Lean Body Mass (LBM)
Lean body mass (LBM) is a part of body composition that is defined as the difference between total body weight and body fat weight. This means that it counts the mass of all organs except body fat, including bones, muscles, blood, skin, and everything else.
While the percentage of LBM is usually not computed, it on average ranges between 60-90% of total body weight. Generally, men have a higher proportion of LBM than women do. The dosages of some anesthetic agents, particularly water-soluble drugs, are routinely based on the LBM.
Lean body mass is also frequently used for calculating estimated body fat percentage, as well as for determining basal metabolic rate (BMR).
LBM Formulas
Adult Formulas
Women: LBM = 0.252W + 0.473H - 48.3
Women: LBM = 1.07W - 148(W/H)²
Women: LBM = 0.29569W + 0.41813H - 43.2933
Child Formula (Peters)
Applicable for children aged 14 years old or younger.
LBM = 3.8 · eECV
Lean Body Mass vs. Fat Free Mass
Lean body mass and fat free mass are often used interchangeably. While this is unlikely to cause issues in most cases, the two are not exactly the same.
Lean body mass includes the combined mass of bones, muscles, water, ligaments, tendons, and internal organs. Internal organs include some essential fat and the mass of this fat is included within lean body mass.
Fat free mass is calculated as the difference between total body mass and all fat mass including essential fat. This is the difference between fat mass and lean body mass. Subtracting the mass of essential fat from lean body mass yields fat free mass.
The difference between lean body mass and fat free mass amounts to approximately a 2-3% difference in men and 5-12% difference in women.