Lean body mass is everything in your body that isn't fat: muscle, bone, organs, water. It's a more useful number than total body weight for tracking fitness, calculating drug doses, and setting protein intake. Our LBM calculator implements three of the most validated anthropometric formulas — Boer (1984), James (1976), and Hume (1966) — and averages them. If you know your body fat percentage, it adds a fourth, more accurate body-fat-based calculation. You get LBM in kg, fat mass in kg, and your lean body mass percentage.
If known, gives most accurate LBM
Input your weight in kg and height in cm.
Sex and age affect formula coefficients.
If you have a recent DEXA or BIA reading, enter your body fat percentage.
Review LBM from each formula plus the average and your LBM percentage.
The three anthropometric formulas (Boer, James, Hume) all estimate LBM from height and weight using sex-specific coefficients. Each was validated against different reference populations and gives slightly different answers. Averaging them reduces single-formula bias. If you know your body fat percentage from a DEXA scan, calipers, or BIA, the body-fat-based calculation is more accurate than any anthropometric estimate.
Boer: 0.407 × W + 0.267 × H − 19.2 (men) • James, Hume use different coefficients
Boer is the most commonly cited formula in clinical and research contexts
If you know body fat % from DEXA/calipers/BIA, that calculation is the most accurate
LBM is more stable than total weight — use it to track progress on a cut
Healthy LBM percentage: men 75-85%, women 65-75% (athletic), lower in average people
LBM is used for chemotherapy and anesthesia dosing because drug distribution scales with lean mass
Protein needs are best calculated against LBM, not total body weight
Sarcopenia (low muscle in older adults) is detected via LBM and lean mass index
Lean body mass (LBM) is your total body weight minus all fat mass — encompassing skeletal muscle, bone, organs, water, and connective tissue. It is a far more useful metric than total body weight for several critical applications. In fitness, LBM determines your baseline caloric needs and protein requirements (most sports nutritionists recommend 1.6-2.2 grams of protein per kilogram of LBM, not total weight). In clinical medicine, drug dosing for anesthetics, chemotherapy agents, and antibiotics is increasingly based on lean mass because fat tissue metabolizes drugs differently than lean tissue. For body composition tracking, two people can weigh the same 180 pounds, but if one has an LBM of 145 pounds (19% body fat) and the other 130 pounds (28% body fat), their health profiles are vastly different. The gold standard for measuring LBM is dual-energy X-ray absorptiometry (DEXA), but validated anthropometric formulas — Boer (1984), James (1976), and Hume (1966) — provide reasonable estimates using only height, weight, and sex. These formulas were derived from regression analysis against hydrostatic weighing data and have standard errors of approximately 2.8-3.5 kg. If you know your body fat percentage from calipers, bioimpedance, or a DEXA scan, a direct calculation (weight times (1 minus BF%)) is more accurate still.
No single LBM equation is perfectly accurate for all body types. Boer was validated against a relatively athletic European cohort, James against a different population, Hume against a third. Each gives slightly different results for the same inputs because they were calibrated to different reference samples. Averaging them produces a more robust estimate that doesn't rely on which population happens to match yours best.
Total body weight is a noisy measurement — it includes water, food, glycogen, and short-term fat fluctuations alongside the things that actually matter (muscle, bone, fat). LBM and fat mass let you track the components separately. If you're lifting and your LBM is going up while fat mass is going down, you're doing exactly what you want even if total weight is unchanged. Tracking LBM is more informative than tracking weight.
Data sourced from trusted institutions
All formulas verified against official standards.