The IRMAA Cliff: Sizing Roth Conversions Around the Two-Year Medicare Lookback

Your client is 66, retired, and enrolled in Medicare. In November he asks the question you have been waiting for: "Should we move some of the IRA to Roth this year?" The bracket math is clean — there is room to the top of his current bracket, the RMD clock starts in a few years, and the long-term case for converting is strong. You convert $100,000. He signs, you document the rationale, and everyone leaves the meeting feeling good.

Fourteen months later he calls with a letter from the Social Security Administration. His Medicare Part B premium just went up. So did his wife's. The letter cites something called IRMAA, and he wants to know if it is a mistake.

It is not a mistake. It is the conversion — arriving two years later, exactly on schedule, as a premium increase nobody priced into the decision. The advisors who keep clients out of that phone call are not the ones who avoid conversions; they are the ones who size every conversion against a second set of thresholds most planning software never shows.

What IRMAA Is — and When It Looks

IRMAA — the Income-Related Monthly Adjustment Amount — is a surcharge on Medicare Part B and Part D premiums for higher-income enrollees. Two mechanics drive everything else in this post:

• The two-year lookback. The premium a client pays in a given year is generally set from their MAGI from two years prior. Premiums for 2026 are keyed to 2024 income. And MAGI for this purpose is generally AGI plus tax-exempt interest — the municipal bond income your client thinks of as invisible counts here.
• It is a household number. For a married couple filing jointly, one joint MAGI sets the surcharge for each spouse enrolled in Medicare. Cross a threshold and the couple does not pay one surcharge — they pay two, one on each premium, for Part B and again for Part D.

The lookback is what turns IRMAA from a billing detail into a planning problem. By the time the letter arrives, the year that caused it closed two tax seasons ago. There is no December maneuver that fixes a premium set from a return already filed. The only place to manage IRMAA is in the year the income happens — which, for a Roth conversion, means the moment you size it.

A Cliff, Not a Phase-In

Most of the thresholds you manage taper. Deductions phase out gradually; each tax bracket taxes only the dollars inside it. IRMAA does none of that. It is a cliff: one dollar over a threshold triggers the full surcharge for that tier, on every month of the year, for both enrolled spouses.

There are generally several tiers above the base premium, and the thresholds are indexed most years — which also means hard-coding last year's numbers into a spreadsheet is its own quiet risk. The exact dollar amounts matter less than the shape. As an illustration: a couple that lands one dollar into the first surcharge tier can pay on the order of $1,800 more in combined Part B premiums over the year, plus a smaller Part D surcharge on top. At the top tiers, an enrollee's Part B premium can run more than triple the base. The marginal cost of that one crossing dollar makes it, effectively, the most expensive dollar in the client's entire tax picture.

Two mercies are worth knowing, because they shape strategy. First, IRMAA is recalculated every year: a single high-MAGI year buys a single year of surcharge, not a permanent penalty, as long as income falls back under the threshold. Second, because it is annual, it is schedulable: a conversion program spread across several years can stay under a threshold every single year, where the same total converted in one year would blow through two tiers.

Sizing the Conversion: Fill the Tier, Never Cross by Accident

The discipline mirrors bracket management, with one extra ceiling stacked on top:

• Project the full-year MAGI before an amount is discussed. Social Security, pension, interest — including tax-exempt — dividends, expected capital-gain distributions, any part-year wages. That total is the floor the conversion stacks on.
• Compute two headrooms. Distance to the top of the current tax bracket, and distance to the next IRMAA threshold. Size the conversion to the smaller of the two. Sometimes the bracket binds and IRMAA never enters the conversation. For retirees with modest ordinary income and large traditional balances, the IRMAA threshold binds far more often than advisors expect — it sits lower than intuition says.
• Leave a buffer. A December mutual-fund capital-gain distribution can wreck a threshold you filled to the dollar in November. A few thousand dollars of margin costs little; the cliff costs more.
• Prefer a multi-year map to one heroic year. Filling the same tier for five consecutive years converts far more in total, at the same premium level, than alternating quiet years with one loud one — and it keeps every year's surcharge a chosen, bounded number.

This is exactly the picture worth putting on screen. WiseNest Connect models IRMAA MAGI inside the household projection, so the tier headroom is visible before you pick the number — the client watches the stair-step fill toward the line and understands, without a seminar on Medicare administration, why the conversion stops where it stops.

Coordinate the Whole Income Picture

A conversion never lands on an empty return. The sizing only works when the rest of the year's income is managed with it:

• Capital gains. Realized gains raise MAGI dollar for dollar. A year with a planned rebalancing gain or a property sale is usually the wrong year to fill a tier with conversions — and vice versa. Decide which event owns the year.
• RMD years. Once RMDs begin, they claim the first slice of headroom and the conversion window narrows. That is the quantitative case for converting in the gap years between retirement and RMD age — the same years when the IRMAA math is tightest.
• QCDs for the charitably inclined. For clients past age 70½, qualified charitable distributions move giving out of AGI entirely — generally the cleanest way to create conversion headroom while keeping the client's giving intact.
• The survivor preview. IRMAA thresholds for a single filer sit at roughly half the married levels. A widowed client can land in surcharge territory on income the couple enjoyed surcharge-free — one more reason conversions while both spouses are alive shrink the account that will be taxed hardest later.

Age 63 Is When Medicare Pricing Starts

Because of the lookback, Medicare pricing begins before Medicare does. A client who enrolls at 65 walks in with premiums set from age-63 MAGI. The year a client turns 63, their tax return quietly becomes a Medicare document.

That changes the review checklist from 63 on:

• Conversions at 63 and 64 are Medicare-priced events, not just bracket events. Price them that way in the recommendation.
• So are the other spikes those years attract — severance, deferred-compensation payouts, the sale of a business or a rental, a large rebalancing.
• When a spike is unavoidable, say so in advance and put the expected first-year premium in the plan. A surcharge that was predicted and explained lands as confirmation of your work; the same surcharge arriving unannounced lands as your error.

When Crossing Is Worth It — On Purpose

None of this means a conversion should never cross a threshold. A surcharge is a one-year, known, bounded cost. The things a larger conversion buys — decades of tax-free compounding, smaller future RMDs, a smaller balance to be taxed at a survivor's single-filer rates — can be worth several multiples of one year of elevated premiums.

The test is not *never cross*. The test is never cross by accident:

• Price it. "This conversion adds roughly this much to your combined Medicare premiums two years from now, once." That sentence belongs in the written recommendation.
• Compare it to what it buys. A one-year surcharge against a permanently smaller RMD stream is usually a lopsided trade — but show the trade, don't assert it.
• Let the client choose it. A surcharge the client selected, with the math in front of them, is a toll they agreed to pay. The same dollars arriving as a surprise are, in the client's eyes, your mistake.

A note on SSA-44 — the appeal that works, and the one that doesn't. When income drops after a life-changing event, Form SSA-44 asks the Social Security Administration to set the premium from a more recent, lower-income year instead of the lookback year. Qualifying events generally include work stoppage or retirement, work reduction, marriage, divorce or annulment, death of a spouse, and a few narrower situations such as loss of pension income. The classic case: a client retires at 66, the lookback lands on a full-salary year, and SSA-44 resets the premium to match the new reality. File it promptly, with documentation, and it generally works.

What SSA-44 does not cover: a Roth conversion. Converting is a voluntary income event, not a life-changing one, and the surcharge stands. That asymmetry is the entire reason this post exists — a retirement-year surcharge can be appealed after the fact; a conversion surcharge can only be managed before it happens, at sizing time.

Monday Morning

• Pull the two-year-back MAGI for every Medicare-enrolled client and note the distance to their next threshold. One column in your review prep turns IRMAA from folklore into a managed number.
• Flag every client aged 63 and up. Their conversions, gains, and income spikes are now Medicare-priced. Say it in the meeting before the first one happens, not after the first letter.
• Before any fourth-quarter conversion, compute both headrooms — top of bracket and next IRMAA tier — and size to the smaller, with a buffer for late distributions.
• Cross only on purpose. When the long-run math justifies a surcharge year, show the cost, the payoff, and the one-year shape of it — then let the client choose it in writing.

The IRMAA letter always arrives two years late. The advisor who sized the conversion against the tier line — and showed the client the stair-step before converting — is the advisor whose phone call, fourteen months later, is a thank-you instead of a complaint.