Lean body mass via three independent formulas (Boer, James, Hume).
Boer (1984), James (1976), Hume (1966) — three independent regressions on weight + height. The body-fat % derives from LBM by subtraction; less precise than calipers or DXA but useful as a stable reference number that doesn't fluctuate with hydration.
Lean body mass (LBM) is the weight of everything in your body that isn't fat: muscle, bone, organs, skin and the water in those tissues. It's a more useful number than total body weight for nutrition, drug dosing, and body-recomposition goals. Two people of the same weight can have radically different LBM — a 75 kg trained man might be 65 kg LBM (87 % LBM, 13 % body fat); a 75 kg sedentary man might be 56 kg LBM (75 %, 25 %). When you "lose weight" you want to lose fat, not LBM; tracking LBM separately is what tells you whether a diet is working in the right direction. Anesthesiologists dose drugs by LBM rather than total weight because most drugs distribute in lean tissue, not fat. This calculator estimates LBM from regression formulas based on weight and height alone — less accurate than calipers or DXA but more stable than scales (which fluctuate with hydration) and free.
Three independent regressions, each fitted to a different population:
The calc returns each formula's output and their arithmetic mean as the headline LBM. Body fat in kg = total weight − LBM; body-fat % = fat_kg / weight × 100. The body-fat % is then categorized using the ACE (American Council on Exercise) bands, which are sex-aware: men's "athletic" is 6–14 %, women's is 14–21 %, and so on across essential, fitness, acceptable and high categories.
Pick sex (changes the formula coefficients). Enter your body weight in kg or lb, and your height in cm, m, or in. The result panel returns the three-formula average LBM as the headline, plus each formula's individual estimate, the implied body-fat mass and percentage, and a sex-aware body-fat-band gauge.
Male, 80 kg, 180 cm.
Regression formulas have ±5 % standard error. The Boer / James / Hume models were fitted on adult, non-athletic populations; the calc averaged 3 formulas to reduce model-specific bias. For elite athletes (very high LBM), the formulas systematically under-estimate; for visibly obese individuals (very high body fat), they over-estimate LBM and under-estimate fat.
Hydration changes total weight by 1–3 kg in a day. LBM is computed from weight, so morning vs evening, pre vs post sauna can shift the estimate by 1–2 kg. Weigh in the morning, post-bathroom, before food, in the same conditions every measurement.
Athletic muscle masks the same way fat does. A 90 kg athlete with 10 % body fat and a 90 kg sedentary person with 30 % body fat will look similar on the scale and even on these formulas (which produce 81 vs 65 kg LBM respectively only after sex-pure regression). The body-fat % the calc returns is from the LBM subtraction, not directly measured — caliper or DXA gives the ground truth.
No height correction for bone density. Two people with the same height and weight but different bone density (genetic + age + sex) have different LBM. The formulas don't capture this, so post-menopausal women with osteoporosis have inflated LBM estimates.
Pediatric and geriatric edge cases. The regressions were fitted on 18–55 year olds. Children, frail elderly with sarcopenia, and pregnant women are out of model.
ACE bands are not perfect. They derive from body composition data of a specific US population in the 1990s. Modern fitness culture has lower visible body-fat targets; the "Athletic" 6–14 % range for men is achievable but not necessarily desirable for long-term health.
Hodgdon-Beckett (US Navy): a different LBM/body-fat regression based on circumferences (neck, waist, hip + height). The calc here uses weight + height only; if you have body-circumference measurements, the dedicated body-fat-percentage calc is more accurate.