Why Your Tax Bracket Isn't the Real Cost of a Roth Conversion
- Jul 16
- 7 min read
By Kenneth M. Ford, AWMA®, AIF®
Last Updated: July 2026
Estimated Reading Time: 8–9 minutes
Quick Answer
Your tax bracket is not what a Roth conversion costs you. Conversion income stacks on top of everything else on your tax return, and as it rises it can trigger four additional costs the bracket does not show:
1. Medicare IRMAA surcharges — a cliff, not a ramp, with a two-year lookback
2. Increased taxation of Social Security — up to 85% of your benefit
3. The 3.8% net investment income tax
4. Capital-gains stacking — losing the 0% long-term capital-gains rate
Your bracket plus whichever of these you’re approaching is your true marginal rate. That is the real cost of a conversion dollar, and it is the only number that should determine how much to convert.
Sometimes the true marginal rate argues for converting more than the bracket suggests. More often — particularly at seven-figure balances — it argues for converting less in any single year and extending the plan across more years.
Why This Matters
Ask most people what a Roth conversion will cost and they’ll name a bracket. “I’m in the 24%, so it costs 24%.”
That answer is almost never right, and the gap between the stated bracket and the real cost is where expensive mistakes happen. A conversion sized perfectly to the top of a bracket can still cross an IRMAA cliff, drag Social Security into taxation, and push capital gains out of the 0% band — all invisible on the tax table.
For a household with a seven-figure pre-tax balance, this is not a rounding error. It is the whole calculation.
Cost One: Medicare IRMAA
IRMAA — the Income-Related Monthly Adjustment Amount — is the surcharge added to Medicare Part B and Part D premiums when income crosses certain thresholds.
Its defining feature is that it is a cliff, not a ramp. One dollar over the threshold and the full surcharge applies for the entire year. There is no phase-in.
In 2026, the first IRMAA tier begins at $109,000 of income for an individual and $218,000 for a couple. The standard Part B premium is $202.90 per month; at the top tier it becomes $689.90 per month.
And IRMAA looks back two years. Your premium at 65 is determined by your income at 63.
That single fact reshapes the timeline. The Medicare planning window effectively closes at 63, not 65 — which is roughly two years before most people start thinking about Medicare at all.
Cost Two: The Social Security Tax Torpedo
Social Security benefits are taxed on a sliding scale tied to your other income. As that income rises, a larger share of your benefit becomes taxable — up to 85% of it.
Here is what makes it a “torpedo”: within a certain income band, one additional dollar of conversion income can cause more than one dollar of benefits to become taxable. Your effective rate on that dollar can briefly exceed your stated bracket by a wide margin.
The mirror image is the opportunity. Roth withdrawals never enter this calculation at all — which is precisely what makes them so valuable in the years after you convert.
Cost Three: The 3.8% Net Investment Income Tax
Conversion income is not itself investment income. But the surtax is triggered by your total income, and a conversion raises it.
So a large conversion can pull your dividends, interest, and capital gains into range for an additional 3.8% they would not otherwise have owed. The threshold is $200,000 for an individual and $250,000 for a couple.
Cost Four: Capital-Gains Stacking
Long-term capital gains sit on top of your ordinary income in the tax stack. If ordinary income is low enough, those gains can be taxed at 0%.
Conversion income is ordinary income. It slides in underneath your gains and lifts them — potentially straight out of the 0% band and into 15%.
And if you’re retired but not yet 65 and buying health insurance through the ACA marketplace, conversion income can also claw back your premium subsidies at an effective rate near 9%.
Roth Conversion Tax Brackets Don't Tell the Whole Story
Consider a hypothetical couple in the 24% bracket, approaching an IRMAA threshold, with Social Security already in the mix.
The tax table says 24%. But the next dollar of conversion income might cross the IRMAA cliff, drag more of their benefit into taxation, and lift a slice of their capital gains out of the 0% rate.
Their true marginal rate on that dollar is materially higher than 24% — and nothing on the tax table would tell them.
Hypothetical illustration. Not a prediction, recommendation, or guarantee. Individual circumstances vary.
Advisor Insight
The single most useful thing we do for a client considering a conversion isn’t calculating a bracket. It’s mapping the thresholds — where the cliffs are, how much room sits beneath each one, and what it would actually cost to cross it.
Sometimes the answer is that crossing an IRMAA threshold is worth it, because eliminating future RMDs is worth more than one year of higher premiums. That’s a legitimate conclusion — but it should be a decision, not an accident.
The people who get hurt are the ones who never knew the threshold was there.
Example
Michael and Susan, both 63, have $1.4 million in traditional IRAs and modest current income. Their advisor identifies room to convert roughly $180,000 while staying inside the 24% bracket.
The conversion crosses a Medicare IRMAA threshold: At $180,000 of income, they exceed the 2026 joint threshold, increasing Medicare premiums when they enroll two years later.
The conversion affects long-term capital gains: A portion of their capital gains is pushed out of the 0% capital gains bracket, increasing the overall tax cost.
Their advisor models an alternative: convert $150,000 this year instead. They stay below the IRMAA threshold, keep their gains in the 0% band, and simply extend the conversion plan by an additional year.
The smaller conversion, spread further, produces a better outcome than the “optimal” bracket-filling conversion.
Common Mistakes
Equating the bracket with the cost. The bracket is the starting point, not the answer.
Converting at 63 or 64 without accounting for the two-year IRMAA lookback. By the time the premium arrives, it’s too late to change it.
Ignoring the Social Security interaction. Once benefits begin, the torpedo can make the effective rate far higher than expected.
Focusing on one year in isolation. A conversion plan is a multi-year exercise; optimizing a single year can damage the whole.
Assuming more conversion is always better. Over-converting is a real error — and it cannot be undone.
Frequently Asked Questions
What exactly is the “true marginal rate”? It’s your stated tax bracket plus the marginal effect of any thresholds your conversion income crosses — IRMAA surcharges, additional Social Security taxation, the 3.8% surtax, and lost capital-gains treatment.
Is the true marginal rate always higher than the bracket? No. If you’re well below all four thresholds, your true marginal rate and your bracket may be the same. The point is that you have to check rather than assume.
Does IRMAA apply if I’m not on Medicare yet? Not immediately — but because of the two-year lookback, income you recognize at 63 determines your premium at 65. So it applies to your future self.
Can crossing an IRMAA threshold ever be the right decision? Yes. If a larger conversion meaningfully reduces future RMDs or lifetime taxes, a temporary surcharge may be a reasonable trade. The point is to make that trade deliberately.
How do I know my true marginal rate? It requires modeling your specific income sources against the current thresholds, year by year. It is not something a bracket table can tell you.
Key Takeaways
Your tax bracket isn't your true Roth conversion cost: It reflects only part of the tax impact.
Several hidden costs can increase the effective tax rate: These include IRMAA, the Social Security tax torpedo, the 3.8% Net Investment Income Tax, and capital gains stacking.
Your true marginal rate should determine the size of a conversion: Evaluate all tax consequences—not just your federal income tax bracket.
IRMAA is a threshold, not a gradual phase-in: Crossing it can increase Medicare premiums, and the calculation looks back two years.
The best Roth conversion is often smaller and spread over multiple years: A multi-year strategy may reduce lifetime taxes while avoiding unnecessary surcharges.
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Final Thoughts
Your bracket is a starting point. Your true marginal rate is the truth.
“Fill up your bracket” is where most advice stops. It is also where the expensive mistakes start — because the bracket is the one number in this decision that is guaranteed to be incomplete.
See Beyond Your Tax Bracket
A tax bracket is only one piece of a Roth conversion decision. The real question is how a conversion could affect Medicare premiums, Social Security taxation, investment income, and your long-term retirement plan.
A Roth Conversion Analysis can help identify your true marginal rate and evaluate how much, if any, may make sense to convert based on your unique financial situation.
Disclaimer
This article is for educational purposes only and should not be considered tax, legal, or investment advice. Roth conversions, Medicare premiums, IRMAA surcharges, required minimum distributions, and retirement income strategies should be evaluated based on your individual circumstances, including your income sources, tax situation, retirement goals, and broader financial plan. All examples are hypothetical, are provided for illustration only, and are not a guarantee or prediction of any particular result; individual results will vary. Figures reflect 2026 federal parameters and are subject to change. Tax laws, Medicare rules, premium amounts, and IRMAA thresholds change over time. Consult qualified tax and financial professionals before making financial decisions.
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