Overpay the mortgage, or invest?

You've got a spare few hundred pounds at the end of the month. The mortgage is sitting there at 5%. Your mate swears investing is the smart move; your dad says clear the debt. Both of them are half right - and neither is looking at your numbers.

It's really one question: guaranteed versus likely

Overpaying your mortgage is one of the only guaranteed, tax-free returns in personal finance. Knock £200 off a 5% mortgage and you've effectively earned a risk-free 5% on that money - no market, no luck, no timing. HMRC doesn't tax it, because it isn't income; it's interest you simply never pay.

Investing might beat that. On average. Over a long enough time. If you don't sell in a panic when the market drops 30% - and at some point in your investing life, it will. That word average is doing an enormous amount of work.

“Overpaying is a certain 5%. Investing is a probable 7% - with a stomach-churning ride to get there.”

The number that actually decides it

Line up two figures: your mortgage rate (you know this one exactly) and your realistic after-tax, after-fee investment return (you're guessing, and you should guess conservatively). A globally diversified tracker has historically returned around 5% a year after inflation - but with years of losses scattered through the good ones.

Then let the tax wrapper tip the scales. A pension adds tax relief on the way in - for a higher-rate taxpayer that's a 40% head start the mortgage can't match. An ISA grows completely tax-free. Both can quietly turn a close call into a clear one.

Your mortgage, your number

Numbers in an article are always someone else's. Drag the slider and watch what an overpayment does to a mortgage in real time - this is the same engine behind our full calculator, just dropped into the page.

Three things the calculator can't tell you

How you handle risk. If a falling market would make you sell at the bottom, the probable 7% isn't yours - you'd lock in the loss. An overpayment never does that to you.

Whether you'll need the money. Cash overpaid into the mortgage is buried in the bricks. You can't easily get it back without remortgaging. Investments in an ISA, you can. Liquidity has a value the spreadsheet ignores - which is also why an easy-access emergency fund comes before either option.

How the debt feels. Some people carry a mortgage without a second thought. Others feel it every month. Being mortgage-free years early is worth something real, even when it isn't the mathematically optimal move - and that's allowed. A decision you'll actually stick with for 20 years beats a theoretically perfect one you abandon after two.

So - which is it?

If your rate is high and certainty helps you sleep: overpay. If your debt is cheap, your horizon is long, and you'll genuinely leave the money invested through the scary bits: invest, ideally inside a pension or ISA. For a lot of people the honest answer is a bit of both - a modest recurring overpayment for the guaranteed win, alongside ISA contributions for the higher long-run potential.

There's no universally correct answer, and anyone who gives you one without asking about your rate, your risk tolerance and your timeline is selling something. This isn't personal financial advice - if the amounts are significant or your situation is complicated, a regulated adviser who can see your whole picture is worth the fee.