Lean Body Mass Calculator
Calculate your lean body mass (fat-free mass) using the Boer, James, and Hume formulas.
Lean Body Mass
Lean Body Mass (LBM) is your total weight minus fat mass. It includes muscle, bone, organs, and water. LBM is useful for calculating calorie needs, protein requirements, and medication dosing. These formulas estimate LBM from height and weight alone; for greater accuracy, consider a DEXA scan or body fat measurement.
About This Tool
Enter your sex, height, and weight. The tool returns lean body mass (the part of body weight that isn't fat) using the Boer, James, and Hume formulas, plus the implied body fat percentage from each.
Reach for it when planning protein intake (target by lean mass, not total weight), checking a body composition trend over time, or sanity-checking a DEXA scan that came back with surprising numbers. The formulas are estimates — they assume average body composition for height and weight — so they're best used as one signal among several.
For competitive athletes, very lean individuals, or anyone outside the average range, expect the formula to disagree with a direct measurement (DEXA, BodPod) by several pounds.
The formulas. Boer (1984): for men, LBM = (0.407 × weight(kg)) + (0.267 × height(cm)) − 19.2; for women, LBM = (0.252 × weight(kg)) + (0.473 × height(cm)) − 48.3. James (1976): different coefficients, similar form. Hume (1966): older but still cited. Each formula was fit to a population of subjects who also had direct body composition measured (typically by densitometry — underwater weighing in the original studies). The coefficients differ because the populations differed.
Worked example. A 180 cm, 80 kg male. Boer LBM = (0.407 × 80) + (0.267 × 180) − 19.2 = 32.56 + 48.06 − 19.2 = 61.42 kg. Body fat = (80 − 61.42) ÷ 80 = 23.2%. James gives 64.3 kg LBM (19.6% BF). Hume gives 62.8 kg (21.5% BF). Three different answers spanning 4 percentage points of body fat. None are wrong — each is the best-fit estimate from a different study cohort. None match a DEXA scan exactly because none were derived for your specific body.
What the formulas assume. Average body composition for the height-weight ratio. The implicit assumption: people of similar size have similar lean-fat ratios. Athletes break this assumption hard. A 180 cm, 80 kg powerlifter at 10% body fat has 72 kg LBM; the Boer formula predicts 61 kg. An 80 kg sedentary person at 30% BF has 56 kg LBM; Boer still predicts 61 kg. The formulas are accurate for the average bell-curve case and progressively wrong toward the tails.
Adjacent ideas: skinfold calipers (cheap, requires technique), BIA scales (very approximate, hydration-dependent), DEXA scans (gold standard for body composition, expensive), BodPod (air displacement plethysmography, less common). For tracking progress over weeks and months, BIA at the same time of day with the same hydration is good enough — the absolute number is approximate, the trend is reliable. For one-time precise composition, DEXA is what to budget for.
Protein intake recommendations are often expressed as g/kg of LBM rather than total weight. A common target for active adults is 1.6-2.2 g protein per kg LBM daily. For a 75 kg person at 60 kg LBM, that's 96-132 g/day — different from the 120-165 g you'd compute against total weight. Using LBM as the basis under-corrects for very lean people and over-corrects for very obese people, but it's still closer to ideal than total weight.
The about text and FAQ on this page were drafted with AI assistance and reviewed by a member of the Coherence Daddy team before publishing. See our Content Policy for editorial standards.