What is the rule of 72?
The rule states that in order to get the estimated doubling time, simply divide
72 by a percentage growth in a single period. For example, if something grows by
5% per minute, then it will double in roughly
72 / 5 = 14.4 minutes. The actual number is closer to 14.2067 – this should give you an idea of how much off you may be when you use the rule of a thumb method.