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The Body Mass Index or BMI is a standard measure of a person's height to weight ratio. It was invented in the 19th century in Europe by a social scientist. However it did not become popular in North America until the 1980's, when health problems related to overweight conditions started to occur. The appeal of the BMI is that it provides an easy way to assess whether we are too thin or too fat. In theory it works for males and females, and short people as well as tall people.

How it works

The body mass index value is obtained by dividing a person's height by the square of their weight. By itself the BMI is just a number and does not really tell us anything. This is like the temperature scale. For example, 90 degrees Fahrenheit doesn't mean much to people in Europe who use the Centigrade scale. However once you realize that 20F is cold, and 90F is hot then it starts to make more sense.

Similarly the BMI values need to be interpreted to have meaning.

Less than 18.5 - Underweight
18.5 to 25 - normal
25 to 30 ' overweight
30 to 40 ' obese
Over 40 ' very obese

In this way the body mass index can give us standard way to assess how thin or how fat we are.

Is it accurate?

A lot of people, including doctors, use the index value for medical assessment, which was not the original purpose of the BMI. In our temperature example, what feels hot for one person, another might consider only 'warm'. The same problem can arise with the BMI. This is especially true if you have a borderline value like 26. You may not in fact be 'overweight' at all, depending on a number of other factors.

Other weight influences

The BMI does not take into account things like body frame type or muscularity. Some people are born with a larger body frame, and have a greater bone and muscle mass. Likewise a body builder would have more muscle and a higher BMI value than an 'average' person. Other things that can influence your BMI value are gender and age.

Your ideal body weight?

The body mass index values are frequently used to construct an ideal body weight chart. If you are using one of these charts to compare your own weight to the 'ideal', try to find a chart that takes some of the other factors (especially body frame size and sex) into account. Otherwise the result may not be appropriate for you.

Conclusion

The BMI is useful as an approximate measure of a person's thinness or fatness, but should not be taken as an absolute indicator of health. Since BMI is calculated using only a person's height and weight it ignores the other factors mentioned above. Some experts claim that the resulting error in BMI values is significant and it should not be used at all in this way. Consult your physician if you think you have a weight problem, and he or she will provide you with the best advice.