Find the optimal Roth conversion to fill a target bracket.
Optimal conversion amount
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Tax owed on conversion
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Your breakdown
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Filling a bracket without spilling over
Federal income tax is progressive, so income is taxed in layers. The dollars that sit inside the 22% band are taxed at 22%, and only the dollars above the band's ceiling jump to 24%. That structure creates an opportunity. In a low-income year, you can deliberately recognize extra income, usually by converting a traditional IRA to a Roth, right up to the top of your current bracket and pay tax at that rate, while stopping short of the higher rate on the next dollar. This calculator measures the room you have between your current taxable income and a bracket ceiling, then prices the tax on filling it.
One framing note that keeps the math honest: the inputs here are taxable income, the figure after your deductions, not gross income. For 2026 the standard deduction is $16,100 for a single filer and $32,200 for a married couple filing jointly, and you subtract that before you arrive at the taxable income this tool expects. Enter the post-deduction number so the bracket ceiling lines up with the actual IRS thresholds.
The room, and the tax on it
The calculation is deliberately simple so the lever is obvious. It subtracts your current taxable income from the bracket ceiling to find the headroom available to convert, then multiplies that headroom by the bracket's marginal rate to estimate the tax the conversion adds. Every dollar you convert inside the band is taxed at that flat marginal rate, which is the whole point of the strategy: you are buying Roth dollars at a known, controlled price.
Filling the 22% bracket to $105,700
Say your taxable income is exactly $50,400, the start of the 22% band for a single filer in 2026. The band ends at $105,700, leaving $55,300 of headroom. Converting that amount keeps the conversion inside the 22% band, for estimated federal income tax of $12,166. This isolates bracket tax only; credits, deductions, IRMAA, ACA subsidies, and state tax can change the real marginal cost.
The traps a flat-rate fill can hide
This tool isolates the bracket math, which is the cleanest way to see the core decision, but a real conversion touches other parts of the return that move at their own thresholds. Added income can push more of your Social Security benefits into taxable territory, raise your Medicare Part B and D premiums two years later through IRMAA, and shrink the income-based ACA premium subsidy if you buy marketplace coverage. None of those show up in a simple bracket fill, yet each can quietly raise the true marginal cost of the conversion above the headline rate. The practical move is to fill the bracket first in this tool, then check those secondary thresholds before you pull the trigger.
The biggest unforced error is converting in December without leaving a cushion. If a year-end mutual fund capital-gains distribution or a late bonus lands after you convert, you can accidentally spill into the next bracket and tax part of the conversion at the higher rate. Pros leave a few thousand dollars of headroom, then top up the conversion in the final days once the year's income is settled. Roth conversions also became permanent once recharacterization was eliminated, so unlike the old days you cannot undo an overshoot, which makes the cushion essential.
Which bracket should I aim to fill?
It depends on the rate you expect to face in retirement. If you are temporarily in the 12% or 22% bracket now, perhaps in an early-retirement gap year before Social Security and required minimum distributions begin, filling that bracket converts dollars cheaply that would later be taxed at a higher rate once RMDs stack on top of benefits. If you are already in a high bracket, the case is weaker. The rule of thumb is to convert when your current marginal rate is at or below your expected future rate.
Where should the tax payment come from?
Ideally from outside the retirement account. Paying the conversion tax with taxable savings lets the full converted balance land in the Roth and compound, which is what makes the strategy powerful. If you withhold the tax from the IRA itself, you convert less and, if you are under 59 and a half, the withheld amount is treated as a taxable distribution that can carry a 10% early-withdrawal penalty. Funding the tax with separate cash is almost always the stronger play.