The "Rule of 72" is a rule of thumb to estimate how long it takes for a number (usually an amount of money) to double in value at a given rate of increase, often expressed as an interest rate or growth rate. Change the rule factor to see the change in error between the rule and the correct number for a given interest rate. Change the interest rate to see the error for a rule factor at different interest rates. Change the interest rate range to see how effective a rule number is for that interest rate range. The idea is to select a rule factor that keeps the calculation easy and minimizes the error for the growth rate range of interest. Is 72 always the best number for the "Rule of 72"?