How to Calculate Average Mortgage Balance for Taxes

To calculate your average mortgage balance for taxes, pick one of three IRS-approved methods in Publication 936, apply it to each mortgage separately, and add the results. The three methods are the first-and-last balance method, the interest-divided-by-interest-rate method, and the monthly lender statements method.¹ Which one you can use depends on whether you made extra principal payments, refinanced, or had rate changes during the year. The average balance you land on then drives the ratio that tells you how much of your mortgage interest is actually deductible.

When You Need to Run This Calculation at All

If your total mortgage debt stayed below the applicable limit for every day of the tax year, skip this. You can deduct all of your qualifying mortgage interest on Schedule A without touching the average balance math. The calculation only matters when your debt exceeded the limit at some point during the year.

The limit depends on when you took out the loan. For mortgages originated after December 15, 2017, the cap is $750,000 ($375,000 if married filing separately). For mortgages between October 14, 1987 and December 15, 2017, the cap is $1,000,000 ($500,000 if married filing separately). Debt from before October 14, 1987 is grandfathered with no dollar limit, though it reduces room under the newer caps for any additional loans.²¹

These caps apply to the combined mortgages on your main home and one second home, not to each loan on its own.¹

One boundary worth naming: the debt has to have been used to buy, build, or substantially improve the home securing it. A HELOC used for a kitchen remodel qualifies; the same HELOC drawn to pay off credit cards does not. If only part of a loan went toward qualifying use, you have to split the balance and run the average only on the qualifying share.¹

Pull These Documents First

Your servicer sends Form 1098 (Mortgage Interest Statement) each January, showing total interest paid and, in most cases, the year-end outstanding principal.³ If you refinanced or hold multiple mortgages, you’ll receive a separate 1098 for each.

For the simplest method, Form 1098 and your January 1 balance are enough. For the other two methods, gather the twelve monthly billing statements from the year. Use the principal balance shown on each statement. That is the amount you still owe on the loan itself, and it is not the same as your escrow balance, which holds funds for property taxes and insurance. Escrow numbers do not belong in this calculation.

Method 1: First and Last Balance

This is the fastest method, and for many homeowners it is the only one they need. You can use it only if all three of these were true during the year:

You didn’t borrow any new amounts on the mortgage. Your original loan amount is fine; new draws are not.
You didn’t prepay more than one month’s principal, whether through extra payments, a refinance, or a sale.
You made level payments at least twice a year at fixed intervals. Payments that adjusted only because the interest rate reset still count as level.

If those conditions hold, the calculation takes four steps:¹

Write down the balance on the first day the mortgage was secured by your home during the year, usually January 1.
Write down the balance on the last day the mortgage was secured by your home, usually December 31, or the payoff date if you sold or refinanced.
Add the two balances.
Divide by 2.

Example: a January 1 balance of $820,000 and a December 31 balance of $795,000 gives ($820,000 + $795,000) ÷ 2 = $807,500.

Method 2: Interest Divided by Interest Rate

Use this method if the mortgage was secured by your qualifying home for the entire year and you paid interest at least monthly. It handles situations the first-and-last method cannot, such as extra principal payments or additional borrowing on the same loan.¹

Take the total interest paid for the year from Form 1098. Do not include points or interest prepaid for a future year.
Take the annual interest rate on the mortgage. If the rate changed during the year, use the lowest rate.
Divide the interest paid by the rate expressed as a decimal.

Example: $38,400 in interest paid at a 6.5% rate gives $38,400 ÷ 0.065 = approximately $590,769. Using the lowest rate when the rate moved during the year produces a slightly higher average balance, which is the conservative result the IRS requires.

Method 3: Monthly Lender Statements

If your servicer’s statements show a closing balance or average balance for each month, add all twelve monthly closing balances and divide by the number of months the home securing that mortgage was a qualifying home.¹ For a full year, that divisor is 12. For any month the mortgage did not secure a qualifying home, treat the balance as zero.

Some servicers publish a pre-calculated annual average balance in their year-end summary or online portal. If yours does, you can use that figure directly. This is often the easiest path when it is available, and it is the most flexible method for messy years.

Refinances and Partial-Year Loans

A mid-year refinance creates two loans for tax purposes: the old one that was paid off and the new one that replaced it. Each gets its own Form 1098 and its own average balance calculation. Divide by only the months each loan was active.¹

The first-and-last method usually will not work for the paid-off loan, because paying off its entire remaining balance through the refinance violates the “no prepayment beyond one month” condition. The monthly lender statements method is the most flexible option for the year of a refinance.

If the refinanced loan replaced a pre-December 16, 2017 mortgage, the higher $1,000,000 limit generally carries over only up to the balance that was outstanding on the original loan. Anything above that is treated as post-2017 debt and falls under the $750,000 cap. That creates a mixed-use mortgage, and you track the average balance for each debt category separately.¹

Multiple Mortgages or Two Homes

Calculate the average balance for each mortgage on its own, using whichever method fits that loan. Then add the averages together. The combined figure is your total average balance and goes on line 12 of the Table 1 worksheet in Publication 936. You fill out one worksheet covering all mortgages across both properties, not a separate worksheet per loan.¹

Example: a $500,000 mortgage on your main home and a $400,000 mortgage on a second home give a combined debt of $900,000. That exceeds the $750,000 limit, so both loans’ averages feed into a single calculation.

Turning the Average Balance Into a Deductible Amount

The average balance is the input. The deduction comes from a ratio:¹

Identify your qualified loan limit. For most post-2017 borrowers with no grandfathered debt, this is $750,000.
Divide the limit by your total average balance. Round to three decimal places.
Multiply your total interest paid during the year by that decimal. The result is your deductible mortgage interest.

Using the earlier example: an average balance of $807,500 against a $750,000 limit gives $750,000 ÷ $807,500 = 0.929. If you paid $52,000 in total mortgage interest, your deductible portion is $52,000 × 0.929 = $48,308. The remaining $3,692 is not deductible.

Common Errors and Penalty Exposure

The most common mistake is not deliberate inflation. It is skipping the limit calculation entirely and deducting the full interest figure from Form 1098. A homeowner with $900,000 in mortgage debt who does that overclaims by roughly 17%. On a large loan, that easily crosses the thresholds for the IRS accuracy-related penalty, which is 20% of the underpayment caused by negligence or a substantial understatement of tax.⁴ For individuals, a substantial understatement means your tax was understated by at least 10% of the correct tax or $5,000, whichever is greater.

Keep the completed Publication 936 worksheet, your monthly statements, and Form 1098 together. You don’t attach the worksheet to your return, but if the interest figure on Schedule A doesn’t match Form 1098, the worksheet is what explains the difference.

1
Internal Revenue Service. Publication 936 (2025), Home Mortgage Interest Deduction
2
Office of the Law Revision Counsel. 26 USC 163 – Interest
3
Internal Revenue Service. About Form 1098, Mortgage Interest Statement
4
Internal Revenue Service. Accuracy-Related Penalty