What Is the Compound Interest Calculator?
A compound interest calculator shows how money grows when your earnings themselves earn returns. By combining an initial amount, regular contributions, a return rate, and time, it projects a future balance and reveals the powerful effect of compounding.
How to use this calculator
Type your numbers into the fields above. The results change the moment you edit any input, so you can try one scenario after another and see exactly what moves. Most calculators show a short summary of the key figures, a line-by-line breakdown underneath, and — where it applies — a year-by-year schedule you can export to a spreadsheet. Everything runs in your browser; nothing is stored or sent anywhere. Treat the output as a planning estimate, not as final word on a real decision.
The Formula
Each period the balance grows by the periodic return rate, and contributions are added on top. Over time the growth accelerates because interest is calculated on an ever-larger balance. The calculator compounds monthly and sums your contributions to separate what you put in from what compounding added.
Worked Example
Start with $10,000, add $300 a month at a 7% annual return for 20 years, and the balance reaches about $180,000. Of that, roughly $82,000 is your contributions and about $98,000 is compound growth — more than your own deposits.
Tips for the Most Accurate Estimate
- Start early; time is the biggest driver of compounding.
- Increase contributions whenever your income rises.
- Reinvest dividends and interest to keep compounding.
- Even a 1% higher return adds up over decades.
- Use realistic return assumptions (6–8% for diversified stocks).
Frequently Asked Questions
Q: What does 'compound' mean here?
It means you earn returns on your returns. Each period's growth is added to the balance, so the base for next period's return is larger.
Q: How important is the return rate?
Very. A small difference compounded for decades produces a large gap in the final balance.
Q: Are the results guaranteed?
No. The calculator assumes a steady return for illustration; actual markets rise and fall.