The body mass index (BMI) is a method of establishing a person's weight classification. It's easy to compute and only requires a measurement of your weight and height. The BMI may also be used in combination with age and gender to assess a child's weight.
Measurements
A person's BMI is the weight in kilograms divided by the height in meters. The height and weight measurements may also be taken in other units of measure and converted into metric units. For example, measurements that use pounds and inches can be converted into metric units by multiplying by 703.
WHO Classification
The World Health Organization (WHO) provides weight classifications based on the BMI. A BMI of less than 18.50 is considered to be underweight. A BMI under 16 is severely underweight, a BMI between 16 and 17 is moderately underweight and a BMI between 17 and 18.50 is mildly underweight. A BMI between 18.50 and 25 is the normal range, and a BMI between 25 and 30 is preobesity. Obese class I is indicated by a BMI between 30 and 35, obese class II is a BMI between 35 and 40. Obese class III is a BMI greater than 40.
Refinements for Females
Some sources modify the BMI classifications from WHO to account for differences between males and females. The Standards Committee for the American Society of Obesity Surgery (ASOS) establishes a BMI below 19.1 as underweight for females, a BMI between 19.1 and 25.8 as normal, a BMI range of 25.8 to 27.3 as marginally overweight, a BMI between 27.3 and 32.3 as overweight and a BMI greater than 32.3 as obese.
Refinements for Males
For males, the ASOS establishes a BMI below 20.7 as underweight, a BMI between 20.7 and 26.4 as normal, a BMI range of 26.4 to 27.8 as marginally overweight, a BMI between 27.8 and 31.1 as overweight and a BMI greater than 31.1 as obese.
Children
The BMI may also be used to compare children of the same age and gender by establishing the child's BMI percentile score. This score indicates the percentage of children of the same age and gender with a BMI score less than that of the subject child. A BMI percentile score of less than 5 indicates the child is underweight, a score between 85 and 95 indicates the child is overweight and a score greater than 95 indicates obesity.
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