U.S. · Tax forms
Form 8606 line 18, taxable conversion.
Line 16 minus line 17, and what to do if the result is zero or less.
Line 18 is the taxable part of your 2025 Roth conversion: line 16 minus line 17. If more than zero, include it on Form 1040, 1040-SR or 1040-NR line 4b. If zero or less, do not include the result on line 4b; put the full distribution on line 4a.
Look up a Form 8606 situation
Result
Part II lines 16 to 18 (2025 form and instructions)
| Line | Meaning | What the IRS says |
|---|---|---|
| 16, Part I done | Conversion amount | Enter the amount from line 8 |
| 16, Part I skipped | Conversion amount | Enter the net amount converted from traditional IRAs to Roth IRAs in 2025 |
| 17, Part I done | Basis in the conversion | Enter the amount from line 11 |
| 17, line 11 not completed | Basis in the conversion | Enter the amount from line 2 (or what you would have entered) plus any contributions on line 1 made before the conversion |
| 18 | Taxable amount | Subtract line 17 from line 16. If more than zero, include on Form 1040 line 4b |
| 18 is zero or less | No taxable amount | Do not include the result on line 4b. Include the full distribution on line 4a |
| Form 1040 | Lines 4a and 4b | 4a is the gross distribution. 4b is the taxable part |
The two paths to line 17
If you completed Part I because you had other IRA money or distributions, line 17 comes from the pro-rata computation (line 11). If you did not complete line 11, the instructions say to use line 2 plus contributions you made before the conversion on line 1. In both cases line 17 is your basis in the converted amount, the part that was already taxed.
Reporting on Form 1040
The converted amount goes in full on Form 1040 line 4a (IRA distributions). Only the taxable amount from line 18 goes on line 4b. The instructions specifically say that when line 18 is zero or less you still report the full distribution on 4a and do not carry the result to 4b.
Check with the pro-rata calculator
Because line 17 depends on every traditional IRA you own on December 31, a small error on line 6 flows straight into line 18. The pro-rata calculator linked below shows the arithmetic with your own figures. This page decodes the IRS lines and does not compute your return.
Worked example
Omar converted $20,000. Line 11 came to $1,240, so line 16 is $20,000 and line 17 is $1,240. Line 18 is $20,000 − $1,240 = $18,760, entered on Form 1040 line 4b, with the full $20,000 on line 4a. A second filer with no pre-tax IRA money who converted a $7,000 nondeductible contribution would show line 17 of $7,000 and line 18 of $0. 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 10, Form 8606 Line 14, 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 is Form 8606 line 18?
The taxable amount of your 2025 Roth conversion, line 16 minus line 17.
Where does line 18 go?
If more than zero, on Form 1040, 1040-SR or 1040-NR line 4b.
What if line 18 is zero?
Do not include the result on line 4b, but include the full distribution on line 4a.
Where does line 17 come from?
From line 11 if you completed Part I. Otherwise from line 2 plus any contributions on line 1 made before the conversion.
Does the conversion go on line 4a too?
Yes. The full distribution is on line 4a and the taxable part is on line 4b.
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.