Double your money with this formula: The Rule of 72.

This rule will determine how long it will take to double your initial investment when you have a compound interest rate that you will receive. AND you can determine what interest rate you want to achieve if you know you want to double your money (initial investment) in a certain amount of time.

It's not exact down to the penny but it's quick/easy math to know if you are close to your desired dollar amount when it comes to your long term investments. Remember: long term is 5 or more years away and most likely still decades away.