world's population doubling time
The Rule of 70. 70 / % growth = doubling time. 70 / doubling time = % growth. For example, if you hear that the population of your town is growing by 2% per year, that means it will double in just 35 years! (70/2=35) It works in reverse, too: divide 70 by the doubling time to find the growth rate. If you hear that U.S. population is due to double in 70 years, you know that it's increasing at 1% per year (70/70=1).