The rule of 72 is a formula you can use to see how long it will take for your investment to double when you factor in the rate of return. The rule of 72 can be used if you’d like to quickly estimate the ending balance without using the more complex compound interest formula. The formula is:
72 ÷ compound annual interest rate (without the %) = the number of years
Using the rule of 72, you would estimate that an investment with a 5% compound interest rate would double in 14 years (72/5).