Backdoor Roth pro-rata calculator.

A backdoor Roth is two steps: put after-tax money in a traditional IRA, then convert it to a Roth IRA. The IRS does not let you choose which dollars you convert. Form 8606 adds up every traditional, SEP and SIMPLE IRA you own at the end of the year and taxes the conversion in the same proportion as pre-tax money sits in that pile. This calculator runs those lines so you can see the bill before you convert.

How the pro-rata rule is calculated

Your basis is the after-tax money you have put into traditional IRAs: this year's nondeductible contribution plus the basis carried from earlier years (Form 8606 lines 1 and 2, added on line 3). That basis is divided by the total of three things: the December 31 value of all your traditional, SEP and SIMPLE IRAs, any distributions you took, and the amount you converted. The conversion and the distributions are added back because they left the accounts during the year. The result is the share of every dollar withdrawn or converted that comes out tax-free.

Multiply the converted amount by that share and you get the tax-free part (Form 8606 line 17). The rest is taxable (line 18). Whatever basis you did not use is carried forward on line 14. The calculator shows each of these and a tax estimate at the rate you enter.

Two points follow from how the lines work. The test uses the value on December 31, not the day you convert, so a conversion in March is judged by a balance you will not know until year end. And money in an employer plan such as a 401(k) is not part of the Form 8606 total, because the form counts IRAs.

Worked examples (hypothetical figures, real rules)

Clean backdoor. You contribute $7,500, convert $7,500 and own no other IRA money. Basis is $7,500, the denominator is $7,500, the ratio is 100% and the conversion is tax-free.

With $50,000 of pre-tax IRA money left. Same $7,500 contribution and conversion, and $50,000 stays in a rollover IRA on December 31. The denominator is $50,000 + $7,500 = $57,500, so the ratio is 13.043%. Only $978.26 of the conversion is tax-free and $6,521.74 is taxable, about $1,565.22 at 24%. The unused basis of $6,521.74 stays with the pre-tax IRA for later years.

Converting everything does not escape it. If you convert the whole $57,500, the ratio is the same, so $50,000.00 is taxable and all $7,500.00 of basis is used. The tax is $12,000.00 at 24%, the price of moving the pre-tax money into a Roth, not a penalty for the backdoor.

Mistakes people make

• Judging the pro-rata rule by the balance on conversion day. The form uses the December 31 value.
• Forgetting SEP and SIMPLE IRAs. They count in the pile even though you never contribute to them yourself.
• Skipping Form 8606. The form is how basis is tracked. Without it the IRS treats the whole conversion as taxable.
• Ignoring earnings. If the contribution earns money before you convert, the earnings are taxable, because they are not basis.

Who this is for, and what it does not cover

It suits high earners who contribute to a traditional IRA without a deduction and convert. It does not model contributions made between January 1 and April 15 for the prior year (Form 8606 line 4), outstanding rollovers, qualified charitable distributions, disaster distributions, the five-year rule for converted money, or state tax. Whether an employer plan accepts money rolled in from an IRA is a plan decision to confirm with the administrator.

Sources

Retrieved 2026-10-26. Each link was opened and checked against the claim next to it.

• IRS Instructions for Form 8606 (2025): what goes on lines 1, 2, 6, 7 and 8 and the Part I and Part II headings.
• IRS Publication 590-B (2025): Distributions from IRAs: Worksheet 1-1, the basis-over-total ratio and the 2025 Form 8606 illustration.
• IRS Notice 2025-67: 2026 amounts relating to retirement plans and IRAs: the 2026 IRA contribution limit of $7,500 and the $1,100 catch-up.