Lean Body Mass Calculator
Lean body mass is everything you weigh that isn't fat: muscle, bone, organs, and water. It is the number behind protein targets, FFMI, and medical dosing. This calculator estimates it with the Boer formula, the equation most often used clinically.
Fill in the fields above to see your result.
How it's calculated
The Boer formula (1984) predicts lean body mass directly from weight, height, and sex: LBM = 0.407 × weight (kg) + 0.267 × height (cm) − 19.2 for men, and LBM = 0.252 × weight (kg) + 0.473 × height (cm) − 48.3 for women. It was derived from body-composition reference data and remains the default in clinical settings — anesthetics and chemotherapy agents are commonly dosed against Boer LBM — because it stays accurate across a wide range of body sizes, including higher body weights where simpler rules drift badly.
The formula's structure encodes two facts about body composition: lean mass grows with total weight (heavier bodies need more muscle and bone to move) but not proportionally — much of added weight at higher body masses is fat, which is why the weight coefficient is well under 1. Height carries independent information about skeletal frame. The sex-specific coefficients reflect average differences in fat distribution and muscularity at a given size.
Because Boer uses no body-fat measurement, it estimates the lean mass of a typical person of your dimensions — a very muscular lifter carries more lean mass than it predicts, and the formula can't see recomposition. If you know your body-fat percentage, weight × (1 − BF%) is the more personal estimate; the FFMI calculator here computes exactly that. The subtraction shown alongside the result (weight − LBM) is your estimated fat mass, a more actionable number to track through a cut than scale weight, which mixes fat, water, and glycogen shifts into one noisy figure.
Frequently asked questions
What is lean body mass exactly?
Everything except fat: muscle, bone, organs, skin, and body water. Muscle is the biggest trainable component, but LBM is not a pure muscle measurement.
How accurate is the Boer formula?
Closest to reference DXA measurements among the common equations for average builds, which is why medicine doses by it. Very muscular people exceed its prediction.
Should I use LBM or bodyweight for protein targets?
The evidence-based 1.6-2.2 g/kg range is expressed per total bodyweight. LBM-based dosing mainly matters at very high body-fat levels, where total weight overshoots.
Can I increase lean body mass while losing fat?
Yes — recomposition is common in newer lifters and those returning from a break: fat mass falls while muscle grows. High protein and progressive training drive it.
Take it with you
Every calculator, plus workout and macro tracking, in the ElevenSix Fitness app for iPhone.