How the calculation works
The conversion tax is the difference between federal income tax on your income with and without the converted amount, using 2026 brackets and the standard deduction ($16,100 single, $32,200 married filing jointly). It then grows both options for the years you enter. The Roth side is tax-free; the Traditional side is taxed at your future rate when spent. If you pay the tax from outside savings, the "keep" side is credited with that money invested in a taxable account instead.
State income tax, Medicare IRMAA surcharges (based on income two years earlier) and the taxation of Social Security benefits can all make a conversion costlier than federal tax alone. Check them before converting large sums.
The conversion window
The years between retiring and starting Social Security or RMDs are often the cheapest time to convert, because taxable income drops. Converting then shrinks future required minimum distributions, which can otherwise push you into a higher bracket in your 70s.
Questions people ask
How is a Roth conversion taxed?
The amount you convert from a Traditional IRA or 401(k) is added to your taxable income for the year and taxed at your ordinary rates. There is no 10% penalty on the conversion itself, but each conversion has its own five-year clock for penalty-free withdrawal of the converted amount before 59½.
How much should I convert each year?
A common approach is to "fill up" your current bracket: convert just enough to reach the top of the 12% or 22% bracket without spilling into the next one. The calculator shows the room left in your bracket.
Should I pay the conversion tax from the IRA?
Paying it from outside money is usually better, because the full converted amount keeps growing tax-free. If you are under 59½, tax withheld from the IRA can also count as an early withdrawal.
What is the break-even for a Roth conversion?
If your future tax rate is higher than the rate you pay on the conversion, the conversion wins from day one in after-tax terms. If you pay the tax from outside savings, it also wins at equal rates because you have effectively sheltered more money.
Can I undo a Roth conversion?
No. Recharacterising a conversion has not been allowed since 2018, so plan the amount carefully, often late in the year once your income is known.