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Form 8606 Calculator

Work out how much of a Roth conversion is taxable under the Form 8606 pro-rata rule.

Updated November 4, 2026. Free, no sign-up; everything runs in your browser and nothing you type is stored.

Form 8606 pro-rata calculator

Basis from prior years, last Form 8606 line 14 ($)Nondeductible contributions this year, line 1 ($)All traditional, SEP and SIMPLE IRAs on December 31 ($)Amount converted to Roth this year ($)Other IRA distributions this year ($)Your tax rate (%)

Taxable part of the conversion

–

–Tax-free part

–Nontaxable percentage

–Estimated tax

–Basis carried forward (line 14)

What Form 8606 does

Form 8606 tracks nondeductible (after-tax) contributions to traditional IRAs, your "basis", so you aren't taxed on that money twice, and reports Roth conversions and Roth IRA distributions. File it for any year you make a nondeductible contribution, convert to a Roth, or take money from an IRA that has basis. Skipping it can cost a $50 penalty and, worse, lost basis.

The pro-rata rule (lines 6-13)

For this form, all your traditional, SEP and SIMPLE IRAs count as one account. Any conversion or distribution is part after-tax basis and part pre-tax money, in proportion:

Nontaxable % = total basis ÷ (December 31 IRA balances + conversions + distributions during the year)

That's why a backdoor Roth is fully tax-free only if you have no other pre-tax IRA money at the end of the year. Rolling pre-tax IRA money into your employer's 401(k) before December 31 is the usual fix.

Worked example

You contribute $7,500 nondeductible and convert it, but also have $92,500 of pre-tax IRA money on December 31. Only 7.5% of the conversion is tax-free: $563 tax-free and $6,937 taxable, and the remaining basis carries forward. With no other IRA money, the whole conversion would be tax-free. See also Roth conversion and traditional IRA limits.

Common questions

What is Form 8606 used for?

Reporting nondeductible IRA contributions (basis), Roth conversions and Roth IRA distributions.

How does the pro-rata rule work?

Conversions are tax-free only in the ratio of basis to all traditional, SEP and SIMPLE IRA money.

Is there a penalty for not filing Form 8606?

$50, and you risk being taxed again on after-tax money.

Sources

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General information, not financial, tax or legal advice. Check important numbers with the official source before acting on them.