What this mortgage deduction calculator answers

The calculator addresses two questions that are often blended together. First, how much of annual mortgage interest is potentially attributable to qualified home acquisition debt under the federal debt limit? Second, how much of the points connected with a refinance belongs in the current year rather than later years? The output combines those two estimated Schedule A components, but it keeps the ledger visible so the user can audit each path.

This calculator uses the final 2025 IRS Publication 936 rules for a 2025 federal return. It is not a 2026 projection. The deduction matters only if the taxpayer itemizes and otherwise qualifies. A dollar of deductible interest is not a dollar of tax savings: its federal tax effect depends on whether total itemized deductions exceed the standard deduction, the applicable marginal rate, and other return limitations.

Federal acquisition-debt ceilings

A refinance does not reset old acquisition debt into a fresh $750,000 qualifying balance merely because a new note is signed. Generally, the new debt retains qualified treatment only up to the old mortgage principal immediately before refinancing, plus qualifying amounts used to substantially improve the home. The selected date category should reflect the debt whose status is being preserved, not automatically the refinance closing date.

Qualified use is more important than the loan label

Federal law focuses on how borrowed money is used and whether the debt is secured by a qualified home. A home-equity loan, cash-out refinance, or line of credit can generate deductible acquisition-debt interest when proceeds buy, build, or substantially improve the secured home and other requirements are satisfied. The same product may generate nondeductible personal interest when proceeds pay credit cards, tuition, a vacation, or living expenses.

The “qualified acquisition use” input is a planning allocation. If 90% of the average balance represents old qualifying principal and qualified improvement spending while 10% represents personal cash-out, the tool applies the ceiling to the 90% qualified portion and allocates entered interest proportionally. Tracing can be more complicated when proceeds are disbursed at different times or balances change. Keep closing disclosures, settlement statements, contractor invoices, bank records, and the old loan payoff statement.

How the interest estimate works

The formula is intentionally transparent but simplified. Publication 936 supplies several methods for average balances and a worksheet for qualified-loan limits. A taxpayer with an original mortgage, a refinance, a second mortgage, and a second home may need to order debts by date and account for grandfathered debt separately. This tool is best for a single representative balance in one date category. It does not allocate lender fees that are not points or determine whether interest was actually paid by the taxpayer who claims it.

Refinance points: current deduction or amortization

The general refinance rule

Points paid to refinance an existing mortgage are generally not fully deductible in the closing year. Instead, they are deducted ratably over the life of the new loan. This calculator divides deferred points by total term months and multiplies by scheduled payments made during the tax year. That produces a practical monthly-equivalent estimate for a level-payment mortgage. Partial months, irregular first payments, or a different accounting method should follow the return instructions.

Main-home improvement exception

A portion of refinance points allocable to proceeds used to substantially improve the taxpayer’s main home may be deductible in the year paid if all IRS tests for deducting points are satisfied. The calculator requires both an allocation percentage and an affirmative eligibility selection. Without that confirmation, it amortizes all new points. The remaining non-improvement portion stays in the amortization pool.

What happens to points from the old loan

When refinancing with a different lender, remaining unamortized points from the paid-off old loan are generally deductible in the payoff year. When the same lender refinances the loan, the remaining old points generally continue to be deducted over the new loan’s life rather than becoming an immediate write-off. This estimator uses the new term and the entered number of payments for that same-lender approximation.

Worked example: cash-out refinance above the limit

Assume a non-MFS taxpayer has a post-2017 mortgage averaging $820,000 during 2025, with 90% traced to qualified acquisition or improvement use. Qualified average debt is $738,000, which is below the $750,000 general ceiling. If $48,600 of total interest was paid, the initial qualified-use allocation is $43,740 and the ceiling ratio remains 100%. The other 10% is treated as personal-use interest in this simplified example.

Now assume $4,200 of points were paid on a 360-month refinance and eight scheduled payments occurred in 2025. With no qualifying immediate-improvement allocation, current new-point amortization is about $93. If $900 of old points remained and the refinance used a different lender, that $900 is included in the current-year ledger. The combined estimate is approximately $44,733 before deciding whether itemizing produces a tax benefit. Changing the lender selection to “same lender” spreads the old points over the new term, sharply reducing the current-year points component.

Closing records that support the estimate

Gather every Form 1098, the original acquisition date and principal, annual beginning and ending balances, refinance payoff records, points shown on closing statements, and evidence of how cash-out proceeds were spent. If two people paid the mortgage but only one received Form 1098, ownership and payment facts matter. Interest on a second qualified home may be included, but the debt ceilings generally apply across the combined qualified debt rather than separately to each property.

For payment planning rather than tax deduction analysis, the published 25-Year Mortgage Calculator on OnlineToolkit provides a separate amortization perspective. That tool should not be used to decide tax deductibility; this calculator should not be used to quote a mortgage payment. The distinct tools are linked because loan cash flow and Schedule A treatment often need to be reviewed together.

Frequently asked questions