Body Mass Index (BMI) is a number calculated from a person's weight and height. BMI provides a reliable indicator of body fatness for most people and is used to screen for weight categories that may lead to health problems.

A frequent use of the BMI is to assess how much an individual's body weight departs from what is normal or desirable for a person of his or her height. The weight excess or deficiency may, in part, be accounted for by body fat (adipose tissue) although other factors such as muscularity also affect BMI significantly (see discussion below and overweight). The WHO regards a BMI of less than 18.5 as underweight and may indicate malnutrition, an eating disorder, or other health problems, while a BMI greater than 25 is considered overweight and above 30 is considered obese.

Measuring Body Mass Index involves simple techniques that can be carried out by an individual before visiting your doctor.
the following simple steps outlined in http://healthweightlosstutors..com/2012/04/health-diet-solution-for-weight-loss.html


These ranges of BMI values are valid only as statistical categories
Category BMI range – kg/m2 BMI Prime Mass (weight) of a 1.8-metre (5 ft 11 in) person with this BMI.
Very severely underweight less than 15.0 less than 0.60 less than 48.6 kilograms (7.65 st; 107 lb)
Severely underweight from 15.0 to 16.0 from 0.60 to 0.64 between 48.6 and 51.8 kilograms (7.65 and 8.16 st; 107 and 114 lb)
Underweight from 16.0 to 18.5 from 0.64 to 0.73 between 51.8 and 59.9 kilograms (8.16 and 9.43 st; 114 and 132 lb)
Normal (healthy weight) from 18.5 to 25 from 0.74 to 0.99 between 59.9 and 81.0 kilograms (9.43 and 12.76 st; 132 and 179 lb)
Overweight from 25 to 30 from 1.0 to 1.19 between 81.0 and 97.2 kilograms (12.76 and 15.31 st; 179 and 214 lb)
Obese Class I (Moderately obese) from 30 to 35 from 1.2 to 1.39 between 97.2 and 113.4 kilograms (15.31 and 17.86 st; 214 and 250 lb)
Obese Class II (Severely obese) from 35 to 40 from 1.4 to 1.59 between 113.4 and 129.6 kilograms (17.86 and 20.41 st; 250 and 286 lb)
Obese Class III (Very severely obese) over 40 over 1.6 from 129.6 kilograms (20.41 st; 286 lb)

The U.S. National Health and Nutrition Examination Survey of 1994 indicated that 59% of American men and 49% of women had BMIs over 25. Morbid obesity—a BMI of 40 or more—was found in 2% of the men and 4% of the women. The newest survey in 2007 indicates a continuation of the increase in BMI: 63% of Americans are overweight or obese, with 26% now in the obese category (a BMI of 30 or more). There are differing opinions on the threshold for being underweight in females; doctors quote anything from 18.5 to 20 as being the lowest weight, the most frequently stated being 19. A BMI nearing 15 is usually used as an indicator for starvation and the health risks involved, with a BMI <17.5 being an informal criterion for the diagnosis of obesity.

BMI-for-age
BMI for age percentiles for boys 2 to 20 years of age.

BMI for age percentiles for girls 2 to 20 years of age.

BMI is used differently for children. It is calculated the same way as for adults, but then compared to typical values for other children of the same age. Instead of set thresholds for underweight and overweight, then, the BMI percentile allows comparison with children of the same sex and age.[11] A BMI that is less than the 5th percentile is considered underweight and above the 95th percentile is considered obese for people 20 and under. People under 20 with a BMI between the 85th and 95th percentile are considered to be overweight.