Kia Ākina

Working towards lifelong recovery from obesity

What is Body Mass Index (BMI)?

BMI is a measure of body size based on weight, which takes height into account. That makes sense doesn’t it, the taller you are the heavier you are likely to be.

The formula was worked out more than 200 hundred years ago by a Belgian genius called Adolphe Quetelet who was interested in astronomy and sociology as well as mathematics and statistics. His formula for working out BMI is as follows:

BMI  =  weight (in kilograms)  /  height (in metres) / height (in metres)

Do you get that? You take your weight in kilograms and divide it by your height in metres, and then divide the result again by your height in metres.

Here is an example.

Let’s take a hypothetical person who is 130kg and 1.72m in height.

Their BMI would therefore be 130 divided by 1.72, divided by 1.72 = 43.9

There are some limitations with the BMI particularly if you’re an elite athlete with a lot of muscle, or you’re either very short or very tall. But most of us are not in any of these three categories and BMI provides us an excellent guide to weight loss endeavours.

Thirty or forty years ago, before the obesity epidemic really got going, 95% of the population had a BMI between 18.5 and 25 and the average would have been in the low 20s, which is why “normal” BMI is generally said to be 18.5 – 25.

The weight of the population overall has increased considerably during this time so that the average BMI of adult New Zealanders is now close to 27 and rising by the year.

In any event, BMI categories continue to be allocated as follows:

Less than 18.5          Underweight

18.5 – 24.9                Normal weight

25 – 29.9                   Overweight

30 – 34.9                   Obesity

35 – 39.9                   Clinical obesity

Greater than 40       Morbid obesity

To be a member of Kia Ākina, when you join Kia Ākina you must have a BMI of at least 30. Most members of Kia Ākina will have a BMI considerably above that, like in the example above, where the person with a BMI of 43.9 would be considered to be morbidly obese.

You will have noticed that we use BMI of 29 as a target marker of recovery from obesity rather than 29.9. A BMI of 29 gives a little buffer.