The origins of the data used to construct some of the formulae in current usage for the calculation of energy expenditure are discussed. The differences in expenditure calculated by the various formulae cover a range of about 3 per cent. This error is large in relation to long-term studies of energy balance, and to the accuracy attainable with modern respiration chambers. The differences stem in part from the use of inappropriate original values and in part from errors in arithmetic. A new set of source data and a derived formula are presented.