Roth Conversion Calculator: Bracket Room and Tax Cost

By Tom, CPA · Updated October 25, 2026

TL;DR: Enter your other income, gains, a proposed conversion and the bracket you want to fill. The calculator shows the room to the top of that bracket and the added federal tax from the conversion, including any qualified dividends or gains pushed from 0% to 15%.

Educational only, not tax, legal, or investment advice. Examples are illustrations, not predictions. Rules and limits change; verify current figures and talk to a qualified professional about your situation.

Convert until it hurts, but only a little. A Roth conversion moves money from a traditional IRA to a Roth IRA; you pay ordinary income tax now so growth and withdrawals can be tax-free later. The question is always how much. This calculator answers two parts: how much room is left in a bracket, and what a given conversion actually costs.

CPA Warning Label: Conversions are permanent; there is no recharacterization. A conversion also raises MAGI for ACA credits and IRMAA two years later.

Inputs

• Filing status: 1 single, 2 married filing jointly.
• Ordinary income before conversion: IRA withdrawals, pensions, interest, taxable Social Security.
• Qualified dividends and long-term gains.
• Conversion amount.
• Target bracket rate: 10, 12, 22, 24, 32 or 35. The room is measured to the top of that bracket.
• Extra deduction.

Bracket room computation

Room = top of the target bracket − (ordinary income − deductions). Gains do not count because they stack above ordinary income. 2026 bracket tops for the 12% bracket are $50,400 single and $100,800 married filing jointly; for 22%, $105,700 and $211,400 (IRS Rev. Proc. 2025-32 as amended). The standard deduction ($16,100 or $32,200) adds to the room when ordinary income is small.

Marginal tax cost

The calculator computes total tax with and without the conversion. The difference is the added tax; divided by the conversion it is the cost rate. If you have gains, a conversion can push them from 0% to 15%, so the cost rate can exceed your bracket rate. That hidden cost is the main reason to run the numbers rather than eyeball a bracket chart.

Sample runs

Illustration 1. Married, no other income, converting $133,000 (the 12% bracket top plus the standard deduction). Added tax: $11,600, a cost rate of 8.72%.

Illustration 2. Married, $40,000 of ordinary income and $30,000 of gains, converting $50,000. Room to the top of 12%: $93,000. Added tax $5,660, cost 11.32%; gains still fit under the 0% threshold.

Illustration 3. Single, $50,000 of ordinary income, converting $20,000. Added tax: $2,750, a 13.75% cost rate, because part lands in 12% and part in 22%.

Choosing a target bracket

The usual targets are the top of the 12% bracket for early retirees with modest spending and the top of the 22% bracket for those with large pre-tax balances, who face big required minimum distributions later. The comparison is your expected future marginal rate: if RMDs, Social Security and a survivor's single filing status will push you into 22% or higher later, converting at 12% now is attractive, and converting at 22% may be too.

Remember that the surviving spouse files single with roughly half the bracket widths, which is one of the strongest arguments for converting while both spouses are alive. Run the calculator as single to see what a widow or widower would face.

Paying the tax

Pay conversion tax from a taxable account if possible, so the full conversion stays in the Roth. Paying it from the IRA shrinks the conversion and, if you are under 59½, the withheld part may face a 10% additional tax. Make an estimated payment or increase withholding to avoid an underpayment penalty.

ACA and IRMAA warnings

As of Oct 2026, households above 400% of the federal poverty level receive no premium tax credit, and excess advance credits must be repaid in full after 2025 (IRS). A conversion that crosses that line can cost more in lost subsidy than in tax. At 63 and later, Medicare IRMAA uses MAGI from two years earlier. The calculator does not model either; check them before converting. See the Roth conversion ladder.

Limits

• Uses 2026 federal brackets, standard deductions ($16,100 single, $32,200 married filing jointly) and capital gains thresholds from IRS Rev. Proc. 2025-32 as amended by the 2025 tax law. Head of household and married filing separately are not modeled.
• Ignores state tax, credits, the 3.8% net investment income tax, AMT, and itemized deductions (enter any excess over the standard deduction as an extra deduction).
• Does not compute taxable Social Security; use the Social Security taxable benefits calculator first and enter the taxable portion as ordinary income.
• Default inputs are examples, not recommendations.

Lavish Larry converted his entire IRA in one year to "get it over with." Frugal Fran converted to the top of 12% each year and paid about a third less.