What exactly is our Body Mass Index? Why is it used as a measurement of our healthiness?  Where do the figures come from?  Why am I clinically obese if my BMI is over 35?

What is Body Mass Index?

Body Mass Index (BMI) is a method of measurement used to compare the individual to a population group using their height and weight. It was invented between 1830 and 1850 by the Belgian polymath, Adolphe Quetelet during the course of developing "social physics" and sometimes known as the Quetelet Index.

The formula for calculating BMI is as follows:

Using kg/m is the prefered way of measuring BMI.  

Why is BMI Used as a Measurement of Our Healthiness?

BMI was never intended to be used to the extent it is used today, it was meant to be used for statistical purposes to broadly categorise population groups (specifically, those groups who followed more sedentary lifestyles), not for diagnosing medical conditions and certainly not to be used to as the only measurement of our healthiness.

So, why is BMI now so commonly used? It makes no sense that we still use BMI.  How did it get so popular?  

What's Wrong with Using BMI?

BMI is fine when used as a broad statistical measurement, but to think that this little number defines us as clinically or morbidly obese is astonishing.  Many factors effect levels of body fat, such as fitness level, muscle mass, bone structure, gender, and ethnicity.

For example, using BMI, many athletes and other very fit but muscley people are classed as clinically obese.

Saying all that, I'm overweight and I lead a relatively sedentary lifestyle, so I'm just the population group BMI was intended to measure.  My BMI is 37.2 right now, but perhaps I have heavier bones or larger muscles than the average? Perhaps I don't? There is no room for such differences by using BMI alone.

If you want to measure your BMI, you'll find plenty of BMI calculators online, such as this one over on the BBC website.

Wikipedia has a good entry for BMI , with further information on all of the above. 

Sources: Wikipedia, Healthline.com, Department of Health