BMI or Body Mass Index is a health indicator given by the following height weight ratio:
BMI = weight (kg) ÷ height ² (m²)
(Metric system: body weight in kgs. divided by the square of the height in meters)
BMI = weight (lb) × 703 ÷ height ² (in²)
(US system: body weight in pounds times 703 divided by the square of the height in inches)
The four main categories of BMI for adult men and adult nonpregnant women, according to the WHO (World Health Organization), and NIH (National Health Institutes, USA), are the following:
BMI Categories
Obesity class
BMI
Underweight
< 18.5
Normal
18.5 - 24.9
Overweight
25 - 29.9
Obesity
I
30 - 34.9
II
35 - 39.9
Extreme Obesity
III
≥ 40
Other currently used predictors of health risks associated with excess weight are Waist-to-Hip Ratio & Waist Circumference that measure the distribution of a person's body fat around the waist or below the waist. Indeed, high WHR and increased waist circumference have been found to be markers for increased health risk even in persons of normal weight.
[Main reference: Preventing and Managing the Global Epidemic of Obesity. Report of the World Health Organization Consultation of Obesity.WHO, Geneva, June 1997.]