BMI - Body Mass Index - an explanation

The Body Mass Index (BMI) is the standard, generally accepted way of expressing weight. In deciding if the weight of a person is excessive, insufficient, or normal, it is difficult to just refer to weight alone as the height of the person is equally important.

The BMI is a calculated value achieved by dividing the height (expressed in kg) by the square of the height (expressed in metres). It has the great advantage of both being a very simple measure to collect, and providing a reasonably strong correlation with the amount of fat in the body in the vast majority of people.

For a typical Western population the following are terms describing weight using the BMI as the measure:

BMI

<18.5 = Underweight
18.5-25 = Normal
25-30 = Overweight
30-35 = Obese
35-40 = Severely obese
40-50 = Morbidly obese
over 50 = Super obese

When BMI can be Misleading

In Athletes: The BMI can be misleading in athletes (such a body builders) who have a low body fat percentage, but deliver an excessive BMI as they have a significant increase in muscle mass. As the index is a calculation of weight and height alone, it assumes that the increase is due entirely to body fat.
In Children: The BMI can be misleading in children, as body fatness changes over the years as they grow, and girls and boys also differ in their body fatness as they mature. In children it is better to express weight as a percentile value, in which it is compared to others at the same age and sex.
In Ethnic Groups: For some ethnic groups the BMI values need to be modified. For Chinese, Indian and Malaysian people, a BMI greater than 23 is regarded as overweight and a BMI greater than 27.5 is regarded as obese.

source: http://www.core.monash.org/bmi.html - There is also a caluclator on this site if you want to check your BMI as your weight decreases!