Body Composition
Lean Body Mass (Hume Equation)
Estimates lean body mass from height and weight using the Hume 1966 prediction equation.
Estimates lean body mass from height and weight using the Hume 1966 prediction equation.
If you log meals, this is the companion number: it uses the same units and daily framing as the diary, which makes comparing your intake against the target a one-glance check rather than a conversion exercise.
About Lean Body Mass (Hume Equation)
Lean body mass (LBM) is the weight of everything in the body that is not fat: skeletal muscle, bones, organs, skin, blood, and water. It is closely related to fat-free mass, though strictly LBM includes essential lipids (about 2 to 3 percent of body weight) whereas fat-free mass excludes all fat. LBM is the metabolically active component of body weight and is the primary determinant of resting energy expenditure, so accurate estimation is important for nutrition planning, drug dosing, and athletic assessment. The Hume equation, published by R. Hume and H. Weyers in 1966 in the Journal of Clinical Pathology, is one of the most widely used anthropometric prediction equations for LBM in adults.How the formula works
The Hume equation predicts LBM from height (cm) and weight (kg) using sex-specific linear coefficients. For men: LBM = 0.32810 x weight + 0.33929 x height - 29.5336. For women: LBM = 0.29569 x weight + 0.41813 x height - 43.2933. The equation was derived by multiple regression against LBM measured by tritium dilution in a sample of Scottish adults. The weight coefficient is smaller than 1.0 because not all of body weight is lean; the remainder is fat. The height coefficient reflects the strong relationship between stature and skeletal and organ mass, and the sex-specific intercept and coefficients capture the dimorphism in body composition between men and women.Alternative equations
Several other anthropometric LBM equations exist. The James equation, used in nutrition support, predicts LBM as 1.1 x weight - 128 x (weight/height)^2 for men and 1.07 x weight - 148 x (weight/height)^2 for women, but it fails in obesity (predicting negative LBM at very high BMI). The Katch-McArdle equation requires body fat percentage: LBM = weight x (1 - bodyfat/100), and is used to compute BMR. The Janmahasatian equation, published in 2005 in the British Journal of Clinical Pharmacology, was derived in more than 300 adults across a wide BMI range and is increasingly preferred for drug dosing in obese patients. For any application requiring accuracy, DEXA, bioimpedance, or skinfold-derived LBM is superior to anthropometric prediction.Evidence and validation
Hume and Weyers' original validation reported a standard error of estimate of about 1.9 kg in men and 1.4 kg in women against tritium dilution. Subsequent validation studies have generally confirmed reasonable performance in normal-weight adults, with errors of 2 to 3 kg, but have noted systematic overestimation of LBM in the obese (because the fixed weight coefficient assumes an average fraction of lean mass per kilogram that does not hold at high BMI). The Janmahasatian equation performs better across the BMI range and is now recommended for drug dosing calculations in clinical pharmacokinetics.Interpretation
Typical LBM values for average-height adults are 55 to 70 kg in men and 38 to 52 kg in women. Resistance-trained men often exceed 70 kg, and elite strength athletes can reach 80 to 90 kg. LBM declines with age due to sarcopenia, at a rate of about 3 to 8 percent per decade after age 30, which is a major contributor to the age-related decline in metabolic rate.When to use it
The Hume equation is useful when a quick LBM estimate is needed from readily available measurements, particularly for drug dosing of hydrophilic medications that distribute into lean mass, for nutrition support calculations, and for athletic assessment. It is also used as the lean-mass input to the Katch-McArdle and Cunningham BMR equations when direct body fat measurement is unavailable.Limitations
The Hume equation was derived in a relatively small Scottish sample and assumes average body composition. It overestimates LBM in obese individuals and underestimates it in very muscular athletes. It does not account for age-related changes in body composition or for ethnic differences in lean mass density. For clinical or research accuracy, DEXA, bioimpedance, or a four-component model is required. The Hume equation should be regarded as a screening estimate, not a substitute for direct measurement.
Editorial and formula review
Formula sources, population limits, and references for this body composition calculator are documented in our public methodology. Read the methodology · Medical disclaimer
Frequently asked questions
Lean body mass includes essential lipids (about 2 to 3 percent of body weight, including cell membranes and nervous tissue), whereas fat-free mass excludes all fat. The two terms are often used interchangeably, but strictly LBM is slightly larger than fat-free mass.
The Hume equation uses a fixed weight coefficient that assumes an average fraction of lean mass per kilogram of body weight. In obesity, the excess weight is mostly fat, so the equation overestimates LBM. The Janmahasatian (2005) equation handles obesity better and is preferred for drug dosing in obese patients.
Hydrophilic drugs such as gentamicin and vancomycin distribute primarily into lean body mass rather than total body weight. Dosing these drugs by total body weight in obese patients can cause toxicity, so LBM-based dosing is safer. The Hume and Janmahasatian equations are commonly used for this purpose in clinical pharmacokinetics.