Taxable part of a backdoor Roth conversion by the pro-rata rule, line by line on Form 8606
Estimates only, not financial advice. What you type stays on your device. Checked against sources on 29 Sept 2026.The pro-rata rule makes 92.5% of a backdoor Roth conversion taxable when $7,500 of nondeductible basis is converted while $92,500 of pre-tax money sits in traditional, SEP and SIMPLE IRAs on December 31: $6,937.50 is taxed, $1,665 at 24%. The calculator runs Form 8606 lines 1 to 18; with no pre-tax balance, none of it is taxed.
Rates and rules read from the sources listed on this page (the earliest on 29 Sept 2026); they may have changed since.
Pick the Tax year, then enter Line 1: nondeductible contribution and Line 2: basis from last year's line 14.
Enter Line 8: amount converted and Line 6: all IRAs on Dec 31, counting every traditional, SEP and SIMPLE IRA.
Add Line 7: other withdrawals and Your marginal tax rate, and turn on 50 or over by December 31 if it applies.
Read the Taxable part of the conversion (line 18) and the tax on it; the lines follow the IRS Form 8606 listed under Sources.
Compare Taxable if IRAs empty on Dec 31, then press Share result to copy a link.
A backdoor Roth is a nondeductible contribution to a traditional IRA followed by a conversion to a Roth IRA. It is tax free only when the traditional IRAs hold nothing but that after-tax money. If you also have pre-tax money in any traditional, SEP or SIMPLE IRA, the IRS treats every conversion as a mix of both, in proportion. That is the pro-rata rule, and Form 8606 is where it is worked out.
The December 31 rule. The proportion uses the value of all your traditional, SEP and SIMPLE IRAs on December 31 of the year of the conversion (line 6), not the value on the day you converted. Pre-tax money that is still in an IRA at year end counts even if you opened that account after converting. Moving pre-tax IRA money into an employer 401(k) that accepts it, before December 31, takes it out of line 6; the result shows what the conversion would owe if line 6 were zero.
The form's arithmetic. Your basis (line 5) is divided by the year-end value plus the year's distributions and conversions (line 9). That ratio, line 10, is rounded to at least three decimal places and capped at 1.000. The converted amount times the ratio is tax free (line 11); the rest is taxable (line 18). Basis not used carries to next year on line 14.
Tax year. The lines are those of Form 8606 (2025). The 2026 form is published early in 2027, so the 2026 figures use the same lines with the dates moved on a year. The IRA contribution limit is $7,500 for 2026 ($8,600 at 50 or over) and $7,000 for 2025, and the page warns if line 1 is above it.
What this does not do: inherited IRAs, Roth IRA withdrawals (Part III), recharacterizations or disaster distributions. The tax figure uses your own marginal rate. An estimate, not tax advice.
When your traditional, SEP and SIMPLE IRAs hold both after-tax and pre-tax money, every conversion is treated as a mix of the two in proportion. Form 8606 divides your after-tax basis by the year-end IRA value plus the year's conversions and distributions, and that share of the conversion is tax free.
Form 8606 line 6 asks for the value of all your traditional, SEP and SIMPLE IRAs on December 31 of the conversion year, plus outstanding rollovers. The balance on the day you converted does not matter, and Roth IRAs and employer plans such as a 401(k) are not included.
The rule only bites when pre-tax money is in an IRA on December 31. Some people move pre-tax IRA money into an employer 401(k) that accepts roll-ins before year end, which takes it out of line 6. Check with the plan first; this page shows the tax both ways.
Line 10 is line 5 divided by line 9, entered as a decimal rounded to at least three places, and capped at 1.000. This calculator rounds to three places, which is the minimum the instructions allow. More places change the answer only by a few dollars on a typical conversion.
Line 14 carries your remaining after-tax basis forward. Next year it goes on line 2, and it is recovered tax free as later conversions and withdrawals are split by the same pro-rata rule. Keep every year's Form 8606, because the IRS relies on it to know your basis.
The 2026 limit is $7,500, plus a $1,100 catch-up at age 50 or over, according to IRS Notice 2025-67. For 2025 the figures were $7,000 and $1,000. The limit covers traditional and Roth IRA contributions together, and a contribution for a year can be made until mid-April of the next year.
Rates and rules change; check the source before you rely on a figure.
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Also works as: Form 8606 Calculator.
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