Yes... Unfortunately, some people think that's the correct way to calculate doubling time for all rates of growth. Some pop investing books even elevate that ru

Yes... Unfortunately, some people think that's the correct way to calculate doubling time for all rates of growth. Some pop investing books even elevate that rule to some kind of magical status. One way to see why it's a good approximation is to do a Maclaurin series: (log 2)/(log(1+r/100)) = c/(log(1+x)) where c=(log 2) and x=(r/100) = c/(x - [x^2]/2 + [x^3]/3 - ...) ~ c/x = 0.693/x = 0.693/(r/100) = 69.3/r ~ 70/r