Overpay Your Mortgage or Invest? The Maths Most People Get Backwards
One option guarantees a tax-free return equal to your mortgage rate. The other has historically paid more but guarantees nothing. Here's the actual comparison.
By Firoz Khan|5 May 2026|Updated 20 September 2026|7 min read
Financial advisers often default to telling everyone to overpay their mortgage, because it sounds responsible and nobody gets blamed for recommending debt reduction. But overpaying is not automatically the better move, and treating it as the safe default ignores decades of data on what a Stocks and Shares ISA has actually returned. The right answer depends on your mortgage rate, your risk tolerance and your time horizon, and it changes when interest rates move. Here is the maths, not the platitude.
What overpaying actually guarantees you
Every pound you overpay on your mortgage earns a guaranteed, risk-free, tax-free return equal to your mortgage interest rate. If your mortgage rate is 5%, overpaying is mathematically identical to an investment that returns 5% a year with zero volatility and no tax to pay, which no ISA or savings account can promise. This is the strongest argument for overpayment when rates are high: a 6% mortgage rate means you would need a taxable investment returning meaningfully more than 6% to beat it after accounting for risk, and that is not guaranteed anywhere.
What investing has historically returned
A globally diversified Stocks and Shares ISA has returned around 7% to 9% a year on average over multi-decade periods, though any single 10-year stretch can land well outside that range, including negative. Unlike overpayment, this return is not guaranteed and the value can fall by 20% or more in a bad year before recovering. The advantage is compounding over a long enough horizon and the tax-free wrapper of the ISA, which shields gains and dividends from tax exactly the way overpayment shields your saved interest from tax.
A worked example over 20 years
Take a £200,000 mortgage balance with 20 years remaining, and suppose you have £300 a month spare. At a 4.5% mortgage rate, overpaying £300 a month typically cuts around 6 to 7 years off the term and saves roughly £28,000 to £32,000 in interest, a result that is certain.
Put that same £300 a month into a Stocks and Shares ISA instead, assuming an average 7% annual return over 20 years, and you would end up with a pot of roughly £150,000, of which about £72,000 is your own contributions and £78,000 is growth. That is a larger headline number than the interest saved, but it came with two decades of market swings and no guarantee the 7% average actually shows up in your specific 20 years.
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How your mortgage rate changes the answer
The comparison flips depending on where rates sit. At a 2% mortgage rate, a common level in 2021, overpaying only guarantees you 2%, and a long-term investment portfolio has historically beaten that comfortably enough to make investing the stronger mathematical choice. At a 6% mortgage rate, the guaranteed return from overpaying gets close enough to typical investment returns, after adjusting for the risk you are taking on, that overpaying becomes far more competitive. There is no fixed rule here. Compare your actual mortgage rate to a realistic, risk-adjusted expected investment return before deciding, and revisit the comparison whenever your mortgage rate changes at the end of a fixed term.
Overpayment caps and early repayment charges
Most fixed-rate mortgage deals cap penalty-free overpayments at 10% of the outstanding balance per year. Exceed that and you typically face an early repayment charge of 1% to 5% of the amount over the limit, which can wipe out the interest saving entirely. On a £200,000 mortgage, that means you can usually overpay up to £20,000 in a year without penalty, but check your specific mortgage offer document, because the percentage and the base it is calculated on both vary by lender.
The psychological case for overpaying that the maths ignores
Being mortgage-free earlier has value that does not show up in a spreadsheet. It removes a fixed monthly obligation, reduces stress around job loss or income disruption, and gives some people genuine peace of mind that a bigger ISA balance does not. If you know that market volatility would make you panic-sell an investment during a downturn, the guaranteed, boring certainty of overpayment may be worth more to you than the mathematically optimal outcome, and that is a legitimate reason to choose it.
The hybrid approach most people should actually use
You do not have to choose one exclusively. A common approach is to overpay up to the 10% penalty-free cap each year while also contributing to a Stocks and Shares ISA, splitting the benefit of guaranteed interest savings and long-term market growth. This also means you are not locking every spare pound into an asset, your home, that you cannot easily access if you need cash, while an ISA can be withdrawn or sold if a genuine emergency arises, subject to market conditions at the time.
Get the emergency fund right before either
Neither overpayment nor investing should come before a cash emergency fund covering three to six months of essential expenses, held somewhere accessible like a savings account or Cash ISA. Overpaid mortgage money is typically difficult or impossible to withdraw again without a further advance or remortgage, and a Stocks and Shares ISA can be down in value at the exact moment you need to sell. A £1,200 boiler repair should never force you to sell investments at a loss or request an emergency mortgage drawdown, and building the buffer first avoids that trap entirely.
A reminder
The FCA risk warning still applies to higher-risk crypto content. Always assess how much risk you are willing to take before buying.
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