Roth Conversion Math for High-Earning Couples: How to Split Income and Avoid Medicare Premium Penalties

A Roth conversion can look like a clean tax win on paper: pay tax now, reduce future required minimum distributions, and create tax-free withdrawals later. But for high-earning couples near Medicare age, the math changes. A conversion that makes sense from a federal income tax standpoint can also push household income over a Medicare IRMAA threshold and raise Part B and Part D premiums two years later.

That is the core tradeoff in Roth conversion math for high-earning couples: tax paid now versus future tax avoided, minus any Medicare premium penalties triggered by higher modified adjusted gross income, or MAGI. This matters most when spouses have uneven earnings, uneven retirement account balances, or a narrow window before one or both spouses start Medicare.

This article focuses on couples because the planning usually happens at the joint-return level. A spouse’s conversion, bonus, capital gain, or business income can affect the Medicare premiums both spouses eventually pay.

Why Roth Conversion Math Changes Near Medicare Age

Before Medicare enters the picture, many Roth conversion decisions come down to a familiar question: is your tax rate today lower than the rate you expect later? If yes, converting some pre-tax money can be sensible.

Near Medicare age, that framework is incomplete. A conversion increases ordinary income in the tax year it happens, and Medicare uses prior-year income data with a two-year lookback to set income-related surcharges. That means a conversion in 2026 can raise Medicare premiums in 2028. A good tax move can become a more expensive move if it lands your joint MAGI just above an IRMAA line.

This is especially relevant for married couples when:

• One spouse is still working and the other is retired.
• One spouse has a much larger traditional IRA or 401(k) balance.
• The couple expects lower-income years after retirement but before Social Security or required minimum distributions begin.
• One or both spouses will start Medicare within the next two years.

In other words, the usual conversion analysis is not enough. You also need to test how much “extra” income your joint return can absorb before Medicare premiums jump.

How Medicare Premium Penalties Work

IRMAA stands for Income-Related Monthly Adjustment Amount. It is an extra charge added to Medicare Part B and Part D premiums for higher-income beneficiaries. Part A is not affected.

The important point is that IRMAA is based on MAGI, not just taxable income. For Medicare purposes, MAGI generally starts with adjusted gross income and adds back certain items, most notably tax-exempt interest. That means income sources that feel harmless for tax planning, such as municipal bond interest, can still matter for Medicare premium planning.

There are two mechanics couples need to remember:

• The two-year lookback: a 2026 conversion generally affects 2028 Medicare premiums.
• The cliff effect: going even slightly over a threshold can trigger the full higher surcharge tier.

For 2026 Medicare premiums, the married filing jointly IRMAA thresholds start once 2024 MAGI exceeds $218,000. The surcharge amounts below are per person, per month and apply on top of the standard premium structure.

Because those surcharges apply per spouse, the household cost can add up quickly. For a married couple with both spouses on Medicare, crossing from one tier to the next can mean several thousand dollars of additional annual premium cost.

Roth Conversion Math for High-Earning Couples

The cleanest way to evaluate a conversion is to compare three numbers:

• Tax paid now on the conversion.
• Tax likely avoided later by shrinking future taxable withdrawals and RMDs.
• Estimated Medicare premium penalties caused by the higher MAGI.

A simple framework looks like this:

Net benefit of conversion = estimated future tax saved – tax paid now – estimated IRMAA cost

To use that formula, couples need a few inputs:

• Current marginal federal and state tax rate.
• Expected future tax rate in retirement or after the first spouse dies.
• Conversion amount being tested.
• Projected joint MAGI for the conversion year.
• Likely IRMAA tier in the premium year two years later.

The couple-level issue is where many households get tripped up. One spouse may think of the conversion as “my IRA,” but IRMAA planning is tied to the joint return. A still-working spouse’s wages, year-end bonus, RSU vesting, consulting income, dividends, interest, and realized gains all use up room under the next threshold.

That is why the last dollars of a conversion can be the most expensive dollars. A couple may be comfortable paying 24% federal tax on an added conversion amount, then discover that the final few thousand dollars also triggered a full IRMAA tier jump for both spouses.

Quick example with uneven earnings

Assume Priya is 67 and retired, while Ethan is 64 and still earning consulting income. They file jointly. Their projected 2026 MAGI before any Roth conversion is $185,000. They want to move $160,000 from Priya’s traditional IRA to a Roth before RMDs begin.

If they convert the full $160,000 in 2026, projected MAGI rises to $345,000. That puts them in the $342,001 to $410,000 IRMAA tier. If both spouses are on Medicare in 2028, they would pay the Tier 3 surcharge for each spouse.

If instead they split the same total conversion into $80,000 in 2026 and $80,000 in 2027, projected MAGI becomes $265,000 in each conversion year. That lands them in the lower $218,001 to $274,000 tier in each affected premium year.

➤ Free Guide: 5 Ways To Automate Your Retirement

Strategic Income Splitting Tactics to Avoid IRMAA

Income splitting in this context does not mean assigning income between spouses arbitrarily. It means managing timing, account sources, and bracket usage so that joint MAGI is spread more efficiently across tax years.

1. Spread conversions across multiple years

This is the most direct tactic. Instead of one large conversion that blows through a Medicare threshold, test several smaller annual conversions. For many couples, the best answer is not “convert everything before Medicare,” but “fill a specific tax bracket and stop short of the next IRMAA cliff.”

2. Keep other income events out of the conversion year

Conversions stack on top of other income. If possible, avoid combining a large conversion with:

• Bonuses or deferred compensation payouts.
• RSU vesting or stock option exercises.
• Large capital gains from a concentrated stock sale.
• Business income spikes or one-time consulting projects.

Sometimes the best Roth conversion year is not the year with the lowest tax bracket in theory. It is the year with the fewest other income surprises in practice.

3. Use low-bracket room efficiently

When one spouse retires before the other, there may be a temporary window where joint taxable income falls. That window can be valuable. If the couple can fill lower brackets with partial conversions before Social Security and RMDs start, they may reduce future taxes without crossing a Medicare threshold.

4. Consider delaying Social Security

Delaying Social Security can create more planning room in some cases. The gap years before benefits start may allow larger conversions while keeping taxable income lower than it would be later. This does not make sense for every household, but it is worth modeling because taxable benefits can reduce your flexibility.

5. Coordinate offsets in the same tax year

Deductions and loss management matter more when they help preserve room under a threshold. Depending on the situation, useful offsets can include:

• Charitable giving, including bunching deductions.
• Deductible IRA contributions, if eligible.
• Tax-loss harvesting to offset realized gains.
• Business deductions timed into the conversion year.

The goal is not just to lower tax. It is to lower MAGI in the specific year that controls Medicare premiums two years later.

Couple-Level Scenario Walkthrough

Here is a simplified side-by-side example showing why timing matters.

Now assume the couple repeats the second $80,000 conversion in 2027. Their 2029 premiums would again reflect the lower tier instead of the higher one. Across the two-year plan, splitting the conversion saves about $4,646 compared with doing the full $160,000 in one year and triggering the higher surcharge tier.

The more striking point is what happens at the margin. In this example, the difference between ending at $342,000 of MAGI and ending at $345,000 is only $3,000 of extra conversion income. But that extra $3,000 pushes the couple into the next IRMAA tier. The added annual surcharge for two spouses is about $3,470. On top of that, they still owe regular income tax on the extra conversion.

That means the effective marginal cost on those last converted dollars can exceed the stated federal tax rate by a wide margin. This is why couples should not stop the analysis at “we are still in the 24% bracket.”

When Income Splitting Helps and When It Backfires

Income splitting and multi-year conversion planning work best when a couple has control over timing and a few lower-income years before RMDs begin. That is often the sweet spot: retirement has started, wages have fallen, Social Security may be delayed, and RMDs have not yet filled the return.

It is less effective when the joint return is already crowded by:

• High wages from one spouse.
• Large pension income.
• Substantial annual capital gains.
• RMDs that already fill the target bracket.

There are also important special cases. State income taxes can materially change the math. So can tax-exempt interest that raises MAGI for Medicare purposes. Once RMDs begin, conversions often become harder to manage because the RMD must generally come out first and cannot be converted. Converting aggressively late in retirement can stack extra income on top of RMDs and create avoidable premium spikes.

Couples should also remember that the survivor of a marriage often faces a less favorable tax picture later. A plan that looks only at today’s joint bracket may understate the long-term value of some conversions, but that does not justify ignoring IRMAA costs in the near term. Both sides of the equation matter.

What To Do Next Before You Convert

Before making a Roth conversion, build a three-year MAGI projection. That gives you a practical view of the tax year, the Medicare lookback year, and the likely premium year. Include both spouses’ income sources, not just the IRA owner’s account.

Then map your projected tax brackets and IRMAA tiers side by side. Test several conversion amounts, not just one. In many cases, the best answer is a number that looks oddly specific because it was chosen to stay under a threshold.

• Review prior-year income sources and expected changes in wages, bonuses, RSUs, gains, and business income.
• Confirm when each spouse will enroll in Medicare and when Social Security may begin.
• Model whether spreading conversions across two or three tax years reduces total cost.
• Check how state taxes and municipal bond interest affect the result.

A Roth conversion can still be a strong move for high-earning couples. The mistake is treating it as a tax-only decision. Near Medicare age, the smart process is to compare income tax, future tax savings, and IRMAA together. That is often the difference between a disciplined conversion plan and an avoidable premium surprise.

This article is for educational purposes only and should not be treated as individualized tax, legal, or financial advice. A CPA or fiduciary financial planner can help verify the assumptions before you convert.