Lean Body Mass (LBM) Calculator
Enter height, weight and sex to estimate your lean body mass (LBM) — everything that is not fat: muscle, bone, organs and water. The calculator runs three established equations side-by-side (Boer, Hume, James) and also reports fat mass and an estimated body-fat percentage you can compare against DEXA or BIA readings. The default Boer formula (1984) is the version most commonly used clinically and for chemotherapy dose calculation.
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Lean body mass (LBM)
55.8
kg
Fat mass
14.2
kg
Estimated body fat %
20.3%
Estimates only — not a true body-fat measurement. For accuracy use DEXA, BIA or skinfold methods. The James formula can be invalid at extreme BMI.
Formula
Boer (recommended) Male: LBM = 0.407 × W + 0.267 × H − 19.2 Female: LBM = 0.252 × W + 0.473 × H − 48.3 Hume Male: LBM = 0.32810 × W + 0.33929 × H − 29.5336 Female: LBM = 0.29569 × W + 0.41813 × H − 43.2933 James Male: LBM = 1.10 × W − 128 × (W/H)² Female: LBM = 1.07 × W − 148 × (W/H)² Body fat % = (W − LBM) ÷ W × 100% (W in kg, H in cm)
- · Boer (1984) was published by Boer P. in J Am Soc Nephrol — originally for body-surface-area correction in kidney patients, it is now the LBM formula most widely used for chemotherapy dose calculation (carboplatin, 5-FU, etc.).
- · Hume (1966) in J Clin Pathol is one of the earliest predictive equations and remains a standard literature benchmark; its accuracy for average-build adults is close to Boer.
- · James (1976), from the British Medical Research Council "Research on Obesity" report, contains a (W/H)² correction and is used in anaesthetic dosing (propofol, rocuronium) to avoid overdosing obese patients.
- · Predictive equations do not separate muscle from bone or water, so "LBM" is not the same as muscle mass. To track training progress use DEXA, BIA scales or skinfold measurements instead.
- · The James formula can produce non-physical values (negative LBM) at extreme BMI; this calculator detects that case and shows an error — fall back to Boer or Hume.
- · The body-fat percentage shown is derived from the LBM estimate, not measured. Expect a systematic bias of roughly ±3–5%; treat it as a rough reference only.
- · Adults only. Not appropriate for children, adolescents, pregnant individuals or elite bodybuilders/athletes with very low fat or very high muscle mass.
- · Sources: Boer P., Am J Physiol 1984; Hume R., J Clin Pathol 1966; James W.P.T., MRC "Research on Obesity" 1976; Janmahasatian S. et al., Clin Pharmacokinet 2005 (LBM in drug dosing).
Frequently asked
What's the difference between lean body mass (LBM) and fat-free mass (FFM)?
Strictly, FFM = body weight − all fat (including the essential fat in brain, nerves and bone marrow). LBM by convention keeps the small amount of essential fat (~3% of weight in men, ~12% in women), so LBM is slightly larger than FFM. In clinical nutrition and drug-dosing contexts the two terms are used almost interchangeably and the difference is under 3%. This calculator uses the Boer, Hume and James formulas, all of which are in the "LBM" family.
Which formula should I pick?
Use Boer (the default) for most cases. It is accurate across normal BMI (18.5–30) in adults and is the version most clinical guidelines specify for chemotherapy dosing. Hume is useful as a literature comparison and slightly overestimates LBM in higher BMI. James includes a (W/H)² correction and tends to track measured LBM better than Boer in obesity (BMI ≥ 30) — it is the anaesthetic-dosing reference — but breaks down at very low or very high BMI. For professional dosing, follow your guideline. For everyday curiosity, Boer is the right default.
How does this compare to a body-fat scale or DEXA scan?
DEXA is the gold standard (±1–2%). Bio-impedance body-fat scales (BIA) typically run ±3–5% and shift with hydration. Boer / Hume / James, which use only height and weight, are accurate to roughly ±3–6% in normal-build adults and worse at the extremes. If you train and want to track lean mass changes, anchor with a DEXA every 6–12 months; use BIA day-to-day, with the formulas as a rough sanity check — don't fixate on tenths of a percent.
Why is my LBM underestimated if I lift weights?
All three formulas are derived from general population statistics. If you carry meaningfully more muscle than a typical adult — long-term lifters, physique athletes — the equations will mis-attribute that extra muscle to "fat", inflating your estimated body-fat % and underestimating LBM. The same bias happens for very lean small-framed individuals in the opposite direction. If your build is far from average, measure directly with DEXA rather than relying on these predictive equations.
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