There are a number of ways of measuring obesity. Probably the most accurate way is percentage body fat (BFP). This is defined as the total mass of fat divided by the total body mass. Body fat includes essential body fat and storage body fat. Essential body fat is necessary to maintain life and reproductive functions and is approximately 2 - 5 % in men and 10 - 13 % in woman. The percentage of essential body fat for woman is greater due to the demands of child bearing and other hormonal functions.

Storage body fat consists of fat accumulation in adipose tissue part of which protects internal organs in the chest and abdomen.

There are number of methods available for determining BFP and probably the most accurate is using bioelectrical impedance analysis.

For practical purposes, in most people, percentage body fat can be estimated from the BMI as there is a good correlation between the two. BMI calculation does not require any special equipment beyond a tape measure and a set of measuring scales. Because of this close correlation, the definitions of obesity have been set using the BMI.

BMI is measured by taking the weight of the individual in kilograms and dividing this by the square of the height in meters.

The range for a healthy BMI is 18.5 – 24.9. Below 18.5 is classified as underweight. Between 25 and 29.9 is overweight. Between 30 and 34.9 is class one obesity. Between 35 and 39.9 is class two obesity (severe obesity) and above 40 is class three obesity (morbid obesity).

Body fat can be estimated from the body mass index by a formula derived by Deurenberg. When making these calculations, the relationship between BFP and BMI must take age and sex into account. It is also different for children.

Adult body fat percentage = (1.20 x BMI) + (0.23 x age) - (10.8 x sex) - 5.4, where sex is 1 for males and 0 for females.

Child body fat percentage = (1.51 x BMI) - (0.7 x age) - (3.6 x sex) + 1.4.

According to the American Council of Exercise there are average percentages that differ according to specified groups. As previously mentioned essential fat accounts for 2 – 5% BFP in men and 10 – 13% in woman. For athletes, the ideal BFP is 6 – 12% for men and 14 – 20% for woman. The average for the rest of us is 17 – 21% for men and 25 – 30% for woman.