What is lean body mass?

Lean body mass (LBM) is your total body weight minus your fat mass. It includes muscle, bone, organs, blood, skin, and water. LBM is important for calculating drug doses, protein requirements, and assessing body composition changes during training or weight loss.

How do I increase lean body mass?

The most effective way to increase lean body mass is progressive resistance training (3–4 sessions per week) combined with adequate protein intake (1.6–2.2 g per kg bodyweight) and a slight calorie surplus of 200–300 kcal/day. Sleep (7–8 hours) is critical for muscle protein synthesis during recovery.

Lean Body Mass Calculator — Free LBM Tool (Boer, James, Hume) 2026

💪 Lean Body Mass Calculator

Boer · James · Hume equations · Fat mass · Lean mass % · Daily protein target

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Weight (kg)

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Boer (kg)

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James (kg)

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Hume (kg)

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Average LBM

Average Lean Body Mass—

Estimated Fat Mass—

Lean Mass Percentage—

Daily Protein Target (1.6g/kg LBM)—

💪 Lean Mass Optimisation Tips

⚠️ LBM formulas estimate lean mass from height and weight only. Results may differ from DEXA or BIA measurements, particularly for athletes and obese individuals.

Lean Body Mass — Complete Guide

Lean body mass (LBM) is everything in your body that is not fat: muscle, bone, organs, blood, skin, and intracellular water. It is a more meaningful measure of fitness than total body weight, because it captures the metabolically active tissue that drives performance, strength, and calorie burning.

Why LBM Matters

Protein requirements: Protein intake recommendations are often based on LBM rather than total bodyweight, for accuracy across different body compositions.
Resting metabolic rate: Lean mass is the primary driver of BMR — more muscle means higher resting calorie burn, which makes maintaining a healthy weight easier long-term.
Drug dosing: Many medications (particularly in critical care) are dosed by LBM to avoid over-dosing obese patients on a total-bodyweight basis.
Training progress: Tracking LBM over time during a cutting or bulking phase shows whether you are gaining muscle or losing it, independent of fat changes.

The Three LBM Formulas Compared

Formula	Year	Notes
Boer	1984	Most commonly used in clinical pharmacokinetics. Tends to give slightly higher LBM estimates.
James	1976	Widely cited. Can underestimate LBM in severely obese individuals.
Hume	1966	Derived from cadaver analysis. Generally gives intermediate estimates.

How to Increase Lean Body Mass

Progressive resistance training: 3–4 sessions per week with progressive overload (increasing weight or reps over time) is the primary stimulus for lean mass growth.
Protein intake: 1.6–2.2 g of protein per kg of LBM daily for muscle building. Spread across 4–5 meals with at least 30–40g per serving to maximise muscle protein synthesis.
Calorie surplus: A modest 200–300 kcal/day above TDEE ("lean bulk") minimises fat gain while providing enough energy for muscle growth. Use our calorie calculator.
Sleep 7–9 hours: The majority of muscle protein synthesis occurs during sleep. Growth hormone peaks during deep sleep. Chronic sleep deprivation reduces testosterone by 10–15%, directly limiting muscle growth.
Creatine monohydrate: The single best-evidenced legal ergogenic aid for lean mass. 3–5g/day increases phosphocreatine stores, improving performance and muscle hydration.

Frequently Asked Questions

These terms are often used interchangeably, but technically differ. Fat-free mass excludes all fat including essential fat. Lean body mass includes a small amount of essential fat (the minimum required for hormone function and cell membrane integrity — roughly 3% for men and 10–13% for women). In practice, the difference is small and both terms are used to mean "everything that is not stored fat."

Natural muscle gain is slow. Beginners can gain 1–1.5 kg of lean mass per month in the first year. Intermediates gain 0.5–0.8 kg/month. Advanced lifters gain 0.2–0.3 kg/month. These rates assume optimal training, nutrition, sleep, and recovery. Any product claiming faster gains than this without drugs should be viewed with scepticism.

Yes, but primarily in specific circumstances: beginners, those returning after a break, and those with significant fat stores (over 25% body fat for men, over 33% for women). In these groups, body recomposition — simultaneous fat loss and muscle gain — is achievable with a slight calorie deficit, high protein, and progressive resistance training. Advanced, lean individuals generally cannot achieve significant recomposition and are better served by dedicated bulking or cutting phases.