How this calculator works
Tax year: 2026, United States. The converted amount is added to your other taxable income and taxed through the ordinary brackets, so the tax is the difference between the tax with and without the conversion. This captures the way a large conversion can push income into higher brackets. The Roth balance grows tax-free and is compared with the traditional balance after the retirement tax rate you enter. If tax is paid from other savings, the whole balance moves to the Roth. If it is withheld from the conversion, the Roth starts smaller. The model assumes both accounts earn the same return, ignores the extra money tied up when tax is paid from savings, and leaves out the five-year rules, the early-withdrawal penalty, required minimum distributions, state income tax, the net investment income tax and any other income changes. It is not tax advice.
Formula
Tax on conversion = tax(other taxable income + converted amount) - tax(other taxable income), using the filing-status brackets. Roth value = converted amount x (1 + return)^years, or (converted amount - tax) x (1 + return)^years when tax is withheld. Traditional after-tax value = converted amount x (1 + return)^years x (1 - retirement tax rate). Advantage = Roth value - traditional after-tax value.
Worked example
A single filer with $60,000 of other taxable income converts $50,000. Tax rises from $7,912 to $18,998, so $11,086 is due, an effective 22.17% of the conversion. With the tax paid from savings and a 6% return for 20 years, the Roth is worth $160,356.77. The traditional balance of the same size is $160,356.77 before retirement tax, or $125,078.28 after 22% tax, so the Roth advantage is $35,278.49.
Frequently asked questions
Is there a penalty for converting?
No. A conversion is taxed but is not an early distribution, so the 10% additional tax does not apply at the time of conversion. Withdrawing converted money within five years and before age 59 and a half can still trigger the penalty.
Should I pay the tax from savings or withhold it?
Paying from savings moves the full balance into the Roth and is usually preferred when the money is available. Withholding reduces the amount that reaches the Roth and can also create a taxable distribution if you are under 59 and a half.
Does a conversion make sense in a lower-income year?
It can. The tax is calculated at the brackets that apply in the conversion year, so a year with lower income may allow more of the conversion to be taxed at lower rates.
What about required minimum distributions?
Roth IRAs have no required minimum distributions for the original owner, while traditional IRAs do. That difference is not valued here, so the Roth advantage shown is only the tax-rate difference.