Invest vs Pay Off Mortgage Calculator

The core trade-off: guaranteed vs probabilistic return

Every pound of spare cash can do exactly one thing. Knock down debt, or buy assets. Overpaying your mortgage is a guaranteed, risk-free return at your mortgage rate. If your fix is at 5%, every £100 you overpay saves £100 of future interest, full stop. Investing the same £100 carries a higher expected return, but the actual outcome lives on a probability distribution. Some years it pays 25%, some years it loses 30%, and the long-run average only emerges if you stay in the seat through both.

That asymmetry is the heart of the decision. A guaranteed 5% from overpayment is not the same product as a 7% expected return from a global tracker, even though the headline numbers look comparable. The right framing is risk-adjusted: how much extra expected return is the market offering for taking equity risk, and is that premium big enough after tax to justify the volatility? Run your own numbers through the mortgage calculator first to see how the total interest bill changes when you accelerate repayment.

Compare after tax, not before

The most common mistake we see is comparing a nominal 7% equity return to a 5% mortgage rate and declaring victory for investing. That comparison is only honest if the investment sits inside a tax wrapper. Outside one, dividend tax and capital gains tax chew the headline return down to something closer to 5.5% for a basic-rate taxpayer and less for higher earners. Suddenly the 2-point spread you thought you had is barely half a point, and the certainty of debt reduction starts to look very attractive.

Inside a Stocks and Shares ISA, all gains and dividends are completely tax-free. The 7% expected return stays a 7% expected return, which gives you a clean 2-point spread over a 5% mortgage. Inside a SIPP with employer-matched contributions, the effective first-year return is dramatically higher because you are getting income tax relief on the way in. Mortgage overpayment cannot compete with a pension match, full stop. That is why we keep saying: never sacrifice the match.

The psychological argument is real (and the spreadsheet can't see it)

A paid-off mortgage means you sleep at night. It means your monthly fixed costs drop by whatever your mortgage payment used to be, which makes a redundancy or a career break survivable on a fraction of your old income. It means you can take risks elsewhere in your life, such as leaving a bad job or starting a business, without the bank knocking on the door if it goes wrong. None of that shows up in the wealth-maximisation maths the calculator runs, but it is real and it matters.

We are not in the business of telling you to ignore the maths. But if you are the kind of person who lies awake worrying about debt, the optimal strategy on paper is the one you cannot stick with. A small expected wealth penalty in exchange for being mortgage-free a decade early is a perfectly defensible trade. The freedom of zero monthly housing costs is one of the most underrated advantages a UK worker can buy themselves, and the spreadsheet undervalues it.

UK-specific nuances that change the answer

Three UK realities tilt the comparison from where a US-centric calculator might land it. First, current fixed mortgage rates sit around 5% for most borrowers, well above the 2 to 3% rates of a decade ago. That has narrowed the spread between mortgage and expected equity return considerably. Second, global equities have returned roughly 7% nominal per year historically, which is about 4 to 5% after UK inflation. The real after-tax spread for a higher-rate taxpayer investing outside a wrapper can be negative. Third, the ISA and SIPP wrappers are unusually generous by international standards: £20,000 a year tax free into an ISA, plus up to £60,000 a year into a pension with tax relief. Most households never come close to filling either.

There is also a step-function effect at remortgage time that the year-by-year maths misses. A lump sum that drops your loan-to-value ratio across a band boundary (for example, from 81% to 79%) re-prices the rate on the entire remaining balance for the next fixed term, not just the overpaid portion. If you are approaching a fix end and a well-timed overpayment moves you into a cheaper LTV band, the effective return on that overpayment can be dramatically higher than your mortgage rate. The calculator above does not model this; it is a manual judgment you make at every remortgage event.

Worked examples

Our opinion: the right answer is usually both

The framing of "invest or overpay" is mostly a false binary. For most UK households the right order of operations is clear and worth saying out loud. Take the full employer pension match first, because nothing else returns 100% on day one. Use our pension match calculator to see how much free money you are leaving on the table if you are not maxing it. Then fill your ISA, because £20,000 a year of tax-free growth compounds into serious money over a working career, as the compound interest calculator will show you. Whatever is left after the match and the ISA can sensibly go into mortgage overpayment, especially at today's higher fixed rates.

The one thing we would never do is sacrifice the pension match to overpay the mortgage. That is taking a guaranteed 5% over a guaranteed 100%, which is a category error rather than a trade-off. Beyond that, the calculator is the right tool to see what the maths says for your specific numbers, but the maths is not the whole answer. A worker who feels safer with a paid-off house is a worker with more bargaining power, and that is worth a couple of percentage points of expected return all by itself.

Frequently asked questions