Overpay vs invest
The kitchen-table classic: the same monthly money against the mortgage or into the market, side by side.
Overpaying earns a guaranteed, tax-free return equal to your mortgage rate.
Overpaying earns a guaranteed, tax-free return equal to your mortgage rate. Investing offers a higher expected return that is uncertain and may be taxed. The calculator puts the same monthly sum into each and compares the benefit at the end of the term.
The comparison is mathematical only. It ignores what usually decides the question in practice: emergency cash, employer pension matching, ISA allowances, and how you would feel in a 30% market fall.
Formula
interest saved at m% vs FV of monthly sum at r%, over the same term
Both series run to the original mortgage end date.
- 01Mortgage rate and investment return held constant.
- 02No tax on investment growth (as if inside an ISA or pension); no fund or platform charges.
- 03No early-repayment charges; overpayments allowed every month.
- 04Ignores pension tax relief, which usually beats both.
Holding everything else at your inputs
Each row changes one variable and shows the result. It illustrates which assumptions the answer is most sensitive to — it does not suggest which to choose.
| Variable | Changed to | Investing minus overpaying | vs your result |
|---|
Reminder: this tool is general education, not a personal recommendation — the right answer depends on rates, tax, temperament and circumstances it can't see. For a decision, consult an FCA-authorised adviser; our toolkit shows how to find one.