Mortgage Overpayment Calculator — Interest Saved & Time Cut
Paying a little extra each month can take years off your mortgage and save thousands in interest. Enter your balance, rate and remaining term, then add a regular overpayment, a one-off lump sum or both. The calculator shows the interest saved, the time cut from your mortgage and your estimated payoff date.
How to use the Mortgage Overpayment Calculator
- Enter your outstanding mortgage balance, interest rate and the time left on the mortgage.
- Add a monthly overpayment, a one-off lump sum, or both, and say which month the lump sum is paid.
- Check the interest saved, the time saved and your new estimated payoff date.
- Use the chart to compare your balance with and without overpayments.
Why overpaying saves so much
Mortgage interest is charged on the outstanding balance. Every pound you overpay reduces the balance immediately, so you pay less interest in every month that follows — and because less interest is added, even more of your regular payment goes towards the loan. This compounding effect is why overpayments made early in a mortgage save the most.
In this calculator your normal monthly payment stays the same, so overpayments shorten the term rather than lowering the payment. Some lenders let you choose to reduce the payment instead.
Check your lender’s overpayment rules
Many fixed and discounted deals allow overpayments of up to 10% of the outstanding balance each year without charge. Going above that can trigger an early repayment charge, which could wipe out the interest saving. Standard variable rate mortgages often have no limit.
It can also be worth comparing overpaying with saving the money, particularly if you have higher-interest debts or no emergency fund.
Assumptions & important information
- The calculator assumes the interest rate stays the same for the rest of the mortgage and charges one-twelfth of the annual rate each month.
- Your usual monthly payment is calculated from the balance, rate and remaining term, then overpayments are added on top.
- A one-off lump sum is applied at the end of the month you choose.
- Early repayment charges, fees and changes to your rate are not included.
- Interest rates shown in the form are examples only, not current market rates. Enter the rates from your own quote.
- This calculator gives estimates for information only. It is not financial advice and not a mortgage or loan offer. Rates, fees and lender criteria vary, so check the details with your lender or a regulated adviser. Your home may be repossessed if you do not keep up repayments on a mortgage or other debt secured on it.
Ideas & examples
- £200 extra every month: On a £200,000 mortgage at 4.5% over 25 years, overpaying £200 a month saves £36,280 in interest and clears the mortgage 6 years 1 month early.
- A £10,000 lump sum after a year: The same mortgage with a single £10,000 overpayment in month 12 saves £18,072 in interest and finishes 2 years 1 month sooner.
- Combining both: Use regular overpayments for steady progress and one-off lump sums, such as a bonus or tax rebate, for an extra boost. Enter both to see the combined effect.
Frequently asked questions
How much will I save by overpaying my mortgage?
Enter your balance, rate, term and overpayment amounts and the calculator shows the interest saved and the time cut from your mortgage.
Is it better to reduce the term or the monthly payment?
Reducing the term saves the most interest. Reducing the payment gives you more breathing room each month. This calculator keeps the payment the same and shortens the term.
Will I be charged for overpaying?
Possibly. Many fixed-rate deals allow overpayments up to a limit (often 10% of the balance a year) before early repayment charges apply. Check your mortgage terms or ask your lender.
When should I make a lump sum overpayment?
The sooner the better — every month a lump sum is paid earlier saves another month of interest. Enter the month number to see the effect of timing.
How accurate is the payoff date?
It is an estimate counted from today. Rate changes, payment holidays and the exact day your lender applies payments will move the real date.
More free tools you might need
- Mortgage Calculator — Work out monthly repayments, total interest and loan-to-value, repayment or interest-only.
- Mortgage Comparison Calculator — Compare two mortgage deals on rates, fees and true cost over the initial period.
- Loan Repayment Calculator — Calculate monthly repayments and total interest, and compare rates and terms.
- Loan Interest Calculator — Compare simple and compound interest and see the total cost of borrowing.