The advice to convert a traditional IRA to a Roth IRA has become a staple of retirement planning. The pitch is simple: pay taxes now at a known rate, then enjoy tax-free withdrawals later. For many, it makes sense. But there is a hidden cost that the cheerleaders rarely mention—and it has nothing to do with the conversion tax itself.

Medicare Part B premiums are income-adjusted. A large one-time Roth conversion can push your Modified Adjusted Gross Income (MAGI) into a surcharge bracket, and the penalty arrives two years after the event. The extra premium can run into thousands of dollars per year, and it persists for as long as the higher income lingers on your tax return. In some cases, the surcharge exceeds the tax paid on the conversion.

This is not a hypothetical edge case. Roughly 7% of Medicare beneficiaries paid the Income-Related Monthly Adjustment Amount (IRMAA) in 2024, according to the Medicare Trustees. The share is growing because the income thresholds are not fully indexed to inflation. A retiree who executes a single large Roth conversion can land in that group for multiple years, turning a supposedly smart move into a costly mistake.

The Roth Rollover Tax Bump That Medicare Never Forgets

A Roth conversion is treated as ordinary income in the year you execute it. If you move $100,000 from a traditional IRA to a Roth IRA, that $100,000 lands on your Form 1040 as a taxable distribution. Your MAGI jumps accordingly.

Medicare Part B premiums are tied to MAGI from two years prior. The Social Security Administration uses the most recent tax return on file to set your premium for the coming year. For 2026 premiums, the relevant return is the 2024 tax return. A conversion done in 2024 shows up in 2026, and the surcharge lasts as long as the higher income remains on your record.

The IRMAA surcharge is not a one-time fee. It is a monthly add-on to your Part B premium, and it applies for the entire calendar year. If your MAGI exceeds the first threshold by even a dollar, you pay the surcharge for twelve months. The brackets are set by statute and adjusted annually, but they rise slowly. In 2026, the first threshold for single filers is roughly $97,000; for married couples filing jointly, it is about $194,000.

The surcharge ranges from about $70 per month to over $400 per month, depending on income. A couple in the top bracket pays an extra $5,000 or more per year, on top of the base Part B premium of around $185 per month. That extra cost can eat up a significant portion of the tax savings from the conversion.

How IRMAA Turns a Roth Strategy Into a Trap

The Income-Related Monthly Adjustment Amount was introduced in 2007 as part of the Medicare Modernization Act. It was designed to shift some of the program's cost to higher-income beneficiaries. The thresholds were initially high enough that few retirees were affected. Over time, as healthcare costs rose and the thresholds were only partially inflation-adjusted, more people fell into the surcharge brackets.

For 2026, the IRMAA brackets are set at roughly $97,000, $123,000, $153,000, and $183,000 for single filers. For married couples, the thresholds are double those amounts. The top surcharge is $419.30 per month, plus the base premium. That means a single filer with MAGI over $183,000 pays about $605 per month for Part B alone.

A Roth conversion of $100,000 can easily push a retiree from the base premium bracket into the top surcharge bracket. Consider a married couple with $150,000 in ordinary income. Adding $100,000 from a conversion brings them to $250,000, which puts them in the second-to-top bracket. Their Part B premium jumps from roughly $185 per month to about $370 per month—an extra $2,220 per year. Over five years, that is $11,100 in surcharges.

The trap is that the surcharge persists for as long as the higher income appears on the tax return used for the look-back. If the conversion is the only spike, the surcharge lasts one year. But many retirees do not realize that the look-back means the penalty arrives two years later, by which time the conversion is irreversible.

The Two-Year Look-Back That Catches Retirees Off Guard

The two-year lag is the source of most surprises. A retiree who converts in 2024 will see the impact on their 2026 Medicare premiums. By then, the tax return is filed, the conversion is done, and there is no way to undo it.

There is an appeal process using Form SSA-44, but it is limited to life-changing events such as marriage, divorce, death of a spouse, or loss of income-producing property. A voluntary Roth conversion does not qualify. The Social Security Administration will not waive the surcharge because you made a choice to increase your income.

The lag also means that a retiree who converts in the year they turn 63 will see the surcharge hit at age 65, when they first enroll in Medicare. That can be a shock for someone who planned a smooth retirement transition. The first premium bill arrives with an IRMAA surcharge they never anticipated.

Some retirees try to game the system by converting in a low-income year before Medicare enrollment. That works if the low-income year is two years before enrollment. But if you convert at age 62, the income shows up on your 62-year-old tax return, which is used for your 64-year-old premiums. If you enroll at 65, the relevant return is from age 63—the conversion year. The timing has to be precise.

Why Advisors Rarely Flag This Interaction

Roth conversion calculators are everywhere, but almost none of them include Medicare premium surcharges. The typical comparison assumes a flat tax rate and ignores the downstream effects on healthcare costs. That is a significant omission.

Part of the problem is that the interaction between Roth conversions and IRMAA is complex. It requires projecting income two years into the future and estimating Medicare thresholds that are adjusted annually. Most financial planning software handles tax brackets well but treats Medicare premiums as a fixed cost. The variable, income-dependent nature of Part B is often a separate module that few advisors activate.

Advisor compensation also plays a role. Many advisors are paid as a percentage of assets under management. A Roth conversion generates a tax bill that reduces the client's portfolio, but the advisor's fee is based on the remaining assets. The incentive is to keep assets in the IRA, not to convert them. At the same time, the advisor may not want to discourage a client from a popular strategy. The result is that the IRMAA risk is underdiscussed.

There is also a cultural bias in the financial planning community. Roth accounts are seen as universally good. The tax-free growth narrative is powerful, and it drowns out more nuanced analysis. A planner who warns about Medicare surcharges might seem overly cautious. But the numbers show that caution is warranted.

Data From the Medicare Trustees Shows the Surcharge Bite

The 2026 Medicare Trustees Report projects that roughly 8% of beneficiaries will pay IRMAA surcharges in 2026, up from about 5% a decade earlier. The share is growing because the income thresholds are indexed to the Consumer Price Index for Urban Consumers (CPI-U), but Part B premiums have risen faster than general inflation. The base premium itself has roughly doubled over the past decade.

The top IRMAA surcharge for 2026 is $419.30 per month, as noted. Combined with the base premium of about $185, a high-income beneficiary pays over $600 per month—more than $7,200 per year. For a couple, that figure doubles. A single Roth conversion that pushes a couple into the top bracket costs them an extra $7,200 per year for as long as the higher income shows up on their tax return.

The conversion itself may trigger a tax bill of $20,000 to $30,000 on a $100,000 conversion, depending on their bracket. The IRMAA surcharge over four years could add another $28,800. Suddenly, the effective tax rate on the conversion is far higher than the nominal rate.

The trustees also note that the IRMAA thresholds are not adjusted for regional cost differences. A retiree in a high-cost area with a modest pension may still hit the surcharge brackets, while someone in a low-cost area with the same income may not. The policy is uniform, but its impact varies.

Partial Conversions and Multi-Year Smoothing as Workarounds

The most straightforward fix is to convert only up to the top of your current IRMAA bracket. If you are single and your MAGI is $80,000, you can convert up to roughly $17,000 before hitting the $97,000 threshold. That keeps you in the base premium bracket.

Spreading the conversion across multiple years is another option. Instead of converting $100,000 in one year, convert $25,000 per year for four years. Each year's income stays below the threshold, and the IRMAA surcharge never triggers. The trade-off is that you pay taxes over a longer period and may face higher future tax rates if the brackets are adjusted downward.

Qualified Charitable Distributions (QCDs) from an IRA can reduce your MAGI. If you are over 70½, you can donate up to $100,000 per year directly from your IRA to a charity. The distribution is excluded from income, lowering your MAGI and potentially keeping you below the IRMAA threshold. This is especially useful if you want to reduce income after a conversion year.

Health Savings Account (HSA) contributions also reduce MAGI, but only if you are enrolled in a high-deductible health plan. For retirees under 65, an HSA can be a powerful tool to lower income. After 65, HSA contributions are still deductible, but you cannot use the funds for non-medical expenses without penalty.

Timing the conversion to a year when your income is naturally low—perhaps the year you retire but before you start Social Security—can help. But you need to account for the two-year lag. If you retire at 63 and convert in that year, the conversion income will hit your 65-year-old premiums. You might be better off waiting until 64 and converting then, so the income falls off your record by the time you enroll at 66.

The Rule of Thumb That Deserves a Caveat

The conventional wisdom is that Roth conversions are beneficial if you expect to be in a higher tax bracket in retirement. That rule of thumb is still correct, but it is incomplete. It ignores the Medicare premium surcharge, which effectively adds a layer of taxation that is not captured by the ordinary income tax brackets.

For a retiree with significant assets and a long time horizon, a Roth conversion can still be a net positive. The tax-free growth over decades can outweigh the upfront tax cost and the IRMAA penalty. But the breakeven point shifts depending on the size of the conversion and the number of years the surcharge applies.

For a retiree near the IRMAA threshold, partial conversion is almost always better than a lump sum. The penalty from hitting the surcharge bracket is so steep that it erodes the benefit of tax-free growth. A side-by-side comparison using realistic assumptions—including Medicare premiums—is essential.

Some advisors argue that the IRMAA surcharge is a small price to pay for the flexibility of a Roth account. Others say it is a stealth tax that the government uses to claw back revenue. Reasonable people can disagree. But the disagreement should be based on actual numbers, not on marketing slogans.

The bottom line is that a Roth conversion is not a set-and-forget strategy. It requires careful planning around Medicare, Social Security, and other sources of income. The two-year look-back means that mistakes are not corrected quickly. Running the numbers with and without IRMAA is the only way to know whether the trade-off is worth it.

This article is for informational purposes only and does not constitute personalized financial, tax, or legal advice. Consult a qualified professional before making decisions about Roth conversions or Medicare enrollment.