Overpay Your Mortgage or Invest in Your ISA? The Maths for 2026's High-Rate World

With mortgage rates and cash ISA rates both sitting well above where they were a few years ago, the old rule of thumb — "investing usually beats overpaying your mortgage" — deserves a second look.

The numbers as they stand today

The best 5-year fixed mortgage rate currently available is 4.51% from HSBC, while the best easy-access cash ISA pays 4.63% AER from Trading 212. On the investing side, the FTSE 100 has returned strongly this year, closing at 10,653 on 2 July, though stock market returns are never guaranteed and can fall as easily as rise.

Why the calculation has changed

When mortgage rates were below 2%, overpaying made little sense for most people, because almost any diversified investment could reasonably be expected to outperform the interest saved. With typical mortgage rates now above 4.4%, the comparison is much closer. Overpaying your mortgage delivers a guaranteed, risk-free return equal to your mortgage rate — in effect, paying down a loan at 4.5% is like earning a guaranteed 4.5% after tax, since mortgage interest isn't deductible for ordinary homeowners in the UK. Investing offers potentially higher long-run returns, but with real volatility, as this week's market swings around US jobs data illustrate.

A framework, not a formula

There's no single right answer, but three factors matter most:

1. Emergency fund and higher-interest debt first. Neither overpaying nor investing makes sense before you have some cash buffer and have cleared any debt charging more than your mortgage rate, such as credit cards or personal loans.

2. Pension tax relief usually comes first. If you're not maximising employer pension matching, that's typically a better use of spare money than either option, since employer contributions and tax relief are close to a guaranteed uplift that neither mortgage overpayment nor ISA investing can match.

3. Your mortgage rate versus your expected investment return. If your mortgage rate is close to or above what you'd realistically expect from a diversified investment portfolio after costs, overpaying starts to look more attractive — especially given it's risk-free and shortens your total borrowing term.

Don't ignore ISA allowances

Because ISA gains and interest are entirely tax-free, and the current £20,000 annual allowance doesn't roll over if unused, some people choose to prioritise filling their ISA allowance and only overpay the mortgage with what's left, particularly since the cash ISA allowance is being capped at £12,000 from April 2027.

How this decision differs internationally

In the US, mortgage interest is tax-deductible for many homeowners who itemise, which tilts the calculation more toward investing in a Roth IRA or 401(k) rather than overpaying, since the effective after-tax cost of US mortgage debt is often lower than the headline rate. The UK offers no equivalent mortgage interest relief for owner-occupiers, which is one reason UK overpayment calculators tend to favour overpaying more readily than their US counterparts at similar interest rates.

Key Numbers

• 4.51% — best 5-year fixed mortgage rate from HSBC
• 4.63% AER — best easy-access cash ISA rate from Trading 212
• £20,000 — annual ISA allowance for 2026/27
• 10,653 — FTSE 100's 2 July 2026 close

Sources

• Uswitch: UK mortgage rates today
• MoneySavingExpert: Best cash ISAs
• BBN Times: FTSE 100 today
• GOV.UK: Individual Savings Accounts
• GOV.UK: Tax on your private pension

Educational content only — not financial advice.