA isn't a ramp, it's a flight of stairs. Cross a threshold by a single dollar and the full surcharge for that tier lands on your entire income, not just the dollar that tipped you over.

One dollar past $218,000 adds $81 a month to each spouse's Part B premium, $162 for the couple, and Part D stacks another $29 on top. Across both parts and both spouses, the first line alone runs about $2,300 a year, and the next tier climbs past $5,700. The line doesn't care that you missed it by a rounding error.

The Needle

This is where the conversion stops being one decision and turns into a yearly balancing act. Every year you convert against two ceilings at once. One is the top of the tax bracket you're willing to fill — income stacks up in tiers, each taxed higher than the last, and this is how high you'll let the conversion climb before it tips into the next tier's rate. The other is the IRMAA cliff hanging just above your income. The move is to fill up to the lower of the two, deliberately, and stop there.

The mistake isn't converting too much. It's converting blind, tripping a line you weren't watching, for income you never even needed to take. Do it deliberately and the surcharge, when it comes, is a price you chose, not one you backed into.

Keep the stakes straight, though. The surcharge is real money, but it's pocket change next to what the conversion is buying. You don't let a $15,000 side effect scare you off a $900,000 cure. You just make sure every dollar of it was spent on purpose.

The Move

Convert. The window is still the best gift the tax code hands you, and the tax on your Social Security is still the bigger enemy by a mile. Just don't do it with your eyes closed. Size each year to the bracket and the cliff, remember the bill lands two years out, and pay the surcharge only when you meant to.

The cure is right. Mind the dosage.

Appendix: Run your own numbers

Two things decide whether a conversion trips IRMAA, and you can check both before you pull the trigger.

What counts as income is MAGI — your adjusted gross income (the income figure at the bottom of your tax return) plus any tax-exempt interest. A Roth conversion lands in it dollar for dollar. Qualified Roth withdrawals later do not, which is the whole point of paying the tax now. Enter the income Medicare will read, two years before you enroll, and see which tier it puts you in and how much room is left before the next cliff.

Notes

• The two-year lookback. Your 2026 premium is set by your 2024 return. So the years that matter for Medicare are the conversion years from about 63 onward, two before you enroll at 65.
• Tax-exempt interest. Interest from municipal bonds is free of federal income tax, but Medicare adds it back into MAGI when it sets your premium. So even "tax-free" interest can nudge you over an IRMAA line.
• Per person. Each spouse enrolled in Medicare pays their own surcharge, both figured on the same joint income. One of you on Medicare means one surcharge; once both are enrolled, it doubles.
• Part D too. The surcharge hits both parts. The same MAGI adds a Part B surcharge and a separate Part D surcharge, and the calculator above combines them into the annual figure.
• Single filers. The thresholds sit at roughly half the married numbers, starting near $109,000, so a solo saver hits the stairs far sooner.

Appendix: When you can appeal

IRMAA isn't always final. Medicare bills you off a two-year-old return because it assumes that return still describes you, and sometimes it badly doesn't. Form SSA-44 lets you ask Medicare to use a more recent year instead. But it only opens for a specific list of life-changing events:

• Work stops or slows. You or your spouse retire, get laid off, or cut your hours.
• Marriage, divorce, or death. Anything that changes who's on the return.
• Income that won't come back. A pension that gets cut, or income property lost to something outside your control.

They all share one shape: something happened to you, and the old return now overstates what you actually live on.

A Roth conversion is the opposite. You did it on purpose, and it pushed your income up, not down. From where Medicare sits, nothing went wrong, so there's nothing to forgive. The surcharge stands.

Which is the whole point of sizing it carefully up front. There's no appeal waiting at the end to bail you out. Plan around the line, you can't appeal your way back under it.