Backdoor Roth Pro-Rata Calculator

Form 8606, § 408(d)(2) · reviewed September 5, 2026

Works out how much of a Roth conversion is taxable when you also have other pre-tax IRAs. About

In plain words. A backdoor Roth is a way for higher earners to fund a Roth IRA: put after-tax money into a traditional IRA, then convert it. The catch is that the IRS treats all your traditional IRAs as one pot. If the pot also holds pre-tax money, the conversion is taxed in proportion to it, and you cannot choose to convert only the after-tax part. This tool does that proportion the way the tax form does.

Why it matters. People do a backdoor Roth expecting no tax and get a surprise bill. Checking the pot first, and emptying it into a 401(k) if possible, is what makes the move clean.

An example. You convert $7,500 of after-tax money, but you also have $92,500 in an old pre-tax IRA. Only 7.5 percent of the pot is after-tax, so about $6,900 of the $7,500 conversion is taxable.

Where it stops. It assumes one conversion and no other IRA withdrawals that year. Rolling the pre-tax money into a 401(k) before December 31 changes the answer completely, and a spouse's IRAs are a separate pot. Everything it leaves out.

All your other traditional, SEP, and SIMPLE IRA balances on December 31

Add them all together, as of the end of the year you convert. A 401(k) does not count, and your spouse's IRAs are counted separately.

Amount converted to Roth this year

After-tax money in your IRAs (your basis)

Contributions you never deducted, so they were already taxed once. Earlier years' total is on your last Form 8606, plus this year's contribution.

Your tax rate on the next dollar of income

Used only for the tax figure. The share that is taxed does not depend on it.

Part of the conversion that is taxed

$6,938

92.5% of every converted dollar counts as taxable income. The law treats all your IRAs as one $100,000 pool, and after-tax money is only 7.5% of it, so the conversion is split in the same proportion. At 24% the tax comes to $1,665, and $6,938 of after-tax money stays behind in the pool for later years.

Part of the conversion not taxed: $563; Shown on Form 8606, line 17
Tax on the conversion at 24%: $1,665; Your rate on the $6,938 that is taxed
After-tax money left for future years: $6,938; Goes on next year's Form 8606, line 14

You cannot get around the rule by converting only the account that holds the after-tax money. The law counts every IRA you own as one pool. You can get around it by moving the pre-tax money out of the pool first. A rollover into a 401(k) that accepts one, finished before December 31 of the year you convert, leaves only after-tax money in the pool, and the next conversion is tax-free.

This follows the Form 8606 rule for splitting a conversion between after-tax and pre-tax money, using every IRA balance at year end plus the amount converted. It leaves out inherited IRAs, employer plans, and your spouse's accounts, which are not in the pool, and any other withdrawal in the same year, which would share the same split. The tax figure is simply your rate times the taxed part. Educational, not advice.

This is a simplified model, not your actual tax return or plan. It only knows what you type in, leaves out rules that may apply to you, and cannot weigh the other facts and trade-offs a real decision depends on. Before you act, talk with a professional who knows your whole situation.

Educational only, not investment, tax, or legal advice. Results are hypothetical estimates that vary with each use and over time and are not guaranteed accurate or complete. Using this tool creates no client relationship with Considerate Capital, and the site that linked here is not affiliated with it. By using it you agree to the tool terms.