Body composition
Lean body mass calculator
Lean body mass and implied fat mass using the Boer or James formula.
Your stats
Lean body mass
135.6lb
Implied body fat 24.7 %.
Fat mass
44.4lb
Body fat (implied)
24.7%
What lean body mass means
Lean body mass (LBM) is everything in your body that is not storage fat — muscle, bone, organs, blood, and water. When physique-focused lifters track progress, LBM is usually a more meaningful metric than total bodyweight, because the goal of most training is to add LBM (muscle) and lose fat.
The two formulas
Boer (male): LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2
Boer (female): LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3
James (male): LBM = 1.1 × weight(kg) − 128 × (weight/height)²
James (female): LBM = 1.07 × weight(kg) − 148 × (weight/height)²Both formulas were validated against measured body composition and tend to agree within a few kilograms for typical adults. Both struggle at the extremes — very lean or very heavy individuals are best measured directly.
Frequently asked questions
What is lean body mass?
Lean body mass is total bodyweight minus essential and storage fat — muscle, bone, organs, blood, and water. It is a more useful target than total weight when you are trying to gain muscle or preserve it during a cut.
Boer or James — which should I use?
Boer is the more commonly cited formula in clinical settings and tends to read slightly higher. James was developed earlier and runs slightly leaner. For most people the two land within 2–3 kg of each other. Pick one and stick with it for tracking change over time.
Why isn't this a true body composition measurement?
Because LBM estimated from weight and height alone can't account for individual variation in muscle mass. A muscular athlete and a sedentary person of the same height and weight will read the same LBM from these formulas, which is obviously wrong. For an actual body-composition reading, use the body-fat calculator or get a DEXA scan.