The Rule of 72 is a formula used to quickly calculate approximately how much time it will take you to double your investment given any compound interest rate. The Rule of 72 for-mula is as follows:

Years To Double Your Money = 72 / (Compound Interest Rate)

To apply this to our example in the compounding section, here is how to calculate how much time it will take you to double your initial investment of $1,000 in a bank account earning 10% compound interest:

72 / 10 = 7.2 years