Updated October 18, 2026. Quick answer: Part II is where a Roth conversion is taxed. Line 16 is the net amount you converted, line 17 is your basis in that amount, and line 18 subtracts line 17 from line 16. If line 18 is more than zero it also goes on Form 1040 line 4b.
What Part II asks for
Part II opens with “Complete Part II if you converted part or all of your traditional IRAs to a Roth IRA in 2025.” The form’s own caption for line 16 is “If you completed Part I, enter the amount from line 8. Otherwise, enter the net amount you converted from traditional IRAs to Roth IRAs in 2025”.
Line 16: the converted amount
If you filled in Part I, line 16 repeats line 8; see the line 8 page. If you did not, the instructions say “If you didn’t complete line 8, see the instructions for that line. Then, enter on line 16 the amount you would have entered on line 8 had you completed it.” For the forms that report conversions, see the Form 5498 boxes 2, 3 and 4 page.
Line 17: your basis in the conversion
The caption reads “If you completed Part I, enter the amount from line 11. Otherwise, enter your basis in the amount on line 16.” For a filer who skipped line 11, the instructions say “If you didn’t complete line 11, enter on line 17 the amount from line 2 (or the amount you would have entered on line 2 if you had completed that line) plus any contributions included on line 1 that you made before the conversion.”
Line 18: the taxable amount
The caption reads “Taxable amount. Subtract line 17 from line 16. If more than zero, also include this amount on 2025 Form 1040, 1040-SR, or 1040-NR, line 4b”. The instructions cover the other direction: “If your entry on line 18 is zero or less, don’t include the result on 2025 Form 1040, 1040-SR, or 1040-NR, line 4b.” They add “Include the full amount of the distribution on 2025 Form 1040, 1040-SR, or 1040-NR, line 4a.”
Where it goes next
Line 18 flows to Form 1040 lines 4a and 4b. For the wider picture of how conversions are taxed, see the Roth conversions and Roth conversion taxes pages. If you also took distributions from a Roth IRA, Part III is next; start with the lines 19 and 20 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. The full form is explained on the Form 8606 page.
Common mistakes
- Entering a number on line 16 without checking line 8. The caption says “If you completed Part I, enter the amount from line 8.”
- Leaving out contributions made before the conversion when you skipped line 11. The instructions say to add “any contributions included on line 1 that you made before the conversion.”
- Putting a zero or negative line 18 result on line 4b. The instructions say “If your entry on line 18 is zero or less, don’t include the result on 2025 Form 1040, 1040-SR, or 1040-NR, line 4b.”
Related: Form 8606, Roth conversions, Roth conversion taxes, Form 1040 lines 4a and 4b, and Form 5498 boxes 2, 3 and 4.
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- 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.