how accurate is the rule of 72 
the rule of 72 formula provides a reasonably accurate but approximate timeline reflecting the fact that it s a simplification of a more complex logarithmic equation to get the exact doubling time you d need to do the entire calculation the precise formula for calculating the exact doubling time for an investment earning a compounded interest rate of r per period is to find out exactly how long it would take to double an investment that returns 8 annually you would use the following equation t ln 2 ln 1 8 100 9 006 yearsas you can see this result is very close to the approximate value obtained by 72 8 9 years 