IRA pro-rata rule calculator (backdoor Roth)

The idea in one sentence

The IRS does not let you pick which dollars come out of your IRAs: every conversion or withdrawal is taxed in proportion to the share of pre-tax money in all of your traditional IRAs together. That share is what separates a nearly tax-free backdoor Roth from one with an unexpected bill.

The formula (Form 8606, Part I)

• Line 3: total basis = nondeductible contributions this year (line 1) + basis from earlier years (line 2).
• Line 6: value of all traditional, SEP and SIMPLE IRAs at December 31 (after the conversion).
• Line 7 / line 8: other distributions / amount converted.
• Line 9 = line 6 + line 7 + line 8. Line 10 = line 5 ÷ line 9, at least three decimal places, and not more than 1.
• Line 11 = line 8 × line 10, the nontaxable part of the conversion. Line 18 = conversion − line 11 is the taxable amount, carried to Form 1040.
• Line 14 = line 3 − (line 11 + line 12) is your basis left for later years.

The calculator uses the exact ratio (more than three decimals, which the form allows) and shows the three-decimal figure for the form.

Worked example

You contribute $7,500 nondeductible to a traditional IRA and convert $7,500 the same year, but you have $93,000 left in a rollover IRA at year-end. Line 3 is $7,500; line 9 is $93,000 + $7,500 = $100,500. The fraction is 0.0746, so $560 of the conversion is tax-free and $6,940 (92.5%) is taxable; at a 24% rate that is about $1,666. These are example numbers, not typical ones.

Common mistakes and limits

• Looking only at the account you converted from. The rule counts every traditional, SEP and SIMPLE IRA you own, including old rollovers. Check all statements for December 31.
• Counting the value before the conversion. Line 6 is the year-end value after the money has moved; the conversion itself is added back on line 8.
• Forgetting Form 8606. You must file it for the year of a nondeductible contribution and for a conversion; it is also how the basis is carried forward.
• Assuming the rule is optional. You cannot treat the conversion as coming from the after-tax dollars alone.
• Federal tax only. The tax amount uses the single rate you enter. A larger taxable part can lift you into a higher bracket and raise state tax; try the Roth conversion tax calculator for bracket-by-bracket detail.

What is the pro-rata rule?

When you convert money from a traditional IRA to a Roth IRA, you cannot choose to convert only your after-tax dollars. The IRS treats all your traditional IRAs as one pot (the aggregate) and each conversion or distribution as a mix of after-tax basis and pre-tax money in proportion to the pot. On Form 8606 the proportion is your total basis (line 3 or 5) divided by the sum of the December 31 value of all your traditional IRAs (line 6), distributions (line 7) and conversions (line 8): line 10 = line 5 ÷ line 9. The nontaxable part of a conversion is the conversion times that fraction (line 11); the rest is taxable (line 18). With $7,500 of after-tax basis, $93,000 of pre-tax money left at the end of the year and a $7,500 conversion, the fraction is 0.0746 and $6,940 of the conversion is taxable.

What is a backdoor Roth and why does pro-rata matter?

A backdoor Roth is a way for a person whose income is too high to contribute to a Roth IRA directly: you make a nondeductible contribution to a traditional IRA and then convert it. If you have no other traditional, SEP or SIMPLE IRA money, nearly all of the conversion is basis and the tax is small: with a zero year-end balance, the fraction is 1 and the taxable part is $0. If you still hold pre-tax IRA money on December 31, the rule takes part of the conversion as taxable. Leaving only $7,500 of pre-tax money makes 50% of the conversion taxable; leaving $93,000 makes 92.5%. The IRA contribution limit for 2026 is $7,500.

Which accounts count in the aggregate?

All of your traditional IRAs, which the IRS says includes traditional SEP IRAs and traditional SIMPLE IRAs, valued on December 31 of the conversion year. Roth IRAs do not count, and neither do employer plans such as a 401(k) or 403(b). Each spouse is figured separately: a spouse's IRA is not added to yours, and each spouse files their own Form 8606. Inherited IRAs are also kept separate from your own. Count outstanding rollovers: money distributed from a traditional IRA near the end of the year and not yet redeposited is added to line 6, except an outstanding rollover to an employer plan, which the instructions say not to include.

Can I avoid the pro-rata rule?

The usual approach is to reduce the pre-tax IRA money before December 31. The Form 8606 instructions leave an outstanding rollover from a traditional IRA to a qualified retirement plan out of line 6, and an employer plan balance is not part of the aggregate, so rolling pre-tax IRA dollars into an employer plan (if the plan accepts them) can leave only basis in the IRAs: in the example the taxable part would fall from $6,940 to $0. A traditional SIMPLE IRA can be rolled into a qualified plan only after two years of participation, according to the same instructions. This page only describes the idea; whether it works for you depends on your plan, the timing and the paperwork, so ask a tax professional.

What does the IRS example look like?

Publication 590-B (2025) works an example in Worksheet 1-1: total basis and contributions of $2,300, a December 31 value of $20,000 and a $5,000 distribution converted to a Roth IRA. The total is $25,000, the fraction is 0.092, the nontaxable part is $460 and the taxable part is $4,540. Enter basis $300 from earlier years, $2,000 this year, $20,000 for the year-end value and a $5,000 conversion to get the same result.

What happens to the basis that is not used?

It stays on your Form 8606. In the example the basis falls by the nontaxable part of the conversion: $7,500 less $560 leaves $6,940 to be used in later years (line 14). That is why people with pre-tax IRA money keep paying tax on a share of every later conversion. Keep your Forms 8606 for every year you made a nondeductible contribution: the IRS asks for your basis carried forward from the last Form 8606 you filed.

Does the rule apply to contributions made after the end of the year?

The December 31 value on line 6 is the value at the end of the year of the conversion, and contributions for that year made between January 1 and April 15 of the following year are treated differently on lines 4 and 5 of the form. This calculator uses the simple case where the contribution is part of your basis for the year. If you make a contribution in the next year's spring for the previous year, or you have a mid-year recharacterization, work through Form 8606 itself.

Estimates for planning only — not tax or financial advice. Official forms and your tax professional decide. How we check our calculators.

Sources, checked October 28, 2026: Instructions for Form 8606 (2025), lines 1, 2, 6, 8 and 14, definitions of traditional IRA (includes traditional SEP and SIMPLE IRAs); IRS Publication 590-B (2025), chapter 1, Worksheet 1-1 and the Rose Green example, and the illustrated Form 8606. The 2026 forms were not yet published when this page was checked; the mechanics are those of the 2025 forms. Related: Roth conversion tax calculator, Roth IRA calculator, traditional IRA calculator and 401(k) calculator. How we check our calculators.