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How to calculate BMI for adults and children

March 06, 2017

How to calculate BMI for adults and children

In the old days, they used to measure your obesity (or not!) by simply calculating your weight in relation to your height. According to that system, taller people are allowed to weigh more than shorter ones - as if nature hasn’t already blessed them enough with the ability to look down on the rest of us.

A similar, but different system is now becoming increasingly popular among weight experts. Called BMI, or Body Mass Index, it measures your weight in relation to the square of your height in metres. For example, let’s say you weigh 70kg and you are 1.65m tall. Your BMI is then calculated as follows:

First, take your height (1.65m) and multiply it by itself. Of course 1.65 x 1.65 = 2.722. Now divide your weight (70 kg) by this figure of 2.722 and you get 25.7. We will discuss the implication of this in a follow-up blog, but for now you can rest assured that you are not terribly overweight - although you are on the border between ‘normal weight’ and ‘overweight’.

BMI is of course a shortcut to arrive at a measurement that can otherwise become very complex. It is an easy-to-perform and relatively cheap way of screening people’s weight into different categories, i.e. underweight, healthy or normal weight, overweight and, heaven forbid, obesity.

The BMI formula is most useful to assess a large number of people (e.g. the whole population) in terms of whether they are overweight or obese. Since calculating it requires only weight and height it is easy to use, and inexpensive for clinicians as well as members of the public. Used in this way it is a screening tool to measure body fatness.

BMI on its own cannot determine whether a particular individual’s health is at risk. Your healthcare provider would have to perform additional tests to reach such a conclusion. This could include your physical activity, your diet, family history and other relevant health screenings.

There are also other ways to measure body fat more directly. These include bioelectrical impedance, skinfold thickness measurements, DXA (dual energy x-ray absorptiometry, densitometry (underwater weighing) and various others.

For adults of 20 years and above, clinicians interpret BMI utilising standard weight categories. The same categories are used for males and females of all ages and body types.

Although the calculation uses the same formula, for children and teenagers BMI is interpreted in a different manner. Their BMI needs to be sex- and age specific since the amount of body fat differs between boys and girls, and it changes with age.

The way this is done is by comparing the child’s personal BMI with that of other children of the same sex and age. If his or her BMI were higher than e.g. 95% of similarly aged boys or girls in the reference group, he or she would be classified as obese.

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