Lean body mass is everything that isn't fat — muscle, bone, organs, water. It's the tissue doing your living: LBM drives resting calorie burn, sets protein needs, and is the thing worth protecting in any diet.
Three formulas, one honest range
Boer (1984), James (1976) and Hume (1966) each fitted height-weight equations on different populations, so they disagree by a couple of kilograms — that spread is the honest answer for formula-based estimates. Boer leads the results here because it's the most used clinically. A 178 cm, 82 kg man scores about 61 kg LBM by Boer — roughly 74% of body weight.
Formula vs measurement
Formulas only see height, weight and sex — a bodybuilder and a sedentary office worker with identical stats get identical answers. If you know your body-fat percentage (from a DEXA scan, calipers, or the body fat calculator's Navy method), skip the formulas: LBM = weight × (1 − BF/100) is direct arithmetic on your actual composition.
What to do with the number
Two practical uses. Protein targeting: recommendations of 1.6–2.2 g/kg make more sense per kilogram of lean mass than total weight, especially at higher body fat. Metabolism: the Katch-McArdle equation in the BMR calculator prices your resting burn from lean mass directly — pair the two for the most personalized calorie math on this site.
LBM is not muscle mass
Skeletal muscle makes up only 40–50% of LBM; bone, organs, skin and water carry the rest. Gaining 5 kg of LBM on a bulk is excellent progress precisely because most of it can be muscle — losing 5 kg of LBM on a crash diet is the opposite story.
Estimates for healthy adults; not medical advice.