The body mass index (weight (kg)/(height (m)²)) is widely accepted as providing a convenient measure of a person's fatness. It gives an index that is broadly independent of height and equally applicable to men and women. A few individuals who are exceptionally muscular may be misclassified as overweight or obese, but otherwise the body mass index provides a rather robust index which has proved exceptionally useful for large scale epidemiological work. Its use is rapidly spreading into adult clinical medicine, where several charts and nomograms are available. Similar charts now exist for children, but their use is less straightforward.

For adults a pragmatic classification system exists based on associations between body mass index and all cause mortality.1 The recently redefined body mass index categories are: underweight <18.5; ideal 18.5-24.9; pre-obese 25.0-29.9; obese class I 30.0-34.9; obese class II 35.0-39.9; and obese class III >40 kg/m². These fixed classifications are not appropriate for children, in whom the 50th centile for body mass index shows profound changes from birth through to early adulthood …