LBM is a part of body composition that is defined as the difference between total body weight and body fat weight. While the percentage of LBM is usually not computed, it is the complement of body fat percentage, and on average ranges between 60-90% of total body weight. Generally, men have a higher proportion of LBM than women do, and the dosages of some anesthetic agents, particularly water-soluble drugs, are routinely based on the LBM. Multiple formulas have been developed for calculating estimated LBM (eLBM) and the calculator above uses the following with the exception of the Peters formula (which is used to determine LBM for children).

The author suggests that this formula is applicable for children aged 13-14 years old or younger. The formula is used to compute an eLBM based on an estimated extracellular volume (eECV) as follows:

Boer P. "Estimated lean body mass as an index for normalization of body fluid volumes in man." Am J Physiol 1984; 247: F632-5

James, W. "Research on obesity: a report of the DHSS/MRC group" HM Stationery Office 1976

Hume, R "Prediction of lean body mass from height and weight.". J Clin Pathol. 1966 Jul; 19(4):389-91.

A. M. Peters, H. L. R. Snelling, D. M. Glass, N. J. Bird "Estimation of lean body mass in children". British Journal of Anaesthesia1 06(5): 719-23 (2011).