The rule of 72 is a simple formula that tells you the approximate amount of time or interest rate needed for an amount to double. The formula is Years X Rate per year = 72.
Here's how it works. If you invest an amount for 8 years at 9% annual interest it will double (because 8 years X 9% = 72). If you invest an amount for 9 years at 8% it will also double (since 9 years X 8% = 72). If your investment earns 6%, it will take 12 years for it to double (since 12 years X 6% = 72; or 72 divided by 6 = 12).
If you invest $1,000 at 12% compounded annually, it will grow to approximately $2,000 in 6 years (6 X 12 = 72; or 72/12 = 6). If the $2,000 continues to earn 12% each year, six years later the investment will be worth $4,000. If the investment continues to earn 12% per year, then in six more years it will have a value of $8,000.
If successful investors were able to earn 18% each year, the value of their portfolios would have doubled every four years (72 divided by 18 = 4). If the investors live a long life and continue to earn 18% compounded annually they will become very wealthy.