Estimate lean body mass, fat-free mass, and body weight composition with USA / UK and metric / imperial options.
This tool provides estimates for informational purposes only. It is not a substitute for medical, nutrition, or fitness advice. Individual results vary based on your health status, training background, body composition, and personal circumstances. Always consult a qualified healthcare or fitness professional before making important decisions based on the output.
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Lean Body Mass (LBM) is everything in your body that is not fat: muscle, bone, water, organs. Three validated formulas: Boer (1984) — most-cited for adults; Hume (1966) — slightly higher in athletes; James (1976) — pediatric-validated. All require height + weight + biological sex. For a 5'10", 180 lb man (178 cm, 81.6 kg): Boer LBM = 0.407 × 81.6 + 0.267 × 178 − 19.2 = 61.5 kg. Body fat = 81.6 − 61.5 = 20.1 kg (24.6%).
Women carry essential reproductive fat that men do not, so the Boer formula uses different coefficients: LBM = 0.252 × kg + 0.473 × cm − 48.3. For a 5'5", 140 lb woman (165 cm, 63.5 kg): LBM = 0.252 × 63.5 + 0.473 × 165 − 48.3 = 45.7 kg. Healthy female body fat range 21–32% means LBM/total varies more than in men.
Strictly speaking, Lean Body Mass includes about 3% essential fat (in cell membranes, nervous system, bone marrow). Fat-Free Mass (FFM) excludes all fat. Most calculators and the gym industry use the terms interchangeably; clinical research distinguishes them. Differences are typically <2 kg for an average adult.
Many medications dose by lean body mass rather than total body weight — particularly anaesthetics, chemotherapy agents, and some antibiotics. Total body weight in obese patients overdoses them; LBM dosing is safer. The Janmahasatian formula (2005) is increasingly used in clinical pharmacology for this reason.
The goal of any cut should be: preserve LBM, lose fat mass. Achieved via (a) adequate protein (0.8–1g per lb body weight), (b) resistance training 3–4×/week, (c) moderate deficit (TDEE − 300 to 500). Aggressive cuts (>1000 cal deficit) lose roughly 50% LBM, 50% fat. Moderate cuts lose ~20% LBM, 80% fat. Recompute LBM every 4 weeks during a cut.
Everything in your body that is not fat — muscle, bone, water, organs. Typically 75–85% of total weight for men, 65–80% for women.
Boer (1984) is the modern standard: Men LBM = 0.407×kg + 0.267×cm − 19.2. Women: 0.252×kg + 0.473×cm − 48.3.
Healthy men: 75–85% of total weight. Healthy women: 68–80%. Athletes typically 85–93% (men) and 75–85% (women).
They are complementary. Body fat % directly answers the health question. LBM helps drug dosing and weight-loss target setting.
No — it includes muscle plus bone, organs, blood, and intracellular water. Muscle is roughly 40% of LBM.
Lean body mass (LBM) is everything in your body that is not fat — muscle, bone, organs, connective tissue and water. It is calculated as total body weight minus fat mass. The calculator estimates it from your height, weight and sex using validated formulas such as Boer and James, which model the typical relationship between body size and lean tissue.
Tracking LBM gives a much clearer view of body composition than weight alone. During a diet, the goal is to lose fat while preserving lean mass; a stable or rising LBM with falling weight is a sign you are losing the right kind of weight.
Lean tissue, especially muscle, is metabolically active and drives a large part of your resting calorie burn — so a higher LBM generally means a higher metabolism. It also underpins strength, mobility and healthy ageing, since muscle naturally declines with age unless maintained.
LBM has practical uses beyond fitness too: clinicians sometimes dose certain medications by lean mass rather than total weight, and athletes use it to set protein needs and track training progress. Pair the estimate with strength training and adequate protein to protect and build lean tissue over time.