Mortgage Overpayment Calculator

How the calculation works

The calculator works out the normal monthly payment for the balance, rate and years left, then runs the mortgage month by month twice: once as it is, and once with the lump sum taken off straight away and the extra amount added to every payment. The difference in total interest is the saving, and the difference in length is the time saved.

It keeps the normal payment the same, so overpayments shorten the term. Some lenders lower the monthly payment instead, which saves less interest. Interest is charged monthly at the annual rate divided by 12 and the rate is assumed to stay the same, so your lender's figures may differ. It also flags when the first year's overpayments go over 10% of the balance, a common annual allowance before early repayment charges apply.

Nothing entered here is sent anywhere. The calculation runs in your browser and no figures are stored, logged or added to the page address.

Mortgage overpayments explained covers allowances and what to check before overpaying, and when my fixed rate ends explains why the end of a deal is often a good time to overpay.