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BMI is Weight divided by Height, again divided by Height. This is true when you measure weight in kilograms and height in meters.

Metric (use kg, m):
BMI = (Weight / Height) / Height

U.S. (use lbs, in):
BMI = (Weight / Height) / Height * 703

The BMI translates into weight categories as follows:

1. Lower than 18.5: underweight
2. 18.5 to 25: normal weight
3. 25-30: overweight
4. 30-35: obese I
5. 35-40: obese II
6. above 40: morbidly obese.

These body mass index (BMI) numbers are used world-wide, by the World Health Organization (WHO), and nationally by the Centers for Disease Control and Prevention (CDC) and National Institutes for Health (NIH), to track of the world-wide and U.S. epidemic of Obesity.

There are a few limitations of using these weight categories based on BMI that you should be aware of: for children and teenagers it doesn't work this way. Their body proportions are still developing and the medical profession will apply adjusted formulas for them. Use the above ranges for people above 20 years old. Also, for people with a lot of muscles, the body mass index (Obesity) is a high number, without them being too fat. Muscles are relatively heavy, and the body mass index (Obesity) gives a misleadingly high number. However, if you were an athlete or body builder, I am sure you would know, and you wouldn't be concerned about your BMI being too high.

Your body mass index ("BMI") measures your height/weight ratio. It is your weight in kilograms divided by the square of your height in meters.

For instance, if your height is 1.82 meters, the divisor of the calculation will be (1.82 * 1.82) = 3.3124. If you weigh 70.5 kilograms, then your BMI is 21.3 (70.5 / 3.3124).

People with BMIs between 19 and 22 live longest. Death rates are noticeably higher for people with indexes 25 and above. (5/1/2001: The U.S. government recently changed these ranges to a minimum of 20 and a maximum of 24. I encourage you to take these numbers with a grain of salt; they are not as accurate as they sound.)

• This is the American version of the calculation.[height in feet and inches]
  
  
• If you're not American, click here for a version of this page that accepts metric values.