How to use this calculator
This tool is designed to be simple.
Just enter a few key details and compare both scenarios.
1. Enter your mortgage details
Add:
- Your remaining mortgage balance
- Your mortgage interest rate
This helps calculate how much interest you could save by overpaying.
2. Add your monthly overpayment
This is the amount you’re considering either:
- Putting towards your mortgage
- Or investing each month instead
3. Choose your investment return
This is your assumed yearly return if you invest instead.
A common long-term assumption is 5–8%, but you can adjust this.
4. Set your time period
Choose how many years you want to compare.
This shows how both strategies play out over time.
What the results show
You’ll see two clear outcomes:
🏡 Overpaying your mortgage
- How much interest you could save
- How much earlier your mortgage could be paid off
This is a guaranteed return, based on your interest rate.
📈 Investing instead
- The potential value of your investments over time
This is not guaranteed, but offers higher upside.
⚖️ The difference
The tool compares both outcomes so you can see:
- Which option could leave you better off financially
- The trade-off between certainty and growth
Important to remember
- Mortgage overpayments = low risk, guaranteed return
- Investing = higher potential, but with risk and volatility
- The “best” choice often depends on your goals, risk tolerance, and time horizon