Updated October 18, 2026. Quick answer: Lines 9 to 14 are arithmetic, with nothing to look up. Line 9 adds lines 6, 7 and 8. Line 10 divides line 5 by line 9, which gives the basis fraction. Lines 11 to 13 apply it to your conversions and distributions, and line 14 is the basis left over for next year.
IRS worksheets read line by line: Pub 590-B Worksheet 1-1.
What each line asks for
The form gives each step in its caption. Line 9 reads “Add lines 6, 7, and 8”. Line 10 reads “Divide line 5 by line 9. Enter the result as a decimal rounded to at least 3 places.” The form adds “If the result is 1.000 or more” to enter 1.000 on the line. Line 11 reads “Multiply line 8 by line 10.” and calls the result “the nontaxable portion of the amount you converted to Roth IRAs.” The instructions have no separate paragraphs headed Lines 9, 10, 11, 12 or 13, so the form’s captions carry the rule.
Lines 12, 13 and 14
Line 12 reads “Multiply line 7 by line 10.” and calls it “the nontaxable portion of your distributions that you did not convert to a Roth IRA.” Line 13 reads “Add lines 11 and 12.” and calls the sum “the nontaxable portion of all your distributions”. Line 14 reads “Subtract line 13 from line 3.” and calls it “your total basis in traditional IRAs for 2025 and earlier years”.
The instructions for line 14 say: “Enter the amount on line 3 reduced by the amount on line 13.” They add a rule if you used the Line 15c Worksheet: “If you completed the 2025 Line 15c Worksheet, include the amount, if any, from line 6 of the worksheet on line 14 of Form 8606 in the next year after 2025 you report traditional IRA distributions on Part I of Form 8606.” That worksheet is covered on the Form 8606 page. Follow the worksheet by name rather than from a summary.
Where the numbers come from
Lines 5, 6, 7 and 8 feed this block. They are covered on the lines 3, 4 and 5, line 6, line 7 and line 8 pages. The idea behind lines 10 to 13 is the same one the backdoor Roth pro-rata calculator uses to show how pre-tax IRA money affects a conversion.
An illustration of the arithmetic
This is a computed illustration, not an IRS example, of how the lines connect. Suppose line 5 is $6,000, line 6 is $94,000, line 7 is $0 and line 8 is $10,000. Line 9 is $94,000 plus $0 plus $10,000, or $104,000. Line 10 is $6,000 divided by $104,000, which is 0.0577 and rounds to .058 on three places. Line 11 is calculated as $10,000 times .058, or $580. Line 12 is $0 times .058, or $0, so line 13 is $580. If line 3 were also $6,000, line 14 would be calculated as $6,000 minus $580, or $5,420. Your own numbers will differ.
Where it goes next
Line 11 is entered again on line 17: “Also, enter this amount on line 17”. Line 12 feeds line 15a, which reads “Subtract line 12 from line 7”. See the lines 15a and 15b page and the lines 16, 17 and 18 page. Line 14 becomes the starting basis for next year’s line 2, covered on the line 2 page.
Edition and line numbers
This page reads the 2025 Form 8606 and its 2025 instructions, the current files irs.gov served on 2026-10-18. They are the files for the tax year 2025 return, filed in 2026. The form says “Attach to 2025 Form 1040, 1040-SR, or 1040-NR.” Check the tax year printed on the form you are filling in and use the line numbers on that year’s form, because a different year’s form can number or word its lines differently.
Common mistakes
- Rounding line 10 to fewer than three places. The form says to enter “the result as a decimal rounded to at least 3 places.”
- Entering more than 1.000 on line 10. The form says “If the result is 1.000 or more” to enter 1.000.
- Forgetting the worksheet adjustment on line 14. The instructions say “If you completed the 2025 Line 15c Worksheet, include the amount, if any, from line 6 of the worksheet on line 14 of Form 8606 in the next year after 2025 you report traditional IRA distributions on Part I of Form 8606.”
Related: Form 8606, backdoor Roth pro-rata calculator, clearing pre-tax IRA money first, and Roth conversion taxes.
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Answer againSources
- Instructions for Form 8606, Nondeductible IRAs (2025): https://www.irs.gov/pub/irs-pdf/i8606.pdf
- Form 8606, Nondeductible IRAs (2025): https://www.irs.gov/pub/irs-pdf/f8606.pdf
Read from the IRS documents linked above on October 18, 2026. General information, not tax advice. Your own facts decide the outcome, and a preparer, the account provider or the IRS is the right place to confirm anything consequential.