The law of 72 is a formula that is used to estimate how long it will take for an investment to double. It is accurate for interest rates that range between 6% and 10%. The formula is shown below: Good to know Years required to double investment = 72/compound annual interest rate For example, if an investment had a 6 percent annual compounded interest rate (72/6) it would take an estimated 12 years for the investment to double. Let's use the example of $1,000 with an interest rate of 6%: