Basal metabolic rate (BMR) has a fairly fixed relationship to body surface area throughout childhood and adult life. For this reason, it was once common practice to use body surface area when calculating drug dosage in childhood. However, while this works reasonably well for children more than a few months old, it is not appropriate in early infancy because BMR rises rapidly in the first 2 or 3 weeks after birth, even though little growth takes place, and BMR is only one of the many factors influencing drug metabolism at this time. Changes in kidney and liver function are of much more relevance, and all the treatment recommendations given in this book are based on what we know of the variable way that these two factors interact to affect how long each drug remains active in the body.

Most paediatric reference texts have, until recently, provided nomograms that allow derivation of surface area from a knowledge of height and weight, but height (or length) is seldom measured with any real accuracy in children who cannot yet stand, and further errors often creep in when nomograms are used without due care. What is more, as Edith Boyd showed in her book on The Growth of the Surface Area of the Human Body (University of Minnesota Press, 1935), is that surface area can be predicted as accurately in very young children from a knowledge of weight alone.

The table relating weight to surface area given in earlier editions of this book utilised Boyd’s data, but the revised figures given here have also made use of the additional data collected by Meban in 1983 (J Anat 137:271–8) (Fig. 2; Table 3

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