SummaryUtilizing available data and several assumptions, the body content of water, minerals, protein, carbohydrate, and fat has been estimated for a 9‐year‐old reference boy. This reference boy is leaner than the reference man (13.4versus15.3% of body weight) and his fat‐free body mass exhibits greater concentration of water (75.5versus73.7%) and lesser concentrations of potassium (66.8versus68.1 mEq/kg) and total minerals (4.7versus6.9%). If agespecific values are used for composition of fat‐free body mass (rather than the conventionally used constants based on adult body composition), it will be possible to avoid underestimation of fat content calculated from data on total body water and overestimation of fat content calculated from40K counting data or from body density determined by underwater weighing.SpeculationBy utilizing age‐specific constants for calculating body fat content from indirect measurement of body composition (i.e., total body water,40K counting, and body density) it will be possible to determine the extent of changes in fat content of the body of normal individuals and of those with specific illnesses or disorders, including obesity.