Rule of 72
72 ÷ return = years for money to double.
The rule of 72 is a quick way to estimate how long it takes money to double: divide 72 by the yearly return.
Using it
At 8% a year, money doubles in about 72 ÷ 8 = 9 years. At 6%, about 12 years. It works the other way too: to double in 10 years you need roughly 7.2% a year.
It is an approximation that works best for returns between about 5% and 10%.