Lean Body Mass Calculator
Calculate your lean body mass (total weight minus fat mass) using three established formulas: Boer, James, and Hume.
Frequently Asked Questions
What is lean body mass and what does it include?
Lean body mass (LBM) is your total body weight minus all fat mass (both essential and stored fat). LBM includes skeletal muscle, bone, organs, skin, connective tissue, and body water — everything that is not fat. For a 75 kg man at 18% body fat, LBM = 75 − (75 × 0.18) = 61.5 kg. LBM is sometimes confused with "fat-free mass" (FFM), but FFM excludes even essential fat, while LBM technically includes essential fat (~3–5% in men, ~10–13% in women).
Why does lean body mass matter for health and fitness?
LBM is the primary driver of Basal Metabolic Rate (BMR) — muscle tissue burns approximately 13 kcal/kg/day at rest, versus ~4 kcal/kg/day for fat. Higher LBM means a higher resting metabolism, making weight management easier. LBM is also critical for: medication dosing (many drugs are dosed on LBM), predicting athletic performance, tracking resistance training progress (muscle gain), and assessing sarcopenia (age-related muscle loss, which accelerates after age 30 at ~3–5% per decade without training).
Which LBM formula is most accurate — Boer, James, or Hume?
No formula is universally the best. Boer (1984) is considered the most accurate for average-weight adults and is widely used in clinical settings. James (1976) tends to overestimate LBM in overweight individuals. Hume (1966) is older and less accurate at extremes of BMI. Studies suggest Boer is the best single estimate for most people, but the average of all three (as shown in this calculator) provides a more robust estimate. For highest accuracy, DEXA scanning remains the gold standard.
How do I increase lean body mass?
The most effective strategy is progressive overload resistance training (weightlifting, bodyweight exercises) combined with adequate protein intake. Aim for 1.6–2.2 g of protein per kg of body weight per day to support muscle protein synthesis. Eat at or slightly above your TDEE (a 200–400 kcal surplus) to provide the energy surplus needed for muscle growth. Sleep 7–9 hours per night — growth hormone release during deep sleep is critical for muscle repair and synthesis. Beginners can gain 1–2 kg of muscle per month; trained individuals gain much less.
Does LBM decrease with age and how can I prevent it?
Yes. Sarcopenia (age-related muscle loss) begins around age 30 and accelerates after 60 — the average sedentary person loses 3–8% of muscle mass per decade. By age 70, many people have lost 25–30% of their peak muscle mass. Prevention: resistance training 2–4 times per week is the most powerful intervention, even for older adults. Protein intake of ≥1.2 g/kg/day (or higher, up to 1.6 g/kg) is especially important for older adults, as muscle protein synthesis efficiency declines with age (anabolic resistance). Creatine supplementation has also shown modest LBM benefits in older populations.