How the Mortgage Interest Calculator works
Estimate total mortgage interest over time. Adjust the inputs above, then use the calculate button to generate a fresh estimate based on the numbers you entered.
The result is designed to give you a quick planning number, plus a short breakdown of the major figures behind it. You can reset the form at any time and try a different scenario.
When to use this calculator
Use this mortgage interest calculator to see the long-term interest cost behind a mortgage payment. It is helpful when comparing amortization lengths, rate offers or extra-payment strategies because a lower monthly payment can still mean more interest over time.
Formula
Total interest = total of all payments - mortgage principal
- Mortgage principal is the balance being repaid.
- The rate determines how much interest accrues each period.
- The amortization controls how long interest has to accumulate.
Worked example
On a $400,000 mortgage at 5% over 25 years, the calculator estimates the monthly payment and subtracts the original mortgage amount from total payments to show estimated interest.
Common mistakes
- Looking only at the monthly payment and ignoring lifetime interest.
- Assuming a 25-year estimate will match reality if the rate changes at renewal.
- Forgetting that prepayments can reduce interest if your mortgage allows them.
FAQs
Why is total interest so high?
Mortgages are repaid over many years, so even moderate rates can create a large total interest cost.
Does this assume the rate never changes?
Yes. It uses the rate you enter for the full amortization, so it is a simplified planning estimate.
How can I reduce mortgage interest?
A lower rate, shorter amortization, larger down payment or extra payments can all reduce interest.