See how much an overpayment really saves. Add a regular extra amount or a one-off lump sum and the calculator shows the interest saved and how many years come off your mortgage.
Overpaying £200/month clears the mortgage 6 years 2 months early.
New payoff18 yrs 10 mo
Interest without£150,754
Interest with£108,911
The normal monthly payment on this balance is £1,169. Overpayments are applied on top of it. Figures assume the rate stays fixed and overpayments continue every month.
Overpaying a mortgage saves money because every extra pound comes straight off the balance, so you pay interest on a smaller debt for the rest of the term. This calculator works out your normal monthly payment, then simulates the loan month by month both with and without your extra payments to show the interest saved and the time cut off.
How overpayment saving is calculated
Monthly payment = B · i · (1+i)ⁿ ÷ ((1+i)ⁿ − 1) where i = annual rate ÷ 12 and n = term in months. The extra amount is added to that payment each month until the balance reaches zero.
Worked example
On a £200,000 balance at 5% over 25 years, the normal payment is about £1,169 a month and you would pay roughly £150,754 in interest. Adding £200 a month clears the loan in about 18 years 10 months and cuts total interest to about £108,911 — a saving of £41,843 and just over 6 years.
Regular overpayment vs a lump sum
A one-off lump sum works hardest early in the term, when the balance — and so the interest — is largest. A regular monthly overpayment compounds its effect over time. You can combine both above. Always check your lender's overpayment limit: many allow up to 10% of the balance per year without an early-repayment charge.
Frequently asked questions
Does overpaying reduce the term or the payment?
By default it shortens the term — you keep paying the same amount but finish sooner. Some lenders instead let you lower the monthly payment; ask which option applies.
Is it better to overpay monthly or save the money?
Overpaying beats saving whenever your mortgage rate is higher than the after-tax interest on your savings. Keep an emergency fund first.
Will I be charged for overpaying?
Many fixed-rate deals allow overpayments up to 10% of the balance each year penalty-free; above that an early-repayment charge may apply. Check your mortgage terms.
Does this account for rate changes?
No — it assumes the rate stays fixed for the whole remaining term. If your rate changes, re-run the calculator with the new figure.
Estimates for general guidance only, not financial advice. Confirm figures and overpayment limits with your lender before acting.
Written by the CalcPine team · Reviewed for accuracy · Last updated 23 July 2026 · Method: standard amortisation simulated month by month, with and without overpayments.
