Estimate lean body mass using the Boer, James and Hume formulas.
Lean Body Mass (LBM) is total body weight minus fat mass. It includes muscle, bone, water, and organs. Unlike total weight, LBM is directly tied to metabolic rate, drug dosing, and protein requirements. Three established formulas are shown because each was validated in a different clinical context — comparing them reveals the degree of uncertainty in any single prediction.
Boer (1984) — male
LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2
Boer (1984) — female
LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3
James (1976) — male
LBM = 1.1 × weight − 128 × (weight/height)²
Hume (1966) — male
LBM = 0.3281 × weight + 0.33929 × height − 29.5336
LBM estimates from height and weight carry ±3–5 kg error compared to DEXA scanning. This tool is for informational purposes only and is not a substitute for clinical body composition assessment.
Each formula was derived from a different population and era. Boer (1984) is based on bioelectrical impedance in European subjects; James (1976) on UK data; Hume (1966) on a small Canadian cohort. No single equation is universally superior, so the average across all three is the most robust single figure to use.
LBM is used to calculate drug doses (especially chemotherapy and antibiotics), GFR (kidney function), and target weights for ICU nutritional support. Total body weight would overdose larger individuals; LBM-adjusted dosing is more accurate and safer.
Fat-free mass (FFM) and LBM are often used interchangeably, but strictly speaking LBM includes essential fat in the nervous system, bone marrow and organs (~3% for men, ~8–12% for women). FFM is zero fat. In practice the difference is small enough that most clinical references treat them as equivalent.