Overpay the mortgage or invest? The honest arithmetic

Overpaying earns your mortgage rate guaranteed and tax-free; investing has to beat it after fees, tax and nerve. How to compare the two properly, including what happens after the mortgage clears early.

One lever among many. Optimising your plan shows every lever, which ones apply to you and how they interact.

The most-asked question in UK personal finance has a two-line core: overpaying your mortgage earns your mortgage rate, guaranteed, tax-free; investing the same money is expected to earn more, but isn't guaranteed to, and won't do it smoothly.

Comparing like with like

A fair comparison needs three honesty adjustments. First, the after-mortgage years: overpaying clears the loan early, and the whole freed payment can then be invested for the saved years. Leaving this out flatters investing. Second, tax: inside an ISA the comparison is clean; outside one, dividend and capital gains tax drag the investing route, and at higher rates the drag is material. Third, your deal's terms: most fixes allow 10% a year penalty-free, with early repayment charges beyond it.

What the numbers usually say

At mortgage rates around 4–5% against equity expectations of 5–7% after fees, investing carries a real but modest expected edge, bought with genuine risk: a bad decade can leave the invested route behind for years. That's why the honest answer is often deliberate diversification: some overpayment (a guaranteed return and falling fixed costs), some investing (the growth engine), with the split set by temperament as much as arithmetic. Two prior claims beat both: expensive debt, and an emergency fund.

Hundred Summers' overpayment calculator runs both routes properly (month-by-month amortisation on the mortgage side, the freed-payment years credited), and the full plan models the overpayment inside your whole lifetime picture, where the real competitor for the money is often the pension.

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