Mortgage Overpayment Calculator
See how much interest and time you could save by overpaying your mortgage each month.
New repayment period
18 yr 10 mo
- Interest saved
- £41,842.60
- Time saved
- 6 yr 2 mo
- Normal monthly payment
- £1,169.18
- New monthly payment
- £1,369.18
Estimates only. Assumes a repayment mortgage at a fixed rate with no fees or overpayment limits.
v1.0 · Reviewed September 2026
How this is worked out
The normal monthly payment is worked out from the balance, rate and remaining term using the standard repayment formula. The balance is then run forward month by month with your overpayment added, and the two paths are compared.
Worked example
- Balance £200,000
- Rate 5%
- 25 years left
- £200 a month extra
normal payment £1,169.18, paying £1,369.18 instead
Clears in 18 yr 10 mo, saving £41,842.60 of interest
Assumptions and limits
- Assumes the interest rate stays the same for the whole remaining term, which rarely happens on a UK mortgage moving between fixed deals.
- A repayment mortgage with interest applied monthly, and every overpayment reducing the balance immediately.
- Ignores early repayment charges and annual overpayment limits, which are commonly 10% of the balance a year on a fixed deal.
- Assumes the term shortens rather than the monthly payment being reduced. Lenders often default to the second, which saves far less.