This calculator compares two possible uses for the same spare monthly amount: overpaying a repayment mortgage or investing the money instead. It is not designed to decide for you. Mortgage overpayments have a more certain interest-saving effect, subject to lender rules and charges. Investing has uncertain returns, can fall in value and puts capital at risk. The result is a simplified comparison to help you think through the trade-off before reading the detail.
What this calculator assumes
- The same monthly amount is used for both routes.
- The mortgage is treated as a repayment mortgage and the overpayment is assumed to be allowed.
- The investment return is assumed, not guaranteed, and annual investment fees are deducted.
- Tax, changing mortgage rates, product fees and personal circumstances are not fully modelled.
- This is educational only and is not financial advice.
What the comparison is showing
The investment side estimates what the monthly amount could grow to over the comparison period using the return and fee assumptions entered. The mortgage side estimates how much interest could be saved if the same monthly amount were paid as a mortgage overpayment during that period. These are different types of outcome, so the numerical difference should not be read as a recommendation. It is a prompt for deeper thinking about certainty, risk, access to cash and your mortgage terms.
Why the mortgage side feels different
Overpaying a repayment mortgage can reduce the balance sooner, which may reduce future interest and shorten the loan. The practical outcome depends on your lender, whether overpayments reduce the term or the monthly payment, any annual overpayment allowance and whether early repayment charges apply. That is why the calculator is only an estimate. Your mortgage offer and lender documents matter more than any calculator output.
Why the investing side is uncertain
The investment estimate uses a smooth assumed return, but real markets do not move smoothly. Investments can fall as well as rise, fees reduce returns and you could get back less than you put in. Investing may also leave money more accessible than a mortgage overpayment, but that access can come with market risk. The calculator keeps the maths simple so you can compare assumptions without treating the output as advice.
Useful official sources
FAQs
Does this tell me whether to overpay my mortgage or invest?
No. It shows a simplified numerical comparison based on the inputs entered. The right decision depends on risk tolerance, mortgage terms, cash needs, tax, time horizon and personal circumstances.
Why is the investment estimate risky?
The investment result depends on an assumed return. Real investment returns can be higher or lower, investments can fall in value and capital is at risk.
Can mortgage overpayments trigger charges?
They can. Some mortgage deals limit how much you can overpay without an early repayment charge. Check your lender documents before making overpayments.