U.S. · Tax forms
Form 8606 line 10, pro-rata percentage.
How the decimal is built and what it does to lines 11 and 12.
Line 10 is line 5 (your remaining nondeductible basis) divided by line 9 (the year-end IRA value plus distributions plus conversions). Round to at least 3 decimal places; if the result is 1.000 or more, enter 1.000. It is the share of a conversion or withdrawal that is tax-free.
Look up a Form 8606 situation
Result
The pro-rata lines (2025 form)
| Line | Meaning | What the form says |
|---|---|---|
| 5 | Basis available | Line 3 minus line 4 (contributions made January 1 to April 15, 2026 are taken out) |
| 6 | Year-end IRA value | Total traditional IRA value on December 31, 2025, plus outstanding rollovers, less certain repayments |
| 7 | 2025 distributions | Distributions from traditional IRAs in 2025, not rollovers or conversions |
| 8 | Net conversion | Net amount converted from traditional IRAs to Roth IRAs in 2025 |
| 9 | Total pool | Add lines 6, 7 and 8 |
| 10 | Percentage | Divide line 5 by line 9. Decimal to at least 3 places. If 1.000 or more, enter 1.000 |
| 11 and 12 | Tax-free pieces | Line 8 times line 10 (conversion). Line 7 times line 10 (distributions not converted) |
Why the denominator includes conversions and distributions
Form 8606 treats your year-end balance as if the money withdrawn or converted had not left. Adding lines 7 and 8 back to line 6 on line 9 means the basis is spread across everything you held, the form’s way of applying the pro-rata rule. Line 13 adds the two tax-free pieces and line 14 subtracts that from line 3 to leave the basis carried to next year.
Rounding
The form says to enter the result as a decimal rounded to at least 3 places. Using more places is allowed by those words. Using fewer is not. This page rounds to three places in its examples.
A pure backdoor Roth case
If your only traditional IRA money is a nondeductible contribution you converted, line 6 is 0 after the conversion, so line 9 equals your basis in line 5 and the percentage hits 1.000. That means the whole conversion is basis and line 18 is zero. Any other pre-tax IRA money enlarges line 9 and shrinks line 10. A full calculator is linked below.
Worked example
Maya has $7,000 of basis (line 5), converts $7,000 (line 8), takes no distributions (line 7 is $0) and has no other IRAs, so line 6 is $0. Line 9 is $7,000 and line 10 is 1.000. Line 11 is $7,000 and the taxable amount is $0. If Maya had $93,000 in another traditional IRA at year end, line 9 would be $100,000, line 10 would be .070, line 11 would be $490 and the taxable part of the conversion would be $6,510. Names and figures are made up.
More Form 8606 line pages
Other pages in this set: Form 8606 Line 2, Form 8606 Line 6, Form 8606 Line 8, Form 8606 Line 14, Form 8606 Line 18, Form 8606 Line 25c.
Sources
- Instructions for Form 8606 (2025): the line-by-line instructions on this page. Page last reviewed or updated April 30, 2026. Read November 2, 2026.
- Form 8606 (2025): the printed line wording and arithmetic on the form. Read November 2, 2026.
Tax year 2025 edition. Line numbers and dollar limits are those printed in the 2025 instructions and form and change by year. Names in worked examples are made up.
Frequently asked questions
What does Form 8606 line 10 calculate?
Line 5 divided by line 9, the nontaxable share of your 2025 distributions and conversions.
How many decimals do I use?
At least 3 places. If the result is 1.000 or more, enter 1.000.
Why is line 8 added into line 9?
Line 9 is lines 6, 7 and 8 added, so the pool includes what you withdrew or converted as well as what remains.
Do my Roth IRA balances count in line 6?
Line 6 is the value of your traditional IRAs per the form and the instructions. Roth IRA balances are not part of it.
Is there a calculator for this?
Yes. The backdoor Roth pro-rata calculator linked below runs the same arithmetic.
Not tax advice
This page summarizes IRS instructions, mostly written for the people who prepare the form, and is not tax, legal or financial advice. A CPA or an enrolled agent can confirm what a line means for your return. Everything runs in your browser; nothing you enter is sent anywhere.