A conversion "in the 12% bracket" can genuinely cost 22%. Four effects drive the gap.
Figures on this page are for the 2026 tax year. Rules that are fixed in law rather than indexed — the Social Security thresholds, for instance — are noted where they come up.
Here's the calculation almost every Roth conversion article tells you to do: find your bracket, multiply, done. Convert $50,000 in the 12% bracket, pay $6,000.
That answer is usually wrong. Not slightly — a conversion nominally "in the 12% bracket" can genuinely cost you 22 cents on the dollar, and one in the 22% bracket can cost 40.
The reason is that a conversion doesn't just get taxed. It raises your income, and a lot of other things in the tax code are watching your income.
Take a retired couple, both over 65, with $48,000 of Social Security and $20,000 of other ordinary income. Here's what each additional dollar of conversion actually costs them, at different conversion sizes:
| Converting | True marginal rate | What's driving it |
|---|---|---|
| $10,000 | 0% | still covered by deductions |
| $30,000 | 22.2% | Social Security dragged into taxable income |
| $50,000 | 12.0% | that effect exhausted — the rate falls |
| $70,000 | 12.0% | genuinely 12% |
| $110,000 | 24.6% | senior deduction phasing out |
Read that middle column again. The marginal cost goes up, then down, then up. It is not a staircase.
Someone who stops converting at $30,000 because "we're at the top of our comfort zone" has stopped at the single most expensive point on the curve — and converting more would have been cheaper per dollar. You cannot get to that table from a bracket chart. It isn't in there.
Social Security isn't taxed at a rate. How much of it is taxable depends on a formula, and that formula keys off your other income.
In the phase-in range, every extra dollar of conversion pulls up to 85 cents of Social Security into taxable income alongside it. Your taxable income rises by $1.85 for every $1 you convert — so a dollar taxed "at 12%" costs 22.2%, and at 22% it costs 40.7%.
This is the tax torpedo, and it's the biggest single distortion most retirees face. It has one redeeming feature: it's exhaustible. Once 85% of your benefits are taxable, it's spent, and further conversions drop back to the plain bracket rate. That's exactly the dip in the table above. Sometimes the right move is to push all the way through it rather than stop inside.
Full detail: the Social Security tax torpedo.
The senior bonus deduction — $6,000 per person aged 65+, created by the OBBBA — phases out at 6% of income above $150,000 for a couple. Inside that range, every dollar you convert costs your bracket rate times 1.06, or times 1.12 for a couple where both qualify. That's the 24.6% in the table: 22% × 1.12.
Worth knowing: this deduction expires after 2028. If it's materially widening your conversion room, that window is short.
Qualified dividends and long-term capital gains are taxed after your ordinary income — they stack on top. A conversion is ordinary income, so it slides in underneath and shoves them upward.
For a couple, the 0% capital gains band runs to $98,900 of taxable income. If your dividends were sitting in that band, a conversion can push them into the 15% one. Each converted dollar then costs your bracket rate plus 15 cents of newly-taxed capital gain that you did nothing to trigger.
Two of them, and they're worse than rates because they're cliffs:
Each effect is individually manageable. The problem is that they interact, and they turn on and off at different income levels.
Social Security taxability depends on your other income, which includes the conversion. The senior deduction phase-out depends on income, which includes the taxable part of your Social Security, which depends on the conversion. Capital gain stacking depends on your taxable income after deductions, which depends on both. IRMAA keys off a modified AGI that includes tax-exempt muni interest — which is otherwise invisible on your return.
That's a system of equations, not an arithmetic problem. It's the reason the calculator sweeps every conversion amount and reports the marginal cost at each one, rather than asking you to pick a bracket and trusting the multiplication.
Look at the shape, not the single number. The useful output isn't "convert $167,000", it's the curve: where the cheap regions are, where the walls are, and which effect is responsible for each.
Convert into the valleys, and stop before the walls. If your marginal cost curve has a hump at $30,000 and a flat stretch from $50,000 to $70,000, the flat stretch is where the cheap money is.
Never land just past a cliff. Right up to a threshold is fine. Well past it can be fine. A thousand dollars over is the worst outcome available, because you've paid the entire penalty for almost none of the benefit.
Leave margin. The thresholds you're aiming at are partly estimates — the IRMAA brackets that your 2026 income determines haven't been published yet — and your income isn't fully known until December. A surprise mutual fund capital gain distribution can push you over a line you've already committed to crossing. And conversions can't be undone.