Updated October 18, 2026. Quick answer: Line 15a is a subtraction: the form tells you to take line 12 away from line 7. Line 15b is the part of that result that came from qualified disaster distributions you reported on Form 8915-F, and it is zero if you had none. Both amounts feed the taxable amount on line 15c.
IRS worksheets read line by line: Pub 590-B Worksheet 1-1.
Retirement forms read line by line: Form 8915-F lines 16 to 26 and Form 8915-F lines 27 to 32.
What lines 15a and 15b ask for
The form’s caption for line 15a reads “Subtract line 12 from line 7”. The instructions have no separate paragraph for line 15a, so the caption is the whole rule. Line 7 is your distributions and line 12 is the nontaxable portion of the distributions you did not convert; see the line 7 page and the lines 9 to 14 page.
The caption for line 15b reads “Enter the amount on line 15a attributable to qualified disaster distributions, if any, from 2025 Form(s) 8915-F.” It adds “Also, enter this amount on 2025 Form(s) 8915-F, line 18, as applicable.”
Where the number comes from
Line 15a comes straight from your own Form 8606: it is line 7 less line 12. Line 15b depends on what kinds of distributions you took. The instructions give three cases. The first is “If you have no qualified disaster distributions in 2025 from a traditional IRA, enter -0- on line 15b.” The second is “If all your distributions in 2025 from those IRAs are qualified disaster distributions, enter the amount from line 15a on line 15b.”
The third case is a mix: “If you have distributions in 2025 unrelated to qualified disasters, as well as qualified disaster distributions, you will need to multiply the amount on line 15a by a fraction.” The instructions describe the fraction this way: “The numerator of the fraction is your total qualified” disaster distributions, and “the denominator is the amount from Form 8606, line 7.” The instructions work this through in two examples with real figures; use those examples rather than guessing. For the distribution form itself, see the 1099-R box 1 page.
Where it goes next
Line 15b has two destinations. The form says to enter it on Form 8915-F, line 18, as quoted above. It also reduces line 15c, whose caption begins “Taxable amount. Subtract line 15b from line 15a.” Continue with the line 15c page. The final taxable amount can reach Form 1040 lines 4a and 4b on your return.
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
- Leaving line 15b blank when there were no disaster distributions. The instructions say “If you have no qualified disaster distributions in 2025 from a traditional IRA, enter -0- on line 15b.”
- Entering all of line 15a on line 15b when only some distributions were qualified disaster distributions. The instructions say that with a mix of distributions “you will need to multiply the amount on line 15a by a fraction.”
- Forgetting the Form 8915-F entry. The form says “Also, enter this amount on 2025 Form(s) 8915-F, line 18, as applicable.”
Related: Form 8606, 1099-R box 1, and Form 1040 lines 4a and 4b.
Sources
- 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.