This guide is for C corporation finance teams, tax preparers, and foreign-owned U.S. entities (including Australian-owned subsidiaries) reviewing interest charges before filing. It covers what Line 18 includes, how to compute the deductible amount, when Section 163(j) and Form 8990 apply, and what records to keep. Figures below refer to the 2025 return unless stated.
Key Takeaways
- Line 18 reports deductible interest expense, never interest income.
- The general ledger total rarely equals the Line 18 amount.
- Tracing, capitalisation, prepayment, related-party rules and Section 163(j) can all change the number.
- Use the current-year instructions and keep support for every figure.
What Does IRS Form 1120 Line 18 Cover?
Line 18 is the deduction line for interest incurred by the corporation during the tax year. Section 163(a) allows a deduction for interest paid or accrued on indebtedness, subject to other restrictions.
The Form 1120 instructions are explicit: do not offset interest income against interest expense on Line 18, and do not offset expense against income on Line 5.
Interest expense versus interest income
- Interest expense goes on Line 18 if it survives the tests below.
- Taxable interest income goes on Line 5 and is typically supported by payer statements.
- Form 1099-INT is a payer information return. It is not a deduction form, and corporate recipients are generally exempt from payer reporting, so a missing 1099 does not excuse omitting income.
Categories to review
- Business loans and lines of credit
- Corporate bonds and notes
- Mortgages on business property
- Equipment financing
- Loans from shareholders or related parties, which are deductible when they are bona fide debt, subject to the related-party rules below
How to Determine the Deductible Amount for Line 18
Classify all interest charged
Start by reconciling the interest expense ledger to lender statements, amortisation schedules, credit-card records and bond documents. Then separate interest tied to the corporation's trade or business from interest on debt that funds something else, such as investments or tax-exempt obligations.
Trace the debt proceeds
Under Temporary Regulation 1.163-8T, interest follows the use of the borrowed money. The books' label doesn't decide it. Tracing the disbursements does.
Example: A corporation borrows US$1,000,000. It uses US$700,000 for inventory and operations, and US$300,000 to buy tax-exempt municipal bonds. Interest on the first US$700,000 is business interest. Interest on the US$300,000 portion is not deductible under Section 265. Booking the full amount to Line 18 would overstate the deduction.
Apply timing and capitalisation rules
- Cash method: deductions generally arise when paid. Accrual method: the all-events test and economic performance apply.
- Prepaid interest covering later periods is capitalised and deducted over the period it relates to (Section 461(g)).
- Production interest on designated property is capitalised into asset cost under Section 263A(f). It never reaches Line 18 as a current expense.
- Original issue discount follows statutory daily portions, not cash payment dates.
Remove amounts that are not currently deductible
Strip out disallowed, deferred, duplicated or waived interest, and document where each amount goes. Then reconcile tax to book. Where the two differ, the gap should show up on Schedule M-1 or M-3, not be hidden by copying the book total.
Interest Expense Limitations and Special Rules
Not every limitation works the same way. Keep these treatments distinct:
| Treatment | Effect |
|---|---|
| Section 265 | Permanent disallowance |
| Section 267 | Deferral until the payee includes the income |
| Section 263A | Capitalised into asset cost |
| Section 163(j) | Annual cap, with carryforward |
Section 163(j) business-interest limitation
For 2025, deductible business interest is generally capped at business interest income plus 30% of adjusted taxable income (ATI) plus floor plan financing interest. For a C corporation, essentially all interest income and expense counts as business interest, other than amounts tied to an excepted trade or business.
Three points trip up preparers:
- ATI is a tax figure, not financial-statement EBITDA.
- The depreciation addback is back. For tax years beginning after 2024, ATI again adds back depreciation, amortisation and depletion. The 2022 to 2024 computation did not.
- Small-business exemption: not a tax shelter, and average gross receipts for the three prior years are US$31 million or less for 2025 (US$32 million for 2026). Controlled groups are aggregated.
Eligible real property and farming businesses can make an irrevocable election out of the limit, with trade-offs on depreciation.
When Form 8990 applies
The Form 8990 instructions generally require the form from taxpayers with business interest expense or a disallowed carryforward, unless an exception applies. Line 30 gives the deductible business interest. Consolidated groups file one Form 8990 for the group.
Carryforwards and prior-year schedules
Disallowed business interest carries forward indefinitely. Form 8990 line 31 feeds the following year's line 2.
Schedules A and B of Form 8990 serve partnership and pass-through situations. They are not the ordinary C corporation carryforward ledger. Tie the opening balance to last year's filed Form 8990 and workpapers.
Foreign-owned and related-party corporations
Interest paid to a foreign parent or other related party raises separate questions. Each test is independent, and meeting one doesn't satisfy another.
- Timing (Section 267): the deduction may wait until the payee includes the income.
- Arm's-length pricing (Section 482): the rate should match what unrelated parties would charge.
- Debt versus equity (Section 385): certain related-group instruments can be treated as stock.
- Hybrid arrangements (Section 267A): deductions can be denied for specified hybrid transactions.
- Withholding: U.S.-source interest to foreign recipients faces 30% withholding unless a treaty or portfolio interest exemption applies (loans from 10% shareholders generally do not qualify). Report on Forms 1042 and 1042-S.
- Reporting: a 25% foreign-owned corporation generally files Form 5472 for reportable transactions, with interest on line 32 at the Section 163(j) allowed amount.
Which forms apply depends on ownership and facts. Not every foreign-owned corporation files the same ones.
How to Report Line 18 and Coordinate Supporting Forms
Prepare the amount from tax records
- Reconcile the general ledger to lender statements.
- Remove non-deductible, deferred and duplicated items.
- Apply capitalisation and timing rules.
- Run the Section 163(j) calculation, if applicable.
- Document the final deductible figure.
Then compare Line 18 with the income statement, balance sheet, debt schedules and the prior-year return. Clean, reconciled books make this faster.
VJM Global's bookkeeping work, including ledger clean-up and year-end financial reports, supports this stage of tax return preparation.
Coordinate supporting forms
Form 8990 shows the allowed, disallowed and carried-forward amounts. Depending on the facts, you may also need:
- Capitalisation schedules for production interest
- Related-party loan agreements
- Interest allocation schedules
- Form 5472 or other international information returns
Reconcile the return
- Schedule L should agree with the corporate books.
- Schedule M-1 or M-3 should explain differences between book and tax interest.
- Schedule M-3 is generally required at US$10 million or more in year-end assets; interest expense is reported in Part III, line 8.
- Small-return exception: if total receipts and year-end assets are both under US$250,000, Schedules L, M-1 and M-2 may be skipped.
Before filing, confirm the tax-year version of the form, attachments, an authorised officer's signature and the filing method. Corporations filing 10 or more returns of any type in the calendar year generally must e-file Form 1120. That test is separate from the Schedule M-3 asset test.
Records, Common Errors, and Review Checklist
Documents to retain
- Lender statements and loan agreements
- Repayment and amortisation schedules
- Accrued-interest calculations
- Proof of payment
- Related-party agreements
- Documentation of how proceeds were used
- Prior-year Section 163(j) workpapers
Payment evidence alone does not establish a deduction, as IRS Publication 583 makes clear. Build a transaction-level schedule that covers:
- Lender, principal and rate
- Interest period and use of proceeds
- Book amount, tax adjustment and current-year deduction
Retain records for at least three years from filing, longer in certain circumstances.
Common Line 18 mistakes
- Reporting interest income as expense, or netting the two
- Deducting the full book amount without testing Section 163(j)
- Missing prepaid or capitalised interest
- Counting the same interest twice, for example in cost of goods sold
- Skipping documentation for related-party debt
- Leaving Schedule M-1 inconsistent with the deduction
- Omitting Form 8990 when it is required
Unsupported estimates and mismatched schedules invite review. An accuracy-related penalty under Section 6662 is generally 20% of the resulting underpayment, not 20% of the interest deducted.
When professional review is appropriate
Get qualified help when you see any of these:
- A large or complex debt portfolio
- Related-party or foreign-party financing
- A merger or acquisition
- Multiple business activities
- Large book-tax differences
- Uncertain Section 163(j) treatment
VJM Global provides accounting, tax compliance and financial review support for U.S. and cross-border businesses. No particular tax result is promised, but a second review of the debt schedule often catches problems before filing.
Frequently Asked Questions
Is interest expense an allowable deduction?
Yes. Qualifying business interest is generally deductible. Section 163(j) and other rules may limit, defer, capitalise or disallow the amount, so check the current IRS instructions.
What IRS form is used for interest income?
Interest income is not reported on a separate form. A corporation reports taxable interest on Form 1120 Line 5. Payer documents such as Form 1099-INT may support the figure.
What are the instructions for using Form 1120-X?
Form 1120-X amends a filed Form 1120. Enter corrected figures, explain changes in Part II, attach affected schedules, and file within the refund window (three years from filing or two years from payment).
Where do you report interest expense on Form 1120?
Deductible interest goes on Line 18. First test the amount for timing, capitalisation, related-party and Section 163(j) limits.
When might Form 8990 be required with Form 1120?
Required when the corporation has business interest expense or a disallowed carryforward, unless an exception such as the small-business exemption applies. Check current-year instructions for exceptions and elections.
