Lean Body Mass (LBM)
$$\text{LBM} = \text{Total Body Weight} - \text{Fat Mass}$$
What is Lean Body Mass (LBM)?
Lean body mass (LBM) is everything in the body that is not fat — including skeletal muscle, bone mineral, organs, blood, skin, and total body water. It is calculated as: $$\text{LBM} = \text{Total Body Weight} - \text{Fat Mass}$$
LBM is the primary determinant of basal metabolic rate (BMR): more lean mass means a higher resting energy expenditure. It is also the basis for drug dosing in pharmacokinetics, as many medications distribute through lean tissue rather than fat — using total body weight for dosing in obese patients would result in dangerous over-dosing of these agents.
The Boer formula is the most widely cited clinical estimation method. For men: $$\text{LBM} = 0.407W + 0.267H - 19.2$$ For women: $$\text{LBM} = 0.252W + 0.473H - 48.3$$ where W is weight in kg and H is height in cm. The James and Hume formulas are alternatives used in specific clinical contexts.
When to use Lean Body Mass (LBM)
Use LBM when calculating drug doses for antibiotics, anaesthetic agents, or chemotherapy drugs that distribute in lean tissue. Use it to set protein intake targets in athletic or clinical nutrition (typically 1.6–2.2g protein per kg LBM for resistance-trained individuals). Use it as an input to FFMI to assess muscularity relative to height independently of body fat level.
Worked examples
| Individual | Total weight | Body fat % | Fat mass | Lean body mass |
|---|---|---|---|---|
| 75 kg man, 15% fat | 75 kg | 15% | 11.3 kg | 63.7 kg |
| 75 kg man, 25% fat | 75 kg | 25% | 18.8 kg | 56.3 kg |
| 65 kg woman, 25% fat | 65 kg | 25% | 16.3 kg | 48.8 kg |
| 65 kg woman, 35% fat | 65 kg | 35% | 22.8 kg | 42.3 kg |
Common pitfalls
LBM is often conflated with muscle mass, but the two are not the same — lean body mass includes bone, organs, blood, and water, which together account for a large fraction of LBM. A person can gain or lose several kilograms of LBM through hydration changes alone without any change in actual muscle tissue.
Frequently asked questions
What is the difference between lean body mass and muscle mass?
Lean body mass is the total weight of all non-fat tissue — muscle, bone, organs, blood, skin, and water. Muscle mass refers specifically to skeletal muscle tissue. In a typical adult, skeletal muscle accounts for roughly 40–50% of LBM in men and 35–40% in women. The remainder is bone, organs, blood, and water.
How do I increase lean body mass?
Progressive resistance training is the most evidence-based method for increasing skeletal muscle. Adequate protein intake (1.6–2.2g per kg of body weight per day) is required to support muscle protein synthesis. LBM decreases with age (sarcopenia) at approximately 3–8% per decade after age 30 without deliberate training.