Body mass index (BMI)

BMI is the standard measure used for clinical diagnosis of obesity.

It predicts health risks for most people accurately. It is not a reliable indicator for strength athletes and body builders (around 2% of the population) because it only takes height and weight into account – not body composition. Bodybuilders with very low levels of fat are often classified as ‘obese’ by this method.

BMI is a quick and easy gauging method. A BMI reading of 30 or more is classified as obese.


Below is a guide to BMI readings:


– < 18.5 = Underweight

– 5 – 25 = Normal

– 25-30 = Overweight

– > 30 = Obese

– > 40 = Morbidly obese

To work out your BMI:
  1. divide your weight in kilograms (kg) by your height in metres (m)
  2. then divide the answer by your height again to get your BMI.


Body composition can be differentiated as:


Fat Mass (FM):

The proportion of total weight that is fat. This includes internal and external fat (adipose tissue).


Fat-free mass (FFM or lean mass):

The proportion of total weight that is not fat. This includes muscle, bone and internal organs.


The percentage of body fat is important information for assessing health.

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