Calculate the federal tax owed on converting Traditional IRA assets to Roth IRA. Use bracket fill-up logic to optimize for low-income years.
Total tax owed on conversion
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Federal tax on conversion
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State tax on conversion
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Marginal rate on conversion
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Net amount entering Roth
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Paying tax now to never pay it again
A Roth conversion moves money from a traditional IRA, where it grows tax-deferred and is taxed on the way out, into a Roth IRA, where it grows and comes out tax-free forever. The catch is that the converted amount is added to your taxable income this year, so you write a check to the IRS now in exchange for never paying tax on that money or its growth again. The whole strategy turns on one comparison: is your tax rate today lower than the rate you expect in retirement? If yes, a conversion is usually a win. There is no annual limit and no income limit on conversions, unlike contributions, so the only real constraint is the tax bill you are willing to trigger.
This calculator does the part people get wrong by hand. It computes your tax on your other income alone, then on your other income plus the conversion, and reports the difference. That stacking approach captures how a conversion can climb through multiple brackets, which a single flat rate never shows.
Filling the 12 percent bracket: a blended-rate example
Set filing status to single, other taxable income to $40,000 (this input is already net of your standard or itemized deduction), and a conversion of $40,000. Your existing income sits inside the 12 percent bracket, whose top for a single filer in 2026 is $48,475. The conversion fills the rest of that bracket and then spills into the 22 percent bracket. Here is how the cost breaks apart.
| Slice of the conversion | Bracket | Tax |
|---|---|---|
| $40,000 up to $48,475 ($8,475) | 12% | $1,017 |
| $48,475 up to $80,000 ($31,525) | 22% | $6,936 |
| Total federal tax on the conversion | blended | $7,953 |
The full $40,000 still lands in the Roth; you pay the $7,953 from outside funds. That works out to a 19.9 percent blended rate on the conversion, even though the last dollar was taxed at 22 percent. The chart shows why the blended cost is the number that matters.
Marginal beats average here
The reason planners obsess over "bracket fill-up" is visible in that example. Converting just the $8,475 that fits inside the 12 percent bracket would have cost only 12 percent. Pushing another $31,525 into the 22 percent band nearly doubled the rate on that slice. So the disciplined move is often to convert only enough to reach the top of a target bracket each year, then stop, and repeat next year. In a low-income window, an early-retirement gap year before pensions and Social Security begin, or a sabbatical, those bracket-fill conversions can be remarkably cheap. The tool lets you test exactly where the rate jumps for your filing status.
Side effects beyond income tax
A conversion raises your adjusted gross income, and several things key off that number. If you are on a Marketplace plan, a large conversion can shrink or wipe out your premium tax credit. If you are 63 or older, the conversion can lift your modified AGI two years later and bump your Medicare Part B and Part D premiums through IRMAA surcharges. It can also push more of your Social Security into taxable territory and raise the rate on your long-term capital gains. None of these appear in the headline tax figure, so a smart conversion plan looks at the whole AGI ripple, not just the income tax. My practical rule: size each year's conversion to a bracket ceiling and to an IRMAA threshold, whichever binds first.
Can I undo a conversion if my situation changes?
No. The Tax Cuts and Jobs Act eliminated Roth recharacterizations of conversions starting in 2018, so a conversion is permanent once done. That is why sizing it correctly the first time matters. A common tactic is to convert late in the year, when your income for the year is nearly certain, so you do not accidentally overshoot a bracket you were trying to fill.
Should the tax come out of the IRA or from other money?
Pay it from outside the IRA whenever you can. If you withhold the tax from the conversion itself, that withheld amount never reaches the Roth, and if you are under 59 and a half it is also treated as an early distribution subject to the 10 percent penalty. Paying with taxable-account cash lets the entire converted balance compound tax-free, which is the whole reason to convert.