Lean Body Mass Calculator
Calculate your lean body mass (fat-free mass) using the Boer and James formulas. Essential for accurate protein targets and body recomposition tracking.
What Is Lean Body Mass and Why It Matters
Lean Body Mass (LBM) is everything in your body except fat — muscle, bone, organs, blood, skin, and connective tissue. It's sometimes called "fat-free mass" (FFM). While body weight tells you your total mass, LBM tells you how much of that weight is metabolically active tissue that burns calories, produces strength, and supports functional health.
Boer formula (females): LBM = 0.252×kg + 0.473×cm − 48.3
James formula (males): LBM = 1.1×kg − 128×(kg/cm)²
James formula (females): LBM = 1.07×kg − 148×(kg/cm)²
Key Uses of Lean Body Mass
- Protein targets: Most sports nutrition research recommends 0.7–1.0g of protein per pound of LBM (not total bodyweight) for muscle preservation and growth during a calorie deficit
- Drug dosing: Many medications — particularly in critical care, anesthesia, and oncology — are more accurately dosed based on LBM than total weight
- Body recomposition tracking: When simultaneously losing fat and gaining muscle, scale weight may not change even as body composition dramatically improves. Tracking LBM separately from fat mass shows progress the scale misses
- Resting metabolic rate: The Katch-McArdle formula calculates the most accurate BMR if LBM is known: BMR = 370 + (21.6 × LBM in kg)
How to Increase Your Lean Body Mass
Muscle is gained through progressive resistance training combined with adequate protein intake and a slight caloric surplus. Key principles: train each muscle group 2–3× per week, progressively increase load or volume over time, consume 0.7–1g protein/lb LBM, and prioritize sleep (most muscle protein synthesis occurs during sleep).