Simple Compound Interest Calculator
Enter initial capital, annual return percentage, and investment time in years. The calculator shows final balance, accumulated interest, total growth, and capital doubling time.
Final balance
Interest earned
Total growth
Doubling time (rule of 72)
What is Compound Interest?
Compound interest means that the return on an investment is added to the principal, so the next year's return is calculated from a larger amount. Over time, growth accelerates exponentially — the longer the investment period, the greater the portion of the final balance is interest, not the original principal.
Formula
Final balance = Principal × (1 + r)ⁿ
where r = annual return percentage as a decimal and n = number of years.
Rule of 72
Quick rule of thumb: divide the number 72 by the annual return percentage, and you get the approximate doubling time in years.
| Annual return | Doubling time |
|---|---|
| 3 % | 24 years |
| 5 % | 14.4 years |
| 7 % | 10.3 years |
| 10 % | 7.2 years |
| 12 % | 6 years |
Example
10,000 € invested at 7 % annual return:
- After 10 years: 19,672 € (growth +97 %)
- After 20 years: 38,697 € (growth +287 %)
- After 30 years: 76,123 € (growth +661 %)
Note how the return almost doubles with each new decade. This is the core of the compound interest phenomenon.
Other Calculators
- Investment Return Calculator — calculate the realized investment return percentage and annual return
- Compound Interest Calculator — more versatile version with monthly additional investments
- Savings Goal Calculator — calculate how much to save monthly