Overpay Mortgage vs Invest Calculator

Should you overpay your mortgage or invest spare cash?

This is one of the most common financial questions for UK homeowners — and there is no single right answer. Overpaying your mortgage reduces the balance you are charged interest on each month, which produces a return equivalent to your mortgage rate. Because that saving is certain, it is sometimes compared to a risk-free return. Investing spare cash instead, for example into a Stocks and Shares ISA or a general investment account, may produce higher growth over the long term, but returns are variable and not guaranteed.

A fair comparison needs to account for what happens in each scenario after the mortgage ends. If you overpay and clear the mortgage early, that monthly outflow becomes available to invest — so the comparison is not simply overpaying versus investing for the same number of years. This calculator models that full picture, showing projected outcomes at 5, 10, and 15 years for both paths based on your own numbers.

Other factors worth considering alongside the numbers include your lender's annual overpayment limit (typically 10% of the outstanding balance for fixed-rate mortgages), access to an ISA allowance, emergency fund status, and any other higher-interest debt. The figures produced by this tool are illustrative only and do not constitute financial advice.

The projections above are estimates based on the assumptions you enter. Investment returns are not guaranteed and past performance is not a reliable indicator of future results. This tool does not constitute financial advice. If you are unsure which option is right for your circumstances, consider speaking with an independent financial adviser regulated by the FCA.