This lean body mass calculator estimates how much of you is not fat — muscle, bone, organs and water — using the three classic formulas (Boer, James, Hume), and derives your fat mass from the result.
Formula estimates vary a few kilograms between methods — for a measurement (not an estimate), use our Body Fat Calculator’s Navy method or a DEXA scan.
What Is Lean Body Mass?
Everything in your body that is not fat: skeletal muscle, bone, organs, blood, skin and water. Athletes chase it (it is the engine that produces force), clinicians guard it (its loss — sarcopenia — predicts frailty and mortality in older adults), and our Body Fat Calculator is its mirror image: fat % + lean % = 100% of you. Formula estimates like these predict LBM from height, weight and sex; they cannot see your training history, which is why three respectable formulas disagree by a few kilos — a feature honestly displayed rather than hidden. Part of our Health Calculators collection.
Worked Examples (Verified)
- Man, 175 cm, 70 kg: Boer gives 0.407×70 + 0.267×175 − 19.2 = 56.0 kg lean (80%) — implying ~14 kg fat (20%), squarely typical for a healthy active man.
- Woman, 165 cm, 60 kg: Boer: 0.252×60 + 0.473×165 − 48.3 = 44.9 kg lean (75%) — ~15.1 kg fat (25%), again typical.
The Three Formulas (and Their Quirks)
- Boer (1984) — the most cited for adults; performs well across normal BMI ranges.
- James (1976) — includes a weight/height² term that bends at high BMI; known to underestimate lean mass in obesity and can even turn non-monotonic — treat its number cautiously at BMI > 35.
- Hume (1966) — the eldest, fitted on smaller samples; still used in radiology dosing contexts.
- All three predate modern body-composition data — DEXA and BIA devices measure rather than infer, and our Navy-method Body Fat tool typically lands within a few percent of them.
Why Lean Mass Is Worth Tracking
- Dieting quality: losing 5 kg with 4 kg from fat is triumph; 4 kg from lean is a warning. LBM trend tells you which diet you are actually on — and argues for protein + resistance training during cuts.
- Protein dosing: sports nutrition often doses protein per kg of lean mass (roughly 2.2–3.0 g/kg LBM in cuts) rather than per kg of total weight.
- Strength standards (squat multiples, etc.) make more sense against lean mass than against a scale number padded by fat.
Related Tools
- Body Fat Calculator — measured estimates incl. the Navy method
- BMI Calculator
- Health Calculators — the full collection
Frequently Asked Questions
How do I calculate my lean body mass?
With the Boer formula: men, LBM = 0.407×weight(kg) + 0.267×height(cm) − 19.2; women, 0.252×W + 0.473×H − 48.3. The calculator also runs the James and Hume formulas for comparison.
What is a good lean body mass percentage?
Equivalently, a healthy body-fat range: roughly 10–20% fat for men (80–90% lean) and 18–28% for women (72–82% lean), varying with age and athleticism. The worked examples land mid-range.
Is lean body mass the same as muscle mass?
No — lean mass includes bones, organs, skin and water, which are roughly half of it. Skeletal muscle alone is typically ~40–50% of lean mass in men.
Why do the three formulas give different answers?
They were fitted on different populations with different measurement methods between 1966 and 1984. Their spread (usually 1–4 kg) honestly reflects estimation uncertainty — a measured method is better when the number really matters.
Can I increase lean mass while losing fat?
Yes — “body recomposition” is realistic for beginners, returning lifters and higher-body-fat trainees given adequate protein and progressive resistance training; it gets slower as you get leaner and more trained.
Is this lean body mass calculator free?
Yes — free, instant, no sign-up. Estimates only, not medical measurements.