Mortgage overpayment calculator — and the honest invest-instead comparison

The interest saved and years cut, computed month by month by the same amortisation the Hundred Summers app uses — then compared, honestly, against investing the same monthly amount: guaranteed saving versus expected-but-uncertain growth. Your figures never leave your browser.

What overpaying really earns

Every pound overpaid stops accruing interest at your mortgage rate for the rest of the term — a guaranteed, tax-free return equal to your rate. No investment offers that certainty. The month-by-month effect compounds: overpayments shrink the balance, which shrinks the interest, which makes every later payment clear more capital, which is why modest monthly sums cut whole years off the term.

The honest comparison with investing

Investing the same money is expected to beat a 4–5% mortgage over long periods — but not guaranteed to, and not smoothly. The fair comparison also credits the overpayment route with what happens after the mortgage clears early: the whole freed payment can then be invested for the saved years. The calculator above does both sides properly. Tax matters too: inside an ISA the comparison is clean; outside one, dividend and capital gains tax drag the investing route.

Two prior claims on the money beat both routes: expensive debt, and an emergency fund. And check your deal's overpayment allowance (typically 10% a year) for early repayment charges. The full Hundred Summers plan models the overpayment inside your whole lifetime picture — the ISA vs pension question is often the real competitor for the money.