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Mortgage overpayment calculator
A little more each month changes the balance on which future interest is calculated. Try an amount that fits your budget and see the difference over the remaining life of your loan.
Illustrative example
Interest you could save
$65,736.37
7 years, 5 months sooner with extra payments
| Plan | Original | With extra |
|---|---|---|
| Monthly payment* | $1,342.05 | $1,542.05 |
| Payoff time | 30 years | 22 years, 7 months |
| Total interest | $233,139.46 | $167,403.09 |
| Total paid | $483,139.46 | $417,403.09 |
*Includes extra principal. The final payment may be smaller.
Download both schedules (CSV)What could an extra payment change?
How to use the overpayment calculator
Enter your current outstanding balance, your annual interest rate and the years remaining. Add the extra amount you would pay toward principal each month. The result compares that plan with making only the scheduled principal-and-interest payment.
Where the interest saving comes from
Each payment first covers the interest for that month. The remainder reduces the loan balance. Extra principal reduces that balance sooner, so less interest accrues in later months. At a 0% rate you can still finish earlier, but there is no interest to save.
Does an overpayment lower my monthly payment?
This model keeps the scheduled payment unchanged and shortens the payoff time. It does not model a lender recasting your loan to lower the required payment. Ask your lender how it applies extra payments and whether fees or early-repayment limits apply.
How these numbers are calculated
We use monthly amortization: M = P × r / (1 − (1 + r)−n), where P is the balance, r is the annual interest rate divided by 1,200 and n is the number of monthly payments. Here, the rate input is the nominal interest rate, not an APR that includes fees. At zero interest, M = P / n.
Interest is calculated on the opening balance each month. Payments arrive at month end; any extra payment reduces principal immediately. Rates stay fixed, payments are never missed and the final payment is capped at the remaining balance plus interest. Calculations retain precision internally; displayed amounts and CSV rows are rounded to cents.
Estimates exclude taxes, insurance, closing costs and early-repayment charges. Lender rounding, daily interest or different compounding conventions can produce different results. These tools are for exploring scenarios, not personalized financial advice.
Sources and verification
The explanations were checked against the following US consumer resources on September 6, 2026. Local rules and your loan agreement may differ.
Built with AI assistance. The calculation model is checked with automated reference and edge-case tests; no independent financial adviser review is claimed. Found an issue? Report a calculation or content error.
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