Lean Body Mass Calculator
Lean body mass (LBM) is everything in you that isn't fat — muscle, bone, organs, blood, water, skin. This calculator estimates it from your weight, height, and gender, and shows the Boer, James, and Hume formulas side by side (plus Peters for children 14 and under) so you can see the spread instead of trusting a single number.
The lean body mass based on different formulas:
Advertisement — In-Article
(Responsive Fluid)
What lean body mass is
Body composition splits your weight into fat mass and everything else. That "everything else" — bones, muscles, blood, skin, organs, water — is lean body mass. It typically makes up 60–90% of total body weight, and men generally carry a higher share of it than women. Day to day, most people care more about the other side of the split — body fat percentage — which is why our Body Fat Calculator exists; LBM matters most in medicine (drug dosing, some clinical exams) and in training, where it tells you whether the weight you gained or lost was the tissue you intended.
The formulas
In all of them, W is weight in kilograms and H is height in centimeters.
Boer (1984)1
Men: eLBM = 0.407W + 0.267H − 19.2
Women: eLBM = 0.252W + 0.473H − 48.3
James (1976)2
Men: eLBM = 1.1W − 128(W/H)²
Women: eLBM = 1.07W − 148(W/H)²
Hume (1966)3
Men: eLBM = 0.32810W + 0.33929H − 29.5336
Women: eLBM = 0.29569W + 0.41813H − 43.2933
Peters (2011)4 — for children
For ages 13–14 and younger, Peters works through estimated extracellular volume: eECV = 0.0215 × W0.6469 × H0.7236, then eLBM = 3.8 × eECV. When "Age 14 or younger" is set to Yes, this calculator adds the Peters row above the adult formulas.
Lean body mass vs. fat free mass
The two terms are often swapped, but they differ by exactly one thing: essential fat. Lean body mass includes the fat woven into organs, bone marrow, and the central nervous system — it is part of the organs' own mass. Fat free mass excludes every gram of fat, essential fat included, so it is always the smaller number: the difference runs about 2–3% of body weight in men and 5–12% in women. In most practical settings the swap is harmless; in clinical and research settings it is not.
References
- Boer P. "Estimated lean body mass as an index for normalization of body fluid volumes in man." Am J Physiol 1984; 247: F632-5.
- James, W. "Research on obesity: a report of the DHSS/MRC group." HM Stationery Office, 1976.
- Hume, R. "Prediction of lean body mass from height and weight." J Clin Pathol 1966; 19(4): 389-91.
- Peters, A. M., Snelling, H. L. R., Glass, D. M., Bird, N. J. "Estimation of lean body mass in children." British Journal of Anaesthesia 2011; 106(5): 719-23.
FAQ
Which formula should I use?
Boer is the usual default for adults in normal weight ranges, and it is the estimate most often used for drug dosing. If the three adult formulas disagree wildly for you, that itself is information: your body composition sits where height-and-weight equations are weakest, and a direct measurement (DXA, Bod Pod) is worth more than any of them.
Can lean body mass exceed my expectations when I gain weight?
Yes — gaining weight almost never adds pure fat or pure muscle. Tracking LBM across a bulk or a cut shows the ratio: if weight rises 5 kg and estimated LBM rises 2 kg, roughly 3 kg was fat.
Why is there a separate formula for children?
Children are not small adults — their water distribution differs, and the adult equations were fitted on adult bodies. Peters (2011) was derived specifically for children, from extracellular volume.