Divide the first number, the lower population figure, by the second number, the higher population figure. For example, if Smallville's population in 2010 was 80, and the population in 2000 was 100, divide 80 by 100. So -- 80/100 = 0.80.
Multiple the result of Step 1 by 100. In our example: 0.8 x 100 = 80.
Subtract the result of Step 2 from 100. In our example, 100 - 80 = 20. The percentage decrease in Smallville's population was 20%.