Rule of 72 will tell you how long it will take your money to double.
To use the Rule, simply divide 72 by the interest rate you are earning and the quotient is the number of years it will take to double your money.
Deposit $1,000 in and account.
The account earns 8% interest and you do not add any more money.
You will have $2,000 in 9 years.
72 / interest rate = # of years until money doubles