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Mortgage Interest Deduction Calculator

Calculate your deductible mortgage interest, accounting for the loan balance cap on the deduction.

Result

Deductible Mortgage Interest
$18,000.00

Mortgages originated before December 16, 2017 can deduct interest on up to $1 million of acquisition debt (grandfathered under old rules); loans from that date forward are capped at $750,000. Interest on balances above the applicable cap isn't deductible - this must also be itemized to provide any benefit, rather than taking the standard deduction.

About the Mortgage Interest Deduction

This calculator determines how much of your annual mortgage interest payment is actually deductible once the loan balance cap is applied. Because the cap depends on when the loan originated, not just how large it is, two homeowners with identical interest payments can end up with very different deductible amounts. It's useful for anyone with a large mortgage who itemizes deductions and wants to know if part of their interest is being disallowed.

How It Works

You enter your annual mortgage interest paid, your average mortgage balance for the year, and whether the loan originated before or after December 16, 2017. The calculator applies the correct acquisition-debt cap, $1 million for older loans or $750,000 for newer ones, figures out what fraction of your balance falls under that cap, and applies that same fraction to your interest payment to get the deductible amount.

Deductible Portion = min(Loan Balance, Debt Cap) / Loan Balance. Deductible Interest = Annual Interest Paid x Deductible Portion. Debt Cap = $1,000,000 if the loan originated before December 16, 2017, otherwise $750,000.

Formula & Methodology

If your loan balance is at or below the applicable cap, the deductible portion is 100% and all your interest is deductible. If your balance exceeds the cap, only the fraction of the balance that falls under the cap is eligible, and that same fraction is applied proportionally to your interest payment, so the calculator doesn't require you to separately track which specific interest dollars relate to which part of the loan. This deduction is also only useful if you itemize rather than take the standard deduction.

Examples

Under the cap

A loan originated after 2017 with a $600,000 average balance and $18,000 in annual interest sits entirely under the $750,000 cap, so the full $18,000 is deductible.

Over the cap

A loan originated after 2017 with a $1,000,000 average balance and $40,000 in annual interest exceeds the $750,000 cap. Only 75% of the balance falls under the cap, so $30,000 of the interest is deductible and $10,000 is not.

Advantages

  • Automatically applies the correct debt cap based on origination date instead of requiring you to look up the grandfathering rule yourself.
  • Converts an oversized loan balance into a concrete deductible-interest dollar figure rather than a vague percentage.
  • Helps homeowners with large mortgages see how much of their interest deduction is actually usable before filing.

Common Mistakes

  • Deducting 100% of mortgage interest on a balance well above the applicable cap without prorating it down.
  • Using the wrong cap by misremembering the origination date cutoff of December 16, 2017.
  • Claiming the deduction while taking the standard deduction, which makes itemized-only deductions like this one provide no additional benefit.

Edge Cases to Watch For

  • Loans with a balance at or below the applicable cap get a 100% deductible portion, since the ratio of the capped balance to the actual balance equals 1 in that case.
  • A balance far above the cap results in a low deductible fraction, meaning a large share of interest paid is simply lost as a deduction rather than carried forward.
  • Loans originated before December 16, 2017 are grandfathered into the higher $1,000,000 cap even under current law, so the origination date field materially changes the result.

Common Use Cases

  • Homeowners with mortgage balances near or above $750,000 checking how much interest they can actually deduct.
  • Buyers comparing the tax impact of a loan taken out before versus after the December 2017 cutoff.
  • Tax preparers verifying a client's Schedule A mortgage interest entry against the correct debt cap.
Written & fact-checked by the Calculateus TeamLast updated August 5, 2026How we verify our formulas

Frequently asked questions

Why does the mortgage interest deduction limit apply to the loan balance rather than the interest amount directly?

The tax code caps the amount of "acquisition debt" that qualifies for deductible interest, so if your mortgage balance exceeds the applicable cap ($750,000 or $1 million depending on origination date), only the interest attributable to the portion of the loan up to that cap is deductible - interest on the excess balance above the cap simply isn't deductible, regardless of how much total interest you paid.

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Conclusion

The deduction cap applies to the loan balance, not the interest amount directly, which is why this calculator first finds what share of your balance qualifies and then applies that share to your actual interest paid. For most borrowers under the $750,000 or $1,000,000 threshold, the result is simply 100% of interest paid; the calculator earns its keep once a loan balance climbs above that line.

References