Lean Body Mass Calculator

Two people with the same body weight can have very different lean mass. A muscular 80-kg adult might have 65 kg of LBM and 15 kg of fat (19% body fat). A sedentary 80-kg adult might have 55 kg of LBM and 25 kg of fat (31% body fat). Same scale weight; very different metabolic profiles, drug pharmacokinetics, athletic capabilities, and health risks.

Drug dosing, especially for water-soluble drugs (which distribute mostly in lean tissue), often performs better when scaled to LBM than to total weight. Some chemotherapy and anesthetic protocols use adjusted body weight (somewhere between actual weight and ideal weight) for the same reason — to better approximate distribution into lean tissue without underdosing very obese patients.

Boer formula (1984): Men: LBM = 0.407 × W + 0.267 × H − 19.2; Women: LBM = 0.252 × W + 0.473 × H − 48.3 (W in kg, H in cm). Among the most accurate of the simple formulas for typical adults.

James formula (1976): Used historically in clinical research; produces slightly different results than Boer, especially at extreme body weights.

Hume formula (1966): Older, similar accuracy to Boer for normal-weight adults; less accurate at extremes.

All of these are estimates from height and weight alone — they can't see your actual body composition. Two adults of the same height and weight will be assigned the same LBM by these formulas, even if one is athletic and one is sedentary. For more accurate LBM, measure body fat directly (DEXA, skinfolds, bioelectrical impedance) and compute LBM = weight × (1 − BF%).

Tracking LBM across weeks and months is one of the cleanest signals of body composition change. Pure weight loss might preserve or lose lean mass — the difference between a successful cut and a poorly executed one is whether LBM is preserved.

Untrained or detrained people can gain lean mass while losing fat ("recomposition") in a slight calorie deficit, given adequate protein (1.6–2.2 g/kg) and consistent resistance training. Trained individuals usually need to choose a direction (cut or bulk) because simultaneous fat loss and muscle gain becomes much harder. LBM measurements over 8–12 weeks reveal whether the chosen approach is producing the desired body composition shift.

Some metabolic-rate formulas (Katch-McArdle, Cunningham) use LBM rather than total weight as the input — and tend to be more accurate for athletes and those with non-typical body composition. The Katch-McArdle BMR: 370 + 21.6 × LBM (kg). For a 65-kg LBM person: BMR = 370 + 21.6 × 65 = 1,774 kcal/day.

These LBM-based BMR formulas typically produce slightly higher numbers than Mifflin-St Jeor for muscular individuals and slightly lower numbers for sedentary individuals — better matching the underlying physiology that lean tissue drives most resting energy use.