Free tool
Mortgage interest calculator with extra payments
Enter a loan amount, fixed annual rate and term to see total mortgage interest and the monthly payment breakdown. Add extra principal payments to compare interest savings and payoff time. The default figures are examples, not current market rates. Everything stays in your browser — nothing you type is sent or stored.
Uses extra payments when entered. Payment includes extra principal; amounts are rounded for display.
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How mortgage interest is calculated
For a fixed-rate loan with monthly payments, interest each month is the opening balance times the annual rate divided by 12. The calculator uses the interest rate, not APR, and excludes lender fees. The rest of the payment reduces the balance. Because interest is charged on a shrinking balance, the interest part falls and the principal part grows every month. Extra payments go straight to principal, so every later month’s interest is a little smaller and the loan finishes sooner.
Using it with buyers and homeowners
- Set extra payments to zero to see total interest over the original term. Choose a year to inspect its monthly split.
- Run an extra $100, $200 and $500 a month and show the years and interest saved.
- Try a one-time lump sum, such as a bonus or tax refund, in a chosen month.
- Remind them to check the loan for prepayment penalties and to ask that extra money be applied to principal.
For the full monthly payment with tax, insurance, HOA and PMI use the mortgage payment calculator. Comparing discount points? Use the mortgage points calculator. Thinking of a new rate? Try the refinance break-even calculator, see equity with the home equity calculator, or check the home affordability calculator. For an annual schedule use the mortgage amortization calculator. More in the free tools for real estate agents.
Common questions
How much interest will I pay on my mortgage?
Total interest is the sum of the interest charged each month on the remaining balance. It depends on the loan amount, fixed annual rate, term and extra principal payments. This calculator shows the total and lets you inspect each month of any year.
How is the monthly payment worked out?
With a monthly rate r (the annual rate divided by 12) and n monthly payments, the payment is loan × r ÷ (1 − (1 + r)^−n). At a 0% rate the loan is simply divided by the number of payments.
How much does an extra mortgage payment save?
Extra money paid toward principal reduces the balance sooner, so less interest is charged every month afterwards. Enter a monthly extra amount, a one-time lump sum, or both, and the calculator shows the new payoff time and the interest saved against the normal schedule. Check with the lender that extra payments are applied to principal and that the loan has no prepayment penalty.
Does this include taxes, insurance or PMI?
No. It covers principal and interest on the loan only. For the full monthly payment with property tax, insurance, HOA and PMI use the mortgage payment calculator.
Can I export every monthly payment?
Yes. Download monthly CSV includes payment, interest, principal and closing balance for every month in the current scenario, including entered extra payments. Changing the displayed year does not restrict the export. Headers use your selected currency, amounts retain extra decimal places, and names and contact details are excluded. It is an estimate, not a lender schedule.
Is this a loan offer or a quote?
No. It is an estimate built only from the figures you enter, not a loan offer, a Loan Estimate or advice. Actual lender schedules can differ slightly in rounding and in how extra payments are applied.
Does the calculator save what I enter?
No. The amounts, names and contact details stay in your browser and are not sent to Kadapt, stored, or used in analytics. Button analytics count actions, never the figures or names on the sheet.