BMI stands for body mass index. According to the National Strength and Conditioning Association (NSCA), BMI is the preferred body composition assessment for obese individuals. The association states that the BMI classifications are as follows: less than 18.5 is considered underweight, 18.5 to 24.9 is normal, 25.0 to 29.9 is overweight and 30.0 to 39.9 is thought to be obese, while any BMI greater than 40 is considered extremely obese.

Determine your weight in pounds. Use a scale if you don't already know your weight.

Determine your height in inches. For example, if you are 6 feet, 1 inch, list your height as 73 inches.

Square your height using a calculator. In other words, multiply your height by your height. For example, if your height is 73 inches, your equation looks like this: 73 x 73. Your height squared is 5,329.

Divide your weight by your height squared. For example, if your weight is 250 lbs. and your height squared is 5,329, your equation looks like this: 250 / 5,329. Your answer is 0.0469131.

Multiply the answer of your weight divided by your height squared by 703. For example, if your weight is 250 lbs. and your height is 73 inches, your equation looks like this: 0.0469131 x 703. The answer is your BMI. In this example, your BMI is 32.9.

Compare your BMI to the table available online by the National Institutes of Health. Using the information given, with a BMI of 32.9, you would be classified as obese.
Tip
If you don't have paper, pencil and a calculator available, the Centers for Disease Control and Prevention and the American Council on Exercise provide an online BMI calculator that's easy to use  all you need to know is your weight and height.
Warnings
If you are athletic or have a muscular build BMI, the American Council on Exercise states that you should use your BMI in conjunction with other body composition assessments. A person with welldeveloped muscles can mistakenly appear overweight or obese according to their BMI score.