By Zencalcy Editorial TeamLast reviewed About us
Lean body mass (LBM) estimates the weight of everything in the body except fat: muscle, bone, organs and water. It is used in nutrition, fitness and some clinical calculations. The Boer and James equations are two common ways to estimate it from height, weight and sex.
Estimate fat-free mass with the Boer or James formula.
Boer men: LBM = 0.407 × weight + 0.267 × height − 19.2 Boer women: LBM = 0.252 × weight + 0.473 × height − 48.3 James men: LBM = 1.1 × weight − 128 × (weight ÷ height)² James women: LBM = 1.07 × weight − 148 × (weight ÷ height)²
Weight is in kilograms and height in centimetres. These are population equations, not measurements. Boer tends to perform well in adults; James behaves differently at very high or low weights. Neither accounts for ethnicity, age or training status directly.
| Symbol | Meaning |
|---|---|
| LBM | Lean body mass in kilograms. |
| Fat mass | Weight minus lean body mass. |
| weight | Body weight in kilograms. |
| height | Standing height in centimetres. |
Boer: 0.407 × 80 + 0.267 × 180 − 19.2 = 32.56 + 48.06 − 19.2 = 61.4 kg lean mass, leaving 18.6 kg fat. James gives 1.1 × 80 − 128 × (80 ÷ 180)² = 88 − 25.3 = 62.7 kg.
A high lean body mass is often treated as a proxy for fitness, but the number mixes muscle, bone, organs and water. A larger person will almost always have more lean mass than a smaller person without being leaner. That is why lean mass is best read alongside body-fat percentage and waist circumference. During weight loss, preserving lean mass is a reasonable goal because muscle supports strength and metabolic rate, but the scale cannot tell you whether the lost weight was fat or muscle. Resistance training and adequate protein help, and the lean-mass estimate can show whether the ratio is moving in the right direction. As with all equations, the absolute number is less useful than the trend measured the same way over time.
Continue with the ABSI Calculator (A Body Shape Index), Adjusted Body Weight Calculator, or the BAI Calculator (Body Adiposity Index) — or browse more body measurements tools and all health calculators.
Also see the Body Fat Calculator.
No. Lean body mass includes muscle, bone, organs and body water. Skeletal muscle is only part of it, so LBM overstates muscle mass.
Every result is a screening estimate from published anthropometric equations, not a diagnosis or a measurement of body composition. Tape measurements should be taken snugly against the skin without compressing it, at the landmarks named in each tool. Equations were derived in specific populations and can be less accurate for athletes, older adults, children, pregnant people and people with significant oedema.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
Content updated: September 2026