BMI has been used by the world health organisation since 1980's to get their obesity statistics. It is considered to be the best proxy for body fat percentages among ratios of weight and height; although it does not actually measure percentage of body fat - it is used to estimate it. This tool is intended to be used on men and women over 18yrs. The BMI of children will be compared against age and gender specific charts. It is calculated by dividing your weight in kilograms by your height in metres squared.
|Adult BMI chart.|
|Child BMI chart|
The BMI's original purpose was to simply classify sedentary individuals using average body compositions. It allowed health professionals to discuss over and under weight problems objectively with their clients. It was never meant to be a tool to diagnose medical conditions.
A problem with this tool is the fact that it does not take into consideration a person's lean muscle mass. A person with a large amount of lean muscle mass may in fact come out as obese using the BMI tool -- when in fact they may be the complete opposite. Therefore, often fit and athletic people tend to be put in the overweight category - even though their body fat percentages fall in the 10-15% category.
People who have a physical disability and are unable to walk may have muscle wasting and their BMI may be low but this does not mean they are underweight.
the percentages are as follows: Australia
18.5 & less - underweight
18.5-25 - normal
25-30 - overweight
30 & over - obese
Different countries use different figures depending on the build of its citizens. For people who are shorter (Asian populations), the cut-offs for overweight and obesity may need to be lower. This is because there is an increased risk of diabetes and cardiovascular disease, which begins at a BMI as low as 23.