IRC Section 163 Interest Deduction: Mortgage, Investment, 163(j)

The IRC Section 163 interest deduction lets you write off most interest you pay during the year, but only if the borrowed money went to one of a handful of qualifying uses: a home mortgage, an investment, a trade or business, a passive activity, or a qualifying student loan. Interest on personal borrowing — credit cards, car loans, consumer installment debt — is not deductible at all. Each qualifying category comes with its own cap, its own carryforward rule, and its own form, and the deduction you actually get depends on tracing the loan proceeds back to what you spent them on.

The Categories That Escape the Personal-Interest Ban

Section 163(h) disallows deductions for “personal interest,” a catch-all that covers most consumer borrowing. The statute then carves out five categories that individuals can still deduct:

Qualified residence interest on a mortgage secured by your home
Investment interest on debt used to buy or carry investment property
Trade or business interest, other than interest you pay as an employee
Passive activity interest, governed by the Section 469 loss rules
Student loan interest under Section 221, up to $2,500 per year as an above-the-line deduction with income-based phaseouts

Which bucket your interest falls into decides everything. Misclassify it and you either lose the deduction or invite an accuracy-related penalty. The rules below cover the four categories most filers deal with; student loan interest sits under its own Section 221 phaseout regime and is not part of the 163 limits.

Home Mortgage Interest

Qualified residence interest under Section 163(h)(3) is the largest exception to the personal-interest ban. It covers interest on debt secured by your main home or one other home you use personally. Rental property doesn’t count as a second home unless you also meet the personal-use threshold. And the deduction only helps if you itemize on Schedule A — for 2026 the standard deduction is $16,100 for single filers and $32,200 for joint filers, so your combined itemized deductions have to clear those numbers to make mortgage interest worth claiming.¹

Acquisition Debt Cap

Acquisition indebtedness is debt used to buy, build, or substantially improve a qualifying residence, secured by that property. The One Big Beautiful Bill Act permanently set the deduction cap at $750,000 of acquisition debt, or $375,000 if married filing separately.² A grandfather rule keeps the older $1 million limit ($500,000 married filing separately) in place for mortgages taken out on or before December 15, 2017.³ If you refinance a grandfathered mortgage, the $1 million cap survives only up to the remaining balance of the original loan.

Home Equity Debt

Home equity indebtedness is debt secured by your home but used for something other than buying, building, or improving it — debt consolidation, tuition, a vacation. The Tax Cuts and Jobs Act suspended the deduction for home equity interest for tax years 2018 through 2025. The One Big Beautiful Bill Act permanently extended the $750,000 acquisition-debt cap but did not extend that suspension, so the pre-TCJA rules for home equity debt are scheduled to return for 2026. Under those rules, interest on up to $100,000 of home equity debt ($50,000 married filing separately) is deductible regardless of how you spent the proceeds, as long as the debt is secured by a qualifying residence.

Points

Points paid at closing are prepaid interest, and the rules for deducting them split by transaction type. On a purchase of your main home, you can deduct the full amount in the year you pay them if you meet each of the conditions in Topic 504 — cash method, principal residence, area-standard practice, amount in line with local norms, cash you brought to closing at least equal to the points, points calculated as a percentage of principal, and clearly labeled as points on the settlement statement.⁴ Seller-paid points also qualify, but you reduce your home’s basis by the same amount.

Points paid on a refinance don’t get deducted upfront. You spread them evenly over the life of the new loan, with one exception: if part of the refinance proceeds pay for substantial improvements to your main home, the portion of points tied to that improvement can come off currently.⁵ If you pay off or refinance the loan early, any unamortized points come off in that year — unless you refinance with the same lender, in which case the leftover balance rolls into the new loan’s amortization schedule.

Investment Interest

Section 163(d) covers interest on debt used to buy or carry property held for investment. Margin loans on a brokerage account are the standard example. The deduction is capped at your net investment income for the year, so investment interest cannot offset wages or business income.³

Net investment income includes taxable interest, ordinary dividends, short-term capital gains, and certain royalties. Long-term capital gains and qualified dividends are excluded by default because they already get the preferential rate. You can elect to include them if you want the larger current deduction, but you give up the lower rate on the amount you include, so the tradeoff needs to pencil out before you make the election.

Investment interest that exceeds net investment income carries forward indefinitely and gets used in the first future year with enough investment income to absorb it. Nothing is lost; the deduction is only deferred.

Passive activity interest is a separate category. Interest tied to a rental property or a business where you don’t materially participate is neither investment interest nor personal interest. It goes into the Section 469 passive activity loss calculation and can only offset passive income. Trying to reclassify passive interest as investment interest to unlock it against portfolio income doesn’t work; the statute draws the line explicitly.

Business Interest Under Section 163(j)

Section 163(j) caps the amount of business interest a taxpayer can deduct in a given year. The limit is the sum of business interest income, 30% of adjusted taxable income (ATI), and floor plan financing interest expense (mostly relevant to auto and boat dealers).⁶ Anything above the cap carries forward indefinitely.

The ATI Add-Back Is Back for 2025

How you calculate ATI drives everything for a capital-intensive business. From 2018 through 2021, depreciation and amortization were added back to taxable income when computing ATI, which produced a larger 30% allowance. From 2022 through 2024, those add-backs disappeared, tightening the limit for companies carrying heavy depreciation. The One Big Beautiful Bill Act reversed that change for tax years beginning after December 31, 2024: depreciation, amortization, and depletion are once again added back.⁶ Businesses with significant equipment or real estate investments will see meaningfully more interest come through in 2025 and 2026 as a result.

Small Business Exemption

Section 163(j) does not apply to a business that meets the Section 448(c) gross receipts test. A business qualifies if it is not a tax shelter and its average annual gross receipts over the prior three years are at or below the inflation-adjusted threshold — $31 million for 2025.⁶ The 2026 figure has not been released yet but will likely tick up with inflation. Businesses under the cap deduct all their interest without applying the 30% limit.

Electing Out

Real property trades or businesses and farming businesses can elect out of Section 163(j) entirely. The election is irrevocable, and the tradeoff is that the electing business has to depreciate certain assets under the Alternative Depreciation System and loses bonus depreciation on them. For real property businesses, that hits nonresidential real property, residential rental property, and qualified improvement property. For farming, it applies to property with a recovery period of 10 years or more.⁶ Because you cannot undo it, model the election before making it.

Interest Tracing: How the IRS Decides Which Category You’re In

Because deductibility turns on what you did with the borrowed money, the IRS requires you to trace loan proceeds to their actual use. Treasury Regulation 1.163-8T supplies the rules, and they matter most when borrowed funds land in an account that also holds other money.

The principle is straightforward: interest follows the money. Borrow $50,000 to buy stock and it’s investment interest. Borrow the same $50,000 to renovate your business office and it’s business interest. The complications start when borrowed proceeds sit in a commingled account before being spent.⁷

For those accounts, the regulation uses an ordering rule: borrowed funds deposited into an account are treated as spent before any other money in the account, whether that money was already there or arrived afterward. A 15-day safe harbor lets you treat any expenditure made within 15 days of the deposit as coming from the loan proceeds, regardless of the strict order.⁷ The cleanest approach is to run loan proceeds through a dedicated account and pay the intended expense directly, which sidesteps the tracing question entirely.

If you redirect debt from one use to another — you borrowed for business equipment, then later use the freed-up cash for personal expenses — the interest has to be reallocated to the new use. When you repay a loan split across categories, the regulation retires personal-use portions first, investment and passive portions next, and trade or business portions last.

Interest You Cannot Deduct

Some interest is disallowed no matter which category it would otherwise land in.

Interest on debt used to buy or carry tax-exempt securities is disallowed under Section 265. The IRS will not let you deduct interest on one side while collecting untaxed income on the other.⁸
Interest incurred during production of certain long-lived assets — real property, or tangible personal property with a class life of 20 years or more, a production period over two years, or a production period over one year with costs exceeding $1 million — must be capitalized into the asset’s basis under Section 263A rather than deducted currently. You recover it through depreciation.⁹
Prepayment penalties are sometimes treated as additional interest, in which case they follow the deductibility of the underlying loan.

Forms and Records

Each category has its own form. Mortgage lenders send Form 1098 for interest and points of $600 or more.¹⁰ If your mortgage is held by a private seller rather than a financial institution, you may not get a Form 1098; you still claim the deduction but need to report the lender’s name, address, and taxpayer identification number on Schedule A.

Investment interest goes on Form 4952, which calculates the deductible amount and tracks any carryforward.¹¹ Businesses subject to Section 163(j) file Form 8990 to apply the 30% ATI cap and track carryforwards.¹² These forms attach to your primary return.

Keep supporting records for at least three years from filing, or six years if you omitted more than 25% of gross income. For property-related interest, hold records until the limitations period runs for the year you dispose of the property, since your basis may reflect capitalized interest or points deducted over time.¹³

What Happens If You Get the Category Wrong

Claiming personal interest as business or investment interest, or otherwise inflating a deductible category, can trigger the Section 6662 accuracy-related penalty. That penalty is 20% of the underpayment attributable to negligence, disregard of rules, or substantial understatement.¹⁴ A substantial understatement for an individual exists when the understatement exceeds the greater of 10% of the correct tax or $5,000; if you claimed a Section 199A qualified business income deduction, the threshold drops to 5%.¹⁵ Interest accrues on the penalty itself until paid. Where the mischaracterization is deliberate rather than careless, the civil fraud penalty of 75% may apply instead.

The protection is documentation. Trace the funds, keep the records, and apply the right category, and there is little for an examiner to adjust. Most audit changes to interest deductions happen where the tracing is thin or the classification does not match what the money actually did.

1
Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026
2
U.S. House Committee on Ways and Means. The One Big Beautiful Bill Section by Section – Section 110008
3
Office of the Law Revision Counsel. 26 USC 163 – Interest
4
Internal Revenue Service. Topic No 504 Home Mortgage Points
5
Internal Revenue Service. Publication 936 Home Mortgage Interest Deduction
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Internal Revenue Service. Questions and Answers About the Limitation on the Deduction for Business Interest Expense
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eCFR. 26 CFR 1.163-8T – Allocation of Interest Expense Among Expenditures (Temporary)
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Office of the Law Revision Counsel. 26 US Code 265 – Expenses and Interest Relating to Tax-Exempt Income
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eCFR. 26 CFR 1.263A-8 Requirement to Capitalize Interest
10
Internal Revenue Service. Instructions for Form 1098
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Internal Revenue Service. Form 4952 – Investment Interest Expense Deduction
12
Internal Revenue Service. About Form 8990
13
Internal Revenue Service. Topic No 305 Recordkeeping
14
Office of the Law Revision Counsel. 26 US Code 6662 – Imposition of Accuracy-Related Penalty on Underpayments
15
Internal Revenue Service. Accuracy-Related Penalty