BMI Body Composition Fact Sheet

Body composition is a key determinant of future health. Being overweight or obese shortens many lives. The most widely used measure of body composition is the body mass index (BMI). BMI is a simple method for estimating the amount of fat and non-fat in the body.


BMI is a measure of body composition that reflects a ratio of weight to height. Weight is a common marker for how fat an individual actually is and is an easy way to track the success of diet and exercise programs. The BMI test is something everyone can do at home with a scale.

The BMI scores reflect whether a person is underweight, of a normal weight, or overweight. One factor in determining whether someone’s score indicates if he or she is overweight is race. In white people, a BMI score between 25 and 29 is categorized as overweight and a BMI score of 30 or more is considered obese. In Asians, a BMI over 22.5 is considered overweight and over 27.5 is obese, reflecting ethnic differences in body shape.

How the Test is Done

The BMI test measures weight in kilograms divided by height in meters squared (multiplied by itself):

Weight in Kg / (Height in M x Height in M)

For example, if a person weighs 68 kg and are 165cm tall, then their BMI is 25 (68 / (1.65 x 1.65)

The formula is slightly different for determining BMI using pounds and inches. People can take their weight in pounds and multiply it by 703. They then square their height in inches (multiply it by itself) and divide the first sum by the second sum:

Weight in Pounds x 703 / (Height in Inches x Height in Inches)

For example, someone who weighs 120 pounds would start by multiplying 120 by 703. This equals 84,360. If they were 5′ 3”, which is 63 inches, tall they would then multiply 63 by 63. The answer is 3,969. To determine the BMI score, one would divide 84,360 by 3,969, which equals 21.3.

A score of 21.3 is within the normal weight range. A normal weight is 18.5 to 24.9. Below 18.5 means a person is underweight. A score of 25 to 29.9 means someone is overweight. A score of 30 or more indicates obesity.

Who Does the Test?

People can do the test themselves, but doctors and other medical professionals may use a BMI test to judge their patients’ health.


A BMI test does not require any special equipment or incur a cost.

When and How Often?

People should weigh themselves every week to monitor their health, but unless there has been a large weight fluctuation, it is not necessary to calculate BMI weekly. However, people who are on a weight loss program may wish to determine their BMI weekly to determine how well their program is working. It is also a good idea to visit a GP monthly when trying to lose weight.


BMI does not take into account muscle mass, which can vary drastically in different people. For example, muscle loss as we get older can result in a lower BMI, and therefore complacency, yet body composition may still be highly abnormal. A BMI measurement can also be highly inaccurate in the young and the elderly.

Many medical professionals may also insist on using BMI as an accurate tool for determining whether someone is overweight or at risk of ill health.


  • Fast Living Slow Ageing; Kate Marie and Christopher Thomas; 2009
  • Clear the Way to Better Health: Your Medical Test Guide:
  • Body Weight and Cancer Risk:

Last reviewed 26/Feb/2014


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Whilst wielding a couple of dumbbells in a gym class in 2003, Kate experienced an epiphany around the lack of accepted best practice guidelines when it came to staying well and avoiding disease. Kate realized that she had no chance of slowing her own aging process unless she became better educated about her options.

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