Updated October 18, 2026. Quick answer: Line 9 is investment interest, which is figured on Form 4952 unless a three-part exception lets you skip it. Line 10 adds line 8e (home mortgage interest and points) and line 9. Together they are the interest you paid that the schedule carries toward the total.
IRS worksheets read line by line: Home Mortgage Interest Limit Worksheet.
What line 9 asks for
The caption reads “Investment interest. Attach Form 4952 if required. See instructions”. The instructions define it: “Investment interest is interest paid on money you borrowed that is allocable to property held for investment.” They add “It doesn’t include any interest allocable to passive activities” and the instructions say to “Complete and attach Form 4952 to figure your deduction.” The form is covered on the Form 4952 page.
When you can skip Form 4952
The instructions open the exception with “You don’t have to file Form 4952 if all three of the following apply.” The first is “Your investment interest expense is less than your investment income from interest and ordinary dividends minus any qualified dividends.” The second is “You have no other deductible investment expenses.” The third is “You have no disallowed investment interest expense from 2024.”
Interest and dividend income are reported on lines 2b and 3b of Form 1040, covered on the Form 1040 lines 2a and 2b and Form 1040 lines 3a and 3b pages. The instructions also note “Alaska Permanent Fund dividends, including those reported on Form 8814, aren’t investment income.”
Where the number comes from
If you file Form 4952, the deductible figure comes from that form. If you meet all three exception conditions, the instructions let you skip the form. For more detail they say “For more details, see Pub. 550.” The general rules for interest are stated at the head of the section: “The rules for deducting interest vary, depending on whether the loan proceeds are used for business, personal, or investment activities.”
Line 10: total interest
Line 10 reads “Add lines 8e and 9”. Line 8e is explained on the Schedule A lines 8b, 8c and 8e page, and the first of its amounts is on the Schedule A line 8a page. The instructions summarize the schedule’s purpose here: “Use Schedule A to deduct qualified home mortgage interest and investment interest.”
Where it goes next
Line 10 is an interest subtotal. The schedule’s total on line 17 reads “Add the amounts in the far right column for lines 4 through 16.” See the Schedule A lines 17 and 18 page for that step and the Schedule A (Form 1040) page for the whole form.
Edition and line numbers
This page reads the 2025 Schedule A (Form 1040) 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 schedule says “Attach to Form 1040 or 1040-SR.” Check the tax year printed on the schedule you are filling in and use the line numbers on that year’s schedule, because a different year’s schedule can number or word its lines differently.
Common mistakes
- Skipping Form 4952 without meeting all three conditions. The instructions say “You don’t have to file Form 4952 if all three of the following apply.”
- Counting Alaska Permanent Fund dividends as investment income. The instructions say “Alaska Permanent Fund dividends, including those reported on Form 8814, aren’t investment income.”
- Forgetting interest carried from last year. One of the three conditions is “You have no disallowed investment interest expense from 2024.”
Related: Schedule A (Form 1040), Form 4952, Form 1040 lines 2a and 2b, and Form 1040 lines 3a and 3b.
Sources
- Instructions for Schedule A (Form 1040), Itemized Deductions (2025): https://www.irs.gov/pub/irs-pdf/i1040sca.pdf
- Schedule A (Form 1040), Itemized Deductions (2025): https://www.irs.gov/pub/irs-pdf/f1040sa.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.