Lean Body Mass Calculator — Boer & Hume Formulas (Free, 2026)
Free lean body mass calculator. Enter height, weight, and sex to estimate your fat-free mass with the Boer and Hume formulas, plus your fat mass and implied body fat %.
Lean body mass is your weight minus your fat — the muscle, bone, organs, and water that make up everything else. This calculator estimates it from your height, weight, and sex using two published formulas.
What is lean body mass?
Lean body mass (LBM), or fat-free mass, is everything in your body that isn't fat: skeletal muscle, bone, organs, connective tissue, and body water. It's the part of your weight that drives your metabolism — more lean mass means a higher resting calorie burn.
When you lose weight, the goal is to lose fat while protecting lean mass. A steady, moderate calorie deficit paired with enough protein and resistance training is what keeps the scale moving down without sacrificing muscle.
How it's estimated
This calculator shows two published prediction equations: Boer (Boer P, American Journal of Physiology, 1984) as the headline and Hume (Hume R, Journal of Clinical Pathology, 1966) for comparison. Both estimate LBM from height, weight, and sex. They're population formulas, so they're best as a ballpark and a way to track change — for a precise reading, a DEXA scan or a body-fat measurement (which we offer separately) gives your actual fat-free mass.
Formula & sources
Boer formula
Hume formula
W = weight in kg, H = height in cm. The headline uses Boer; Hume is shown alongside for comparison.
Sources
- Boer P (1984) Estimated lean body mass as an index for normalization of body fluid volumes in humans. Am J Physiol 247(4):F632–F636.
- Hume R (1966) Prediction of lean body mass from height and weight. J Clin Pathol 19(4):389–391.
Lean body mass FAQ
How accurate is a lean body mass calculator?
Formula-based estimates like Boer and Hume are good ballparks for typical builds and great for tracking change over time, but they can be off for very muscular or very lean people. For a precise number, measure your body fat percentage (and subtract it from your weight) or get a DEXA scan.
What's the difference between lean body mass and muscle mass?
Muscle is part of lean body mass, but LBM also includes bone, organs, and water. Skeletal muscle is usually around 40–50% of lean mass, so LBM is always larger than your muscle mass alone.
How do I increase lean body mass?
Resistance training plus enough protein (roughly 1.2–2.0 g per kg of body weight for active adults) and adequate total calories. Building lean mass while in a calorie deficit is possible but slow — most people gain it fastest at maintenance or a slight surplus.
Why does lean mass matter for weight loss?
Lean mass burns more calories at rest than fat does, so preserving it keeps your metabolism higher as you lose weight. Crash diets that ignore protein and training tend to shed lean mass along with fat, which makes weight regain easier.