Rule of 72 Calculator

The Rule of 72 is a simple formula: divide 72 by the annual interest rate to estimate how many years it takes for an investment to double. For example, at 6% annual return: 72 / 6 = 12 years to double. It's derived from the compound interest formula ln(2)/ln(1+r) and works as a convenient mental math shortcut.

The Rule of 72 is most accurate for interest rates between 6% and 10%, where the error is less than 1%. At very low rates (1-2%) or very high rates (20%+), the approximation becomes less precise. This tool shows both the Rule of 72 estimate and the precise mathematical calculation so you can see the difference.

Yes. The Rule of 69.3 is mathematically more precise (since ln(2) ≈ 0.693) and works better for continuously compounded interest. The Rule of 70 is sometimes used for simplicity. 72 is popular because it's divisible by more numbers (2, 3, 4, 6, 8, 9, 12), making mental math easier.