Mortgage Overpayment Calculator
This calculator shows what regular monthly overpayments, a one-off lump sum, or both would save on your mortgage. It assumes your interest rate stays the same for the remaining term and that overpayments reduce the term rather than the monthly payment. Check your lender's overpayment allowance before paying extra — early repayment charges can apply.
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Overview
The mortgage overpayment calculator shows what paying a little extra would really do to your mortgage. Enter your current balance, interest rate and remaining term, then add a regular monthly overpayment, a one-off lump sum, or both. The calculator works out your contractual monthly payment, re-runs the repayment schedule with the extra money going straight at the balance, and reports the interest saved and how much sooner the mortgage clears.
Overpayments are powerful because every pound paid early stops interest being charged on it for the whole of the remaining term. On a typical repayment mortgage, even a modest monthly overpayment made consistently removes years from the term — and because the balance falls faster, each later payment clears more capital than it otherwise would. The calculator assumes your rate stays the same for the remaining term and that overpayments shorten the term (the way most UK lenders apply them by default) rather than reducing the monthly payment.
How it works
The outstanding balance, your current annual interest rate and the term you have left in years and months — all three are on your latest mortgage statement.
A regular monthly overpayment, a one-off lump sum paid now, or both. The lump sum comes off the balance immediately; the monthly amount is paid on top of every instalment.
We simulate the mortgage month by month with and without the overpayments, and report the interest saved, the new payoff term and how much earlier you're mortgage-free.
Worked example
£200,000 at 4.5% over 25 years, overpaying £100 a month
A £200,000 mortgage at 4.5% with 25 years to run costs about £1,111.66 a month, and roughly £133,500 in interest over the full term. Overpaying £100 a month — about 9% extra — clears the mortgage in around 21 years and 6 months instead.
That single change saves about £21,142 in interest and makes you mortgage-free 3 years and 7 months earlier. The earlier in the term you start overpaying, the bigger the saving, because the balance — and therefore the interest — is at its largest in the early years.
Frequently asked questions
Is it better to overpay monthly or with a lump sum?
How much can I overpay without a penalty?
Should I overpay my mortgage or save the money instead?
Does overpaying reduce my term or my monthly payment?
What if my interest rate changes during the term?
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