Estimate your fat-free mass — muscle, bone, organs, and water — using three established clinical formulas.
Reviewed by Sarah Mitchell, BSc Nutrition · Last updated: July 2026
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Average estimated lean body mass
Estimated fat mass:— · Lean mass %:—
By Formula
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What Is Lean Body Mass?
Lean body mass (LBM), also called fat-free mass, is everything in your body that isn't stored fat — muscle, bone, organs, connective tissue, and water. It's a more useful reference point than total body weight for setting protein targets, estimating resting energy needs, and tracking whether weight change reflects fat loss or muscle loss.
The Three Formulas Used
Boer Formula (1984): Men: LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2. Women: LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3. Widely used in clinical and pharmacological dosing contexts.
James Formula (1976): Men: LBM = 1.10 × weight(kg) − 128 × (weight/height)². Women: LBM = 1.07 × weight(kg) − 148 × (weight/height)². Incorporates a body-mass-relative adjustment.
Hume Formula (1966): Men: LBM = 0.3281 × weight(kg) + 0.33929 × height(cm) − 29.5336. Women: LBM = 0.29569 × weight(kg) + 0.41813 × height(cm) − 43.2933. One of the earliest validated formulas.
No single formula is universally superior — showing the average across all three gives a more realistic estimate than relying on just one.
💡 Have a body fat percentage already? A more personalized calculation is: Lean Mass = Total Weight × (1 − Body Fat %). Use our Body Fat Calculator first, then apply this formula for a result specific to you rather than population averages.
Lean Body Mass vs Muscle Mass
These terms are often used interchangeably but aren't the same. Lean body mass includes all non-fat tissue — muscle, bone, organs, and water. Muscle mass refers specifically to skeletal muscle. LBM will always be a larger number than muscle mass alone, since bone and organs typically make up 15-20% of total body weight on their own.
Why Lean Body Mass Matters
Protein targets: Many nutrition guidelines recommend protein intake per kg of lean mass (e.g. 1.6-2.2 g/kg) rather than total weight, which is more accurate for people with higher body fat percentages.
Resting energy needs: Lean tissue is far more metabolically active than fat tissue, so lean mass is a stronger predictor of your BMR than total weight alone. Try our BMR Calculator.
Tracking body recomposition: If your weight is stable but lean mass is increasing, you're likely building muscle while losing fat — a sign of progress the scale alone won't show.
Limitations
These formulas estimate lean mass from height, weight, and sex alone — they don't account for individual differences in muscularity, ethnicity, or age-related muscle loss (sarcopenia). Someone with unusually high or low muscle mass for their height and weight will see less accurate results. Formulas based on measured body fat percentage (see the callout above) are generally more personalized.
References:
Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. American Journal of Physiology. 1984;247(4):F632-636.
James WPT. Research on Obesity: A Report of the DHSS/MRC Group. London: HMSO, 1976.
Hume R. Prediction of lean body mass from height and weight. Journal of Clinical Pathology. 1966;19(4):389-391.
Lean body mass (LBM), also called fat-free mass, is your total body weight minus fat mass. It includes muscle, bone, organs, water, and connective tissue — essentially everything in your body except stored fat.
The Boer, James, and Hume formulas were each developed independently using different population studies. The Boer formula (1984) is widely used in clinical practice and drug dosing. The James formula (1976) accounts for body mass index in its calculation. The Hume formula (1966) is one of the oldest and most established. No single formula is universally accepted, so showing all three gives a more balanced estimate.
Lean body mass includes muscle, but also bone, organs, water, and connective tissue. Muscle mass refers specifically to skeletal muscle tissue. LBM is always higher than muscle mass alone, since it includes everything in the body that isn't fat.
These formulas use height, weight, and sex to estimate lean mass without requiring circumference measurements, but they do not account for individual differences in muscularity. If you have measured your body fat percentage, multiplying it by your total weight and subtracting from your total weight gives a more personalized estimate of lean mass.
Lean body mass is often used to set more accurate protein targets (grams per kg of lean mass rather than total weight) and to estimate resting energy expenditure, since lean tissue burns significantly more calories at rest than fat tissue. Tracking lean mass over time also helps confirm whether weight changes reflect fat loss/gain versus muscle loss/gain.
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