The 72 Rule is a great mental math shortcut to estimate how long will it take to double a certain amount of money at a given fixed annual rate of interest. It can be used for several financial estimates.

For example, the rule of 72 states that at 6% interest, your money takes 72/6 or 12 years to double. Starting with a specific time horizon, let's say 10 years, in order to double your money you need an interest rate of 72/10 or 7.2%. A country growing at 3% a year, doubles its economy in 72/3 or 24 years. Inflation estimates can also calculates, for example, if inflation rates go from 2% to 3%, money will lose half its value in 36 or 24 years.