About this calculator
Paying a little extra each month can take years off a repayment mortgage and save thousands in interest. Enter the amount borrowed, the interest rate, the term and a monthly overpayment to see the effect.
Leave the overpayment empty to see the standard payment and total interest. This assumes a fixed rate with interest added monthly, which is close to how a standard UK repayment mortgage works.
Worked examples
Real numbers, worked out by the same calculator. Press “Use these numbers” to try one above.
£200,000 at 4% over 25 years, no overpayment
- Standard monthly payment
- £1,055.67
- Total interest
- £116,702.10
- Total you pay back
- £316,702.10
Borrowing £200,000.00 at 4% over 25 years costs £1,055.67 a month and £116,702.10 in interest. Add a monthly overpayment to see how much you would save.
Show the working
- Standard payment = P × i ÷ (1 − (1 + i)^−n) with i = 4% ÷ 12 and n = 300: £1,055.67.
- No overpayment entered.
- This assumes a fixed rate, interest added monthly, and no early-repayment charges. Check your lender's overpayment limits.
Same mortgage with £200 a month extra
- Standard monthly payment
- £1,055.67
- Payment including overpayment
- £1,255.67
- Paid off after
- 19 years
- Time saved
- 6 years
- Total interest with overpayments
- £85,635.03
- Total interest without
- £116,702.10
- Interest saved
- £31,067.08
Overpaying £200.00 a month clears the loan in 19 years instead of 25 years and saves £31,067.08 in interest.
Show the working
- Standard payment = P × i ÷ (1 − (1 + i)^−n) with i = 4% ÷ 12 and n = 300: £1,055.67.
- Each month interest is added to the balance, then £1,255.67 is paid. The balance reaches zero after 228 months.
- This assumes a fixed rate, interest added monthly, and no early-repayment charges. Check your lender's overpayment limits.
£150,000 at 5.5% over 20 years, £100 extra
- Standard monthly payment
- £1,031.83
- Payment including overpayment
- £1,131.83
- Paid off after
- 17 years 1 months
- Time saved
- 2 years 11 months
- Total interest with overpayments
- £81,428.39
- Total interest without
- £97,639.43
- Interest saved
- £16,211.04
Overpaying £100.00 a month clears the loan in 17 years 1 months instead of 20 years and saves £16,211.04 in interest.
Show the working
- Standard payment = P × i ÷ (1 − (1 + i)^−n) with i = 5.5% ÷ 12 and n = 240: £1,031.83.
- Each month interest is added to the balance, then £1,131.83 is paid. The balance reaches zero after 205 months.
- This assumes a fixed rate, interest added monthly, and no early-repayment charges. Check your lender's overpayment limits.
Why overpaying works
Interest is charged on the balance you still owe. Every pound you overpay cuts the balance immediately, so you pay less interest next month, and that saving compounds. Overpaying early in the term has the biggest effect.
Check your limits
- Many fixed-rate deals allow up to 10% of the balance a year without a penalty
- Above that, an early repayment charge may apply
- Ask your lender whether an overpayment shortens the term or lowers the monthly payment (this calculator shortens the term)
- Compare the interest rate on your mortgage with what savings pay: overpaying is a guaranteed return equal to your mortgage rate
Limits of this calculator
It assumes one fixed rate for the whole term. Real mortgages often switch to a variable rate after a fixed period, and lenders calculate interest daily or annually. Treat the result as a good estimate and check with your lender.
Frequently asked questions
How much can I save by overpaying my mortgage?
On £200,000 at 4% over 25 years, £200 a month extra saves about £31,000 of interest and finishes six years early.
Is it better to overpay or save?
If your savings rate is below your mortgage rate, overpaying usually wins, but keep an emergency fund first.
Will overpaying lower my monthly payment?
In this calculator it shortens the term. Ask your lender if you can choose to lower the payment instead.
Are there penalties for overpaying?
Often an annual limit, commonly 10%, applies on fixed deals. Check your terms.
Does it include fees?
No. Only interest and capital.
Formulas tested against hand-worked answers. Last reviewed 29 September 2026. These calculators do arithmetic only; they are not financial, tax or legal advice.