Body Mass Index
Today, obesity is most often measured by using a mathematical formula called a Body Mass Index. BMI can be determined by dividing your weight in pounds by your height in inches squared and then multiplying by 705, and is almost always expressed in the unit kg / m2, which is therefore often left out. For example, a person who is 5'6" and weighs 190 Lbs would have a BMI of 31.
Height of 5'6" = 66 inches
66 squared = 4,356
190 divided by 4,356 = 0.0436
0.0436 x 705 = 30.75 (which would be rounded up to a BMI of 31)
BMI, for all its approximation, is a good tool to use in determining whether you need to lose weight, gain weight, or congratulate yourself for being just right.
An individual is considered "Underweight" if their BMI is less than 18.5.
A BMI of 18.5 to 23.9 is considered a "Normal" weight.
A BMI of 24 to 29.9 is considered "Overweight".
Charts and calculators to determine your BMI are easily found on most health and diet sites. These calculators are rough translations of true percentages, and there are a number of factors that might influence whether or not your BMI is a true reflection of your total body fat.
Muscle is denser than fat and takes up less space. Therefore a heavily muscled person might weigh more than a same sized over-weight person, or two individuals with identical BMI might have widely different percent body fat.