Body Mass Index, BMI for short, is a mathematical method that can be used to estimate a person's body mass relative to their height. This is done by first determining a value for weight and a value for height; in kilograms and metres respectively.
The weight is then divided by the square number of the height. The value thus obtained, and which is usually around 20, is the Body Mass Index.
Depending on the gender and age of the person for whom the BMI is being determined, there are various tables for interpreting the BMI.
Men aged between 18 and 24 years with a Body Mass Index of between 20 and 25 are considered to be of normal weight. A value of below 20 indicates the person is underweight. With a BMI of over 25, the person is slightly overweight, at over 30 they are overweight and at over 40 they are considerably overweight. Elevated BMI limits apply for older men.
Women between 18 and 24 years are of normal weight when they have a BMI of between 19 and 24. Below 19, they are tending towards underweight, 24 is the start of overweight. Overweight becomes considerable at a BMI of 39 or earlier. As with men, higher limit values apply to women as they get older.
Absolute figures are not consulted to establish whether a child's body weight lies within normal limits using the BMI. Instead, the BMI of a child is compared to the average of other children.
This ensures that the development stages which children in certain age groups experience are taken into account. If a child has a higher Body Mass Index than 97 percent of all children of the same age, they are considered to be overweight.
Essentially, the Body Mass Index is merely a guide, especially for adults, since in no way does it allow for a person's particular physique. For example, the BMI of people with high bone density, highly-developed muscles or broad shoulders can be higher than the BMI of people of the same height.