A mortgage overpayment calculator that shows exactly how much interest you save and how many years earlier your mortgage clears when you pay a fixed amount extra each month. For homeowners deciding whether to overpay, by how much, and whether it beats leaving the money in savings.
How it works
The tool builds two full amortisation schedules and compares them. The contractual monthly payment comes from M = P · r(1+r)ⁿ ⁄ ((1+r)ⁿ − 1), with r the monthly rate (annual rate ÷ 12 ÷ 100) and n the remaining months. It then simulates each month twice — once paying just the contractual amount, and once paying that plus your overpayment, which is applied straight to principal. The difference in total interest and in months to clear is your saving. Because the extra cuts the balance that interest is charged on, the effect compounds over the rest of the term.
Example
A £200,000 balance at 4.5% over 25 years has a payment of about £1,112/month. Overpay by £150/month:
- The mortgage clears in about 21 years instead of 25 — roughly 4 years early
- Interest saved: about £24,000
| Overpayment | Years saved | Interest saved |
|---|---|---|
| £50/mo | ~1y 8m | ~£9,000 |
| £150/mo | ~4y | ~£24,000 |
| £300/mo | ~7y | ~£40,000 |
(Figures for £200,000 at 4.5% over 25 years.) It is privacy-first: nothing leaves your browser. Check your lender’s annual overpayment limit (often 10%) before committing.