Lean Body Mass Calculator — 3 Scientific Formulas

Calculate your lean body mass (LBM) using Boer, Hume, and James formulas. Get estimated fat mass and body fat percentage instantly. Free, science-based calculator.

Medically reviewed by Dr. Ivan IbáñezNº Col. 17/05487Mar 31, 2026

How to use this calculator?

CalcVita. (2026). Lean Body Mass Calculator — 3 Scientific Formulas. CalcVita. Retrieved October 17, 2026, from https://calcvita.com/en/calculators/lean-body-mass

Lean Body Mass: What It Is, Why It Matters, and How to Measure It

Lean body mass is the weight of everything in your body that is not fat — muscles, bones, organs, and water. Learn how to calculate it using Boer, Hume, and James formulas, and why it matters more than total body weight for health and performance.

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How does this calculator work?

This calculator uses three validated formulas — Boer (1984), Hume (1966), and James (1976) — to estimate lean body mass from weight, height, and sex. Lean body mass (LBM) is the portion of your body weight that is not fat. It includes muscles, bones, organs, and body water. The average of all three formulas is displayed as the primary result.

About the formulas

Boer (1984) published his equation in a study of 66 healthy adults, in which lean body mass estimated from weight and height proved the best index for normalizing blood and extracellular fluid volumes; it is widely used in clinical settings. Hume (1966) derived his from lean body mass calculated from total body water (antipyrine space) in 29 men and 27 women. The James formula (1976) is simple, but because of its squared term it underestimates LBM at high BMI and stops working above a BMI of about 43 in men or 36 in women. All three are used in drug dosing, sports science, and body composition research.

Limitations

These formulas were derived from population samples and may not accurately reflect individual body composition. They do not account for ethnicity, fitness level, or disease state. For precise measurements, clinical methods such as DEXA scanning or underwater weighing should be used. Consult a healthcare professional for medical decisions.

Sources

Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. Am J Physiol 1984;247(4 Pt 2):F632-6. PMID: 6496691
Hume R. Prediction of lean body mass from height and weight. J Clin Pathol 1966;19(4):389-391. PMID: 5929341
James WPT. Research on Obesity. Her Majesty's Stationery Office, London, 1976.

Frequently asked questions

What is lean body mass?: Lean body mass (LBM) is the total weight of your body minus all fat mass. It includes muscles, bones, organs, skin, and body water. LBM is a key metric in sports science, clinical nutrition, and drug dosing calculations.
Which LBM formula is most accurate?: No single formula is best for everyone. At usual body weights the three usually differ by only a few kilograms. The James (1976) formula, however, breaks down at high weights: because of its squared term, its estimate stops rising and then falls once BMI exceeds about 43 in men or 36 in women, so in people with obesity it is better to rely on Boer (1984) and Hume (1966). Comparing all three shows how much uncertainty the estimate carries.
How is lean body mass different from muscle mass?: Lean body mass includes all non-fat tissue: muscles, bones, organs, fluids, and glycogen. Muscle mass is just one component of LBM. To measure muscle mass precisely, methods like DEXA scanning or bioelectrical impedance analysis are needed.
Why does lean body mass matter?: LBM determines your basal metabolic rate, influences drug dosing in clinical settings, predicts athletic performance, and is used to set protein and calorie targets. Higher LBM is associated with better metabolic health, stronger bones, and lower risk of chronic disease.

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Frequently asked questions

What is lean body mass?

Lean body mass (LBM) is the total weight of your body minus all fat mass. It includes muscles, bones, organs, skin, and body water. LBM is a key metric in sports science, clinical nutrition, and drug dosing calculations.

Which LBM formula is most accurate?

No single formula is best for everyone. At usual body weights the three usually differ by only a few kilograms. The James (1976) formula, however, breaks down at high weights: because of its squared term, its estimate stops rising and then falls once BMI exceeds about 43 in men or 36 in women, so in people with obesity it is better to rely on Boer (1984) and Hume (1966). Comparing all three shows how much uncertainty the estimate carries.

How is lean body mass different from muscle mass?

Lean body mass includes all non-fat tissue: muscles, bones, organs, fluids, and glycogen. Muscle mass is just one component of LBM. To measure muscle mass precisely, methods like DEXA scanning or bioelectrical impedance analysis are needed.

Why does lean body mass matter?

LBM determines your basal metabolic rate, influences drug dosing in clinical settings, predicts athletic performance, and is used to set protein and calorie targets. Higher LBM is associated with better metabolic health, stronger bones, and lower risk of chronic disease.