Lean Body Mass Calculator

Lean Body Mass (LBM) is your total body weight minus fat mass — everything that is not stored fat, including muscle, bone, organs, blood, connective tissue, and the water held inside all of them. For most people it works out to roughly 70–90% of body weight. LBM is a far more meaningful fitness metric than the bathroom-scale number alone, because two people of identical weight and height can have completely different body compositions. This calculator estimates your LBM, your fat mass, and your body-fat percentage from just your height, weight, and sex, and then averages three published formulas so you are not relying on any single equation's blind spots.

Who it is for. Anyone tracking a body-recomposition goal — losing fat while holding onto muscle — gets more signal from LBM than from weight. It is useful for setting a realistic protein target, for judging whether a diet is costing you muscle, and for gym-goers who want a baseline before a training block. Clinically, LBM is also used to scale the dosing of certain anaesthetic and chemotherapy drugs, which is why these formulas exist in the first place. It is not a diagnostic tool: treat the output as a screening estimate, and for anything medical, confirm body composition with a clinician.

The formulas this tool uses

Three peer-reviewed equations are computed and averaged. All take weight in kilograms (W) and height in centimetres (H):

• Boer (1984) — Men: LBM = 0.407 × W + 0.267 × H − 19.2; Women: LBM = 0.252 × W + 0.473 × H − 48.3. Generally the most accurate across a wide range of body weights.
• James (1976) — Men: LBM = 1.1 × W − 128 × (W/H)²; Women: LBM = 1.07 × W − 148 × (W/H)². Simple, but it underestimates — and can even turn negative — at very high BMIs.
• Hume (1966) — derived from body-density and total-body-water measurements; a useful third opinion that anchors the average.

Body fat is then derived: Fat mass = total weight − LBM, and body-fat % = fat mass ÷ total weight × 100.

Worked example

Take a 30-year-old man weighing 80 kg at 178 cm. Boer gives 0.407 × 80 + 0.267 × 178 − 19.2 ≈ 60.9 kg. James and Hume land nearby, and the three-formula average comes to roughly 61 kg of lean mass. That leaves about 19 kg of fat mass, or a body-fat percentage near 24% — in the "average" band for men. His daily muscle-building protein target would be about 1.0–1.1 g per lb of LBM, i.e. roughly 134–149 g per day. Change the sex to female with the same numbers and the female-specific coefficients return a higher fat percentage, which is expected: essential fat is naturally higher in women.

Asia-Pacific context

Body-fat thresholds matter more for Indian and other South-Asian bodies than the global figures suggest. South Asians tend to carry more visceral fat and less muscle at a given BMI — the so-called "thin-fat" phenotype — so a "normal" weight can still hide an unhealthy body-fat level. The WHO has proposed lower BMI action points (23 and 27.5) for Asian populations for exactly this reason. Use the body-fat percentage here as a more honest signal than weight or BMI alone, and read the "healthy range" bands as a starting point rather than a verdict.

Tips for a better estimate

• Weigh yourself first thing in the morning, after using the bathroom and before eating or drinking, for the most repeatable number.
• Measure height without shoes; even a centimetre shifts the result.
• Re-measure on the same scale at the same time of day when tracking progress — the trend matters more than any single reading.
• If your goal is muscle gain, watch whether LBM rises while fat mass holds steady; if you are cutting, you want fat mass to fall while LBM barely moves.

Common mistakes

• Treating it as exact. Formula-based LBM carries roughly ±2–4% error; a DEXA scan or hydrostatic weighing is the gold standard.
• Setting protein off total weight. For people carrying extra fat, dosing protein on LBM avoids massively over-shooting the target.
• Trusting James at a high BMI. It is the formula most likely to read low for heavier individuals, which is exactly why the tool averages it with two others.
• Chasing the "essential fat" band. Very low body fat is not a health goal — it sits at the edge of what the body needs to function.