What lean body mass is — and why it's the number that matters
Lean body mass (LBM) is everything that isn't fat: muscle, bone, organs, water. It's the metabolically active part of you — which is why the best-targeted nutrition numbers are anchored to LBM rather than total weight. Protein targets during a cut (1.6–2.2 g per kg of lean mass), the Katch-McArdle BMR, and muscle-preservation planning on GLP-1 medications all start from this number. Two people at 90 kg with different LBM need genuinely different plans.
Three formulas, honestly compared
The Boer formula (1984) is the best-validated general-purpose estimate and our default — it's also the standard in clinical drug-dosing contexts. James (1976) tends to overestimate at higher body weights because of its squared weight-to-height term. Hume (1966) is the oldest and runs conservative. The spread between them — typically 3–6 kg — is an honest picture of formula uncertainty. If you know your body fat percentage, the direct calculation (weight × (1 − BF%)) beats all three: get a tape estimate from the Navy-method body fat calculator in two minutes.
What to do with the number
Set protein from it with the protein calculator, run Katch-McArdle for a body-composition-aware BMR, and if you're losing weight, track LBM alongside the scale — weight falling while lean mass holds is the actual goal. A DEXA scan is the gold standard if you want the formula uncertainty settled.
Frequently asked questions
Which lean body mass formula is most accurate?
Boer is the best-validated default and the clinical standard. But any formula is ±a few kg — a measured body fat percentage (even tape-based) gives a more direct answer, and DEXA settles it.
What is a normal lean body mass percentage?
Typically 60–90% of total weight depending on sex, training and body fat — men commonly 75–85%, women 65–75%. The trend matters more than the snapshot: preserving LBM while losing weight is the goal.
Why do protein recommendations use lean mass instead of body weight?
Because muscle is what protein maintains. At higher body fat, total-weight targets overshoot wastefully; lean-mass targets (1.6–2.2 g/kg LBM when cutting) scale to the tissue actually at stake.
Can I increase lean body mass while losing weight?
Yes — “recomposition” is realistic for beginners, the detrained, and people with higher body fat: resistance training plus adequate protein in a modest deficit. Advanced lifters usually need dedicated phases instead.
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Sources
- Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. Am J Physiol. 1984. [link]
- Hume R. Prediction of lean body mass from height and weight. J Clin Pathol. 1966. [link]
- James WPT. Research on Obesity: A Report of the DHSS/MRC Group. HMSO, London. 1976.
- Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men and women from body circumferences and height. Naval Health Research Center. 1984. [link]