The decision: a guaranteed return vs a risky one

When you have spare cash each month, two common moves are: overpay the mortgage or invest it. Overpaying is like earning a guaranteed, tax-free return equal to your mortgage rate. Investing offers a potential higher return, but the value can fall. This is a risk trade-off, not a maths problem with one answer.

This example uses the mortgage calculator's default: £240,000 at 5% over 25 years.

The numbers: overpay vs invest

Extra per monthOverpay: interest savedOverpay: cleared earlyInvest at 7%: pot after 25y
£200£43,9775y 4m sooner£162,014
£300£58,5227y 3m sooner£243,022

Baseline: at 5% over 25 years the monthly payment is about £1,403 and you pay £180,905 in total interest. Adding £200 a month to the mortgage saves about £43,977 of interest and clears the loan 5 years 4 months early. The same £200 a month invested in an ISA at a hypothetical 7% could grow to about £162,014 after 25 years.

Why the two numbers are not directly comparable

The £43,977 of interest saved is guaranteed — it is a tax-free return on your money with no risk. The £162,014 ISA figure is a hypothetical outcome at a constant 7%: markets fall as well as rise, and the actual amount could be much lower (or higher). The honest framing is:

  • Overpaying is a risk-free, tax-free return at your mortgage rate (5% here). Its downside: your money is locked into the house — less liquid, and you miss out if investments do better.
  • Investing targets a higher expected return but carries volatility and no guarantee. Its upside: liquidity, and potentially more money over the long term.

A common rule of thumb: if you expect after-tax investment returns to comfortably beat your mortgage rate, investing may come out ahead over a long horizon — but only if you can stick with it through downturns. If you value guaranteed, tax-free savings and a faster path to a debt-free home, overpaying is hard to beat.

Practical points before you decide

  • Keep an emergency fund first — before overpaying or investing, have 3–6 months of expenses in easy access.
  • Check your overpayment limit — most lenders allow 10% a year without an early-repayment charge; above that you may pay one.
  • Pension employer match beats both — if your workplace pension offers matching, that is usually the best first move.
  • Tax matters — mortgage overpayments are effectively tax-free; taxable investments need to beat the mortgage rate after tax.

Frequently asked questions

Is it better to overpay the mortgage or invest?
Overpaying gives a guaranteed, tax-free return equal to your mortgage rate. Investing offers a higher potential return but with risk. If you expect investments to beat your mortgage rate after tax over the long term — and can stomach the swings — investing can win; otherwise overpaying is the safer choice.
What return do I need from investing to beat overpaying?
Your investments need to beat your mortgage rate after tax. At a 5% mortgage, a basic-rate taxpayer needs investments to clear roughly 6.25% before tax to match — a high bar for low-risk investments.
Should I clear my mortgage before investing?
Not necessarily. A pension with employer matching, an emergency fund and possibly an ISA for long-term goals can all be reasonable alongside a mortgage. The right order depends on your rates, tax situation and risk tolerance.
Does this include fees or taxes?
No. Investment platform fees, fund charges and any early-repayment charges on overpayments are not included. Fees can meaningfully reduce the investment side of the comparison.
Educational tools only — not financial advice. Example figures assume constant returns, no fees and no inflation; investments can fall as well as rise. Rules and rates can change — always verify against the latest official guidance.