Thin capitalization rules are the U.S. tax provisions that limit how much interest a business can deduct when it finances itself with a lot of debt relative to equity. The main tool is Section 163(j) of the Internal Revenue Code, which generally caps the business interest deduction at 30 percent of a company’s adjusted taxable income for the year.1Office of the Law Revision Counsel. 26 USC 163 – Interest Two other doctrines sit alongside that cap: the IRS can recharacterize a purported loan as equity, and it can adjust the interest rate on a related-party loan that doesn’t reflect market terms. Interest payments reduce taxable income while dividends do not, so without these rules a company could load up on intercompany debt and quietly drain its tax base.
The 30 Percent Interest Deduction Cap
Under Section 163(j), business interest deductible in any tax year cannot exceed the sum of three amounts: the taxpayer’s business interest income, 30 percent of adjusted taxable income, and any floor plan financing interest expense.2Internal Revenue Service. Questions and Answers About the Limitation on the Deduction for Business Interest Expense Business interest means interest on debt tied to a trade or business. It does not include investment interest, and it does not include interest that another Code provision requires you to capitalize.1Office of the Law Revision Counsel. 26 USC 163 – Interest
The 30 percent piece draws the attention, but the other two matter. Interest income from lending or deposits raises the ceiling dollar for dollar. Floor plan financing interest, the cost dealers pay to carry vehicle or heavy equipment inventory, is fully deductible on top of the 30 percent allowance, so auto dealers and similar businesses are not squeezed by a rule aimed at a different problem.2Internal Revenue Service. Questions and Answers About the Limitation on the Deduction for Business Interest Expense
How Adjusted Taxable Income Works in 2026
Adjusted taxable income (ATI) is not the same as EBITDA, though they overlap. ATI begins with taxable income, strips out items that aren’t connected to a trade or business, and then removes business interest expense, business interest income, net operating loss deductions, and the Section 199A qualified business income deduction.1Office of the Law Revision Counsel. 26 USC 163 – Interest The variable that has moved over time is whether depreciation, amortization, and depletion get added back.
From 2018 through 2021, the law allowed those add-backs, making ATI look EBITDA-like. For tax years 2022 through 2024, the add-back expired, and the cap tightened for capital-intensive businesses. The One Big Beautiful Bill Act permanently restored the add-back for tax years beginning after December 31, 2024, so 2026 filings once again include depreciation, amortization, and depletion in ATI.1Office of the Law Revision Counsel. 26 USC 163 – Interest That materially raises the deduction ceiling for businesses with large depreciable asset bases. One other 2026 change works in the opposite direction: ATI no longer includes certain income inclusions from controlled foreign corporations, which can lower the cap for U.S. shareholders of CFCs.3Internal Revenue Service. Instructions for Form 8990
Who Is Exempt from the Cap
Not every business has to run the calculation. Small business taxpayers, generally those with average annual gross receipts under $32 million for the three prior tax years for 2026, are exempt from the limitation entirely. For a closely held company that stays under the threshold, interest deductions work as they did before Section 163(j) was tightened.
Certain industries can also elect out of the cap, though each election comes with a cost. A real property trade or business can deduct all of its interest, but it must switch to the Alternative Depreciation System for its real property assets, which lengthens recovery periods and permanently disqualifies the property from bonus depreciation. A farming business can make a similar election with longer depreciation lives on certain assets. Certain regulated utilities are excepted without needing to elect at all. And the trade or business of performing services as an employee is always outside the rule.2Internal Revenue Service. Questions and Answers About the Limitation on the Deduction for Business Interest Expense
The real property and farming elections are irrevocable. With the depreciation add-back restored for 2026, some businesses that elected out under the tighter rules may find the cap generous enough that the election is no longer worth its trade-off. They cannot reverse the choice.
What Happens to Disallowed Interest
Interest that exceeds the cap in a given year is not permanently lost. Section 163(j)(2) treats disallowed business interest as paid or accrued in the following tax year, which creates an indefinite carryforward.1Office of the Law Revision Counsel. 26 USC 163 – Interest Each year the carryforward gets added to that year’s interest and tested against the new year’s cap. A company that has a lean year with a tight cap can still absorb the deduction later when profitability recovers. The mechanics get more complicated for partnerships and S corporations because disallowed interest allocates out to owners and follows special tracking rules; Form 8990’s instructions walk through those adjustments.3Internal Revenue Service. Instructions for Form 8990
When the IRS Reclassifies Debt as Equity
The 163(j) cap controls the volume of interest deductions. A separate and often more damaging issue is whether the underlying instrument is really debt. Section 385 authorizes Treasury to distinguish debt from equity by reference to factors including whether there is a written unconditional promise to pay a sum certain with fixed interest, whether the instrument is subordinated to other creditors, the corporation’s debt-to-equity ratio, whether the instrument is convertible into stock, and the relationship between stock ownership and holdings of the instrument in question.4Office of the Law Revision Counsel. 26 USC 385 – Treatment of Certain Interests in Corporations as Stock or Indebtedness
In practice courts look at a broader set of indicators: whether there is a fixed maturity date, whether repayment depends on the company’s earnings, whether the lender can enforce collection, whether the instrument confers management rights, and whether a bank would have made a comparable loan. A shareholder who advances funds in proportion to stock ownership, with no realistic repayment schedule and interest paid only when earnings allow, has arguably made a capital contribution rather than a loan.
If the IRS wins the reclassification, the consequences cascade. Every interest payment the corporation deducted becomes a nondeductible distribution, essentially a constructive dividend. The corporation loses the deduction and owes back tax on the increased income, plus interest on the underpayment. Any later transfer of corporate funds to the shareholder is exposed to the same recharacterization risk as a disguised dividend rather than a loan repayment.
Arm’s Length Terms on Related-Party Loans
Even when a loan is genuinely debt, the IRS can adjust its terms. Section 482 gives the agency authority to allocate income, deductions, and credits between related organizations when necessary to prevent tax evasion or clearly reflect income.5Office of the Law Revision Counsel. 26 USC 482 – Allocation of Income and Deductions Among Taxpayers The authority reaches interest rates, principal amounts, and every other term of an intercompany loan.
The test is what an unrelated lender would have done. If a U.S. subsidiary borrows $100 million from its foreign parent at 2 percent when the market rate for comparable credit risk is closer to 6 percent, the IRS can impute a higher rate and reallocate the income. If no outside lender would have made the loan at all because the borrower cannot support the debt, the deduction can be disallowed outright. Defending against a Section 482 adjustment usually requires a contemporaneous transfer pricing study identifying comparable arm’s length loans and showing that the rate, tenor, and covenants fall inside a defensible range. Companies that set intercompany rates by internal convenience are relying on not being audited.
Withholding Tax When Interest Becomes a Dividend
Reclassification cuts across borders too. Under Section 1441, dividends paid to nonresident aliens and foreign corporations are subject to a default withholding rate of 30 percent.6Office of the Law Revision Counsel. 26 USC 1441 – Withholding of Tax on Nonresident Aliens If a foreign parent treated a payment as interest, possibly at a reduced treaty rate for interest, and the IRS later recharacterizes it as a dividend, the withholding obligation shifts and under-withholding penalties can apply.
Bilateral tax treaties may reduce the dividend rate, sometimes to 5 or 15 percent depending on ownership thresholds. Treaty benefits require proper documentation, typically a Form W-8BEN-E on file, and they do not fix a payment that was reported as interest at the time. The payor bears responsibility for withholding at the correct rate, which means a reclassification can leave the paying corporation on the hook for the shortfall.
Reporting on Forms 8990 and 5472
Any taxpayer with business interest expense, a disallowed interest carryforward, or excess business interest expense from a partnership must file Form 8990 with its return. Pass-through entities that allocate excess taxable income or excess business interest income to owners also file, even when the entity has no interest expense of its own.3Internal Revenue Service. Instructions for Form 8990 Small business taxpayers and those whose only interest is from an excepted trade or business are generally excused.
Foreign-owned U.S. corporations get an additional layer. A domestic corporation with at least one direct or indirect 25 percent foreign shareholder must file Form 5472 to report transactions with foreign related parties, including amounts borrowed, amounts loaned, and interest paid.7Internal Revenue Service. Instructions for Form 5472 The form also captures payments subject to Section 267A hybrid arrangement disallowances and base erosion payments under Section 59A. Intercompany loans between a foreign parent and a U.S. subsidiary are the most common thin capitalization fact pattern, so Form 5472 is often the first document an IRS examiner asks to see.
Penalties for Getting It Wrong
Losing the deduction is not the whole cost. If the IRS disallows interest through the 163(j) cap, a debt-equity reclassification, or a Section 482 adjustment, the resulting underpayment can trigger an accuracy-related penalty of 20 percent of the underpaid tax when the understatement is attributable to negligence or a substantial understatement of income.8Office of the Law Revision Counsel. 26 USC 6662 – Imposition of Accuracy-Related Penalty on Underpayments Statutory interest compounds on the underpayment from the original due date of the return.9Internal Revenue Service. Accuracy-Related Penalty
Reporting failures are penalized separately. A corporation that fails to file Form 5472 when required faces a $25,000 penalty per form. If the failure continues more than 90 days after IRS notification, an additional $25,000 applies for each 30-day period the failure persists, per related party.7Internal Revenue Service. Instructions for Form 5472 A company with three foreign related parties that ignores IRS notices for six months can accumulate hundreds of thousands of dollars in penalties before the substance of the intercompany transactions is examined.
The evidence that keeps most of this from happening is prepared before the return goes out, not after an audit letter arrives. A transfer pricing study benchmarking the rate and terms of any intercompany loan, a clean Section 163(j) computation supporting the deduction, and correct withholding documentation on cross-border payments together form a defensible position. Reconstructing that record later is expensive and much less persuasive to an examiner.