Body Mass Index or BMI is probably the most common measure for assessing your weight range. To calculate your BMI your body weight measured in kilograms is divided by the square of your height in meters. For example, if you are 70kgs and measure 170cms (1.7m) in height your BMI would be as follows:
70/1.7 x 1.7 = 70 /2.89 = 24.22 (normal weight range)
The ranges vary according to ethnicity (covered in another blog) but are generally:
| Severely underweight | less than 16.0 |
| Underweight | from 16.0 to 18.5 |
| Normal | from 18.5 to 25 |
| Overweight | from 25 to 30 |
| Obese Class I | from 30 to 35 |
| Obese Class II | from 35 to 40 |
| Obese Class III | over 40 |
Unfortunately, BMI can be misleading. Take the two people above. Both have a BMI of 40 (obese range). However, the man on the left has a BMI of 40 because he is solid muscle and muscle is twice as heavy as fat. He should not technically be in the obese range. However, the man beside him, should be.
So, while there is usually a good correlation between body fat and BMI there are exceptions such as the one depicted above.
BMI ranges span large weight zones. For the example of the person who is 170cms above when they have a BMI of 20 there weight is 57.8kgs, but when your BMI is 25 (the top of the normal range) your weight is 72.25kgs which is a 14kg difference! Now some people who are 170cms and weigh 72kgs look fit, healthy and muscular, while others may look overweight. To help you interpret your BMI look in the mirror and give yourself a pinch test. Allow common sense to prevail in determining whether you need to lose a few kilos or not.
Interestingly, there is a group of low BMI individuals that researchers call thin-fat. Although their BMI indicates they are lean, when their fat percentage is measured it can indicate that they are actually overweight or even obese. Studies suggest that as many as 29% of people in the normal weight range are obese, and up to 80% in the overweight range are obese. Those who already fall in the BMI range for obesity are usually obese (except for weight lifters and elite sports people).