Rule of 72

I always used to wonder how long it would take for money to double at any given rate of interest. I seem to have found this easy way out

Assume that you are promised 8% rate of interest .Now, as per this "Rule of 72" , you would need to divide this number 72 by 8 to get at that (72/8 = 9). That would mean, you would need 9 years for your invested money to double in value , provided you invest it for that period of time and let the amount compound annually.

Please correct me if i'am wrong