Mortgage Overpayment vs Invest Calculator
Should you overpay mortgage or invest the difference.
Compare mortgage overpayment vs investing extra money — see which produces the better long-term financial outcome at your rates.
What this tool does
This calculator models two alternative uses for a fixed monthly sum: overpaying a mortgage or investing it elsewhere. It estimates the financial outcome of each path by comparing the effective return rate of mortgage interest avoided against projected investment returns over your chosen timeframe. The result shows the cumulative advantage—in your currency—of choosing one option over the other. Your mortgage rate, expected investment return percentage, monthly amount available, and time horizon are the primary drivers of the outcome. A typical scenario might compare overpaying a home loan versus investing in a diversified portfolio. The calculation assumes consistent monthly contributions, stable rates, and does not account for tax implications, transaction costs, inflation adjustments, or changes in circumstances. Results are illustrative approximations for educational purposes and should not be viewed in isolation from your full financial picture.
Enter Values
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Formula Used
Spotted something off?
Calculations or display — let us know.
Disclaimer
Results are estimates for educational purposes only. They do not constitute financial advice. Consult a qualified professional before making financial decisions.
Mortgage overpayment vs investing comes down to rate vs expected investment return. If mortgage rate (after tax benefits) exceeds expected investment return, overpay wins. If investment return higher, invest. Risk-adjusted comparison matters too — mortgage overpayment carries less uncertainty.
What the result means
Better option for the inputs. Real-world consideration: mortgage overpayment certain return, investment return uncertain. Risk-adjusted, mortgage often wins even at slightly lower 'return'.
Run it with sensible defaults
Using mortgage rate of 5%, expected investment return of 7%, monthly extra amount of 300, time horizon of 15, the calculation works out to Invest. The defaults are meant as a starting point, not a recommendation.
The levers in this calculation
The inputs — Mortgage Rate, Expected Investment Return, Monthly Extra Amount, and Time Horizon — do not pull with equal force. The rate and the time horizon usually dominate — compounding means a small change in either reshapes the final figure more than a similar shift in contribution size. Test this by doubling one input at a time.
How the math works
Compares effective return rates. Overpayment 'returns' the mortgage rate (interest avoided). Investment returns expected market return.
Why this matters before you sign
A mortgage is usually the biggest single financial commitment a person makes. The difference between a well-chosen product and a hasty one can run into tens of thousands over the life of the loan. Running the numbers here before committing is the cheapest form of due diligence available.
What this doesn't capture
The figure excludes arrangement fees, valuation costs, legal fees, insurance, and any early-repayment charges — those can add several thousand to the headline cost. Rate changes at renewal for fixed-term deals will shift the picture further. Use this for the core interest/principal math and add the other costs on top.
Investing £300 monthly over 15 years years at 7% return versus overpaying a 5% mortgage results in Invest.
Inputs
This example uses typical values for illustration. Adjust the inputs above to match a specific situation and see how the result changes.
Sources & Methodology
Methodology
Compares effective return rates. Overpayment 'returns' the mortgage rate (interest avoided). Investment returns expected market return.
Frequently Asked Questions
Is the math really that simple?
What about emotional benefit?
Can I do both?
Risk consideration?
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