Lean Body Mass Calculator — LBM by Boer, James & Hume Formula

What Is a Lean Body Mass Calculator?

A lean body mass calculator estimates how much of your total body weight is made up of everything that isn't fat — muscle, bone, organs, water and connective tissue. Rather than measuring this directly, which normally requires equipment such as a DEXA scan, this free lean mass calculator uses three well-established formulas — Boer, James and Hume — that estimate lean body mass (LBM) from your height, weight and sex alone.

Lean body mass is a useful reference point for anyone tracking body composition changes during training, weight loss or weight gain, because it separates fat loss from the loss of muscle and other lean tissue. It also underpins several other calculations used in sports science and clinical nutrition, including some approaches to estimating calorie needs.

If you want a general weight-based screening tool, use the main BMI Calculator NHS. For a single ideal weight estimate based on height, see our ideal body weight calculator. If you specifically want a body fat percentage using measurement-based methods, our body fat calculator UK covers that in more depth.

How to Use the Lean Body Mass Calculator

1. Choose metric (cm/kg) or imperial (ft/in, lb) using the unit toggle.
2. Select your sex — all three formulas use sex-specific constants.
3. Enter your height and current weight.
4. Select Calculate My Lean Body Mass to see results from all three formulas, plus estimated fat mass and body fat percentage, instantly with no page reload.

The Lean Body Mass Formulas Explained

Each lean body mass formula combines your height and weight with a sex-specific constant, reflecting that men typically carry proportionally more lean tissue than women at the same height and weight. This lean body mass calculator applies all three automatically:

Worked Example

Take a male adult who is 180 cm tall and weighs 85 kg. Applying each formula:

• Boer: (0.407 × 85) + (0.267 × 180) − 19.2 = 34.6 + 48.1 − 19.2 = 63.5 kg
• James: (1.10 × 85) − 128 × (85/180)² = 93.5 − 28.5 = 65.0 kg
• Hume: (0.3281 × 85) + (0.33929 × 180) − 29.5336 = 27.9 + 61.1 − 29.5 = 59.4 kg

The average across all three formulas is approximately 62.6 kg of lean body mass. Subtracting this from the total weight of 85 kg gives an estimated fat mass of around 22.4 kg, or approximately 26% estimated body fat.

Lean Body Mass by Weight — Boer Formula Reference Table (2026)

The table below shows approximate lean body mass using the Boer formula at a fixed height of 170 cm, across a range of body weights. Use it as a quick reference, or enter your own exact height, weight and sex in the calculator above for a precise result across all three formulas.

Figures rounded to one decimal place, based on the Boer formula at 170 cm only. For your exact height, weight and sex across all three formulas, use the calculator above.

Interpreting Your Lean Body Mass Result

• High lean mass relative to total weight: generally indicates a higher proportion of muscle, bone and other lean tissue — common in trained individuals and naturally muscular body types.
• Lower lean mass relative to total weight: may reflect lower muscle mass, higher body fat, or both — worth reviewing alongside your BMI vs body fat percentage comparison.
• Tracking over time: lean body mass is most useful when tracked over weeks or months, particularly during resistance training or a structured weight change programme, rather than interpreted as a single snapshot.

Common Mistakes When Using a Lean Body Mass Calculator

• Confusing LBM with muscle mass: lean body mass also includes bone, organs, water and connective tissue — not just muscle.
• Expecting exact precision: formula-based LBM is an estimate; direct methods like DEXA scans or bioelectrical impedance are more accurate for individual tracking.
• Ignoring formula differences: Boer, James and Hume can differ by several kilograms for the same person — always check which formula a source uses.
• Applying the formulas to children: these equations were derived from adult population data and are not appropriate for under-18s.
• Treating a single result as diagnostic: use LBM as a general reference or a trend-tracking tool, not a clinical diagnosis.

Benefits and Limitations of Lean Body Mass Formulas

Benefits

• Well-established formulas with decades of use in body composition research
• Quick to calculate from just height, weight and sex — no special equipment needed
• Useful for tracking trends over time during training or weight change programmes
• Comparing multiple formulas gives a useful sense-check rather than relying on one figure

Limitations

• Does not directly measure muscle, bone or fat tissue — all figures are estimates
• Less accurate for very muscular or very lean individuals, such as athletes and bodybuilders
• Not designed for children, pregnant women, or frail older adults
• Should be considered alongside, not instead of, direct body composition measurement where precision matters

Lean Body Mass vs Body Fat Percentage: What's the Difference?

Real-Life Example: Tracking Lean Mass During Weight Loss

Consider a female adult who is 165 cm tall and weighs 78 kg at the start of a weight loss programme. Her average LBM across the three formulas is approximately 51 kg, giving an estimated fat mass of 27 kg and an estimated body fat percentage of around 35%.

After three months of combining a moderate calorie deficit with regular resistance training, she now weighs 72 kg. Recalculating her LBM shows it has only dropped slightly to around 49.5 kg — meaning most of the 6 kg lost came from fat mass rather than lean tissue. This is exactly the kind of comparison a lean body mass calculator is useful for: distinguishing fat loss from muscle loss during a weight change. For guidance on pacing weight loss safely, see our safe rate of weight loss per week guide.

Expert Tips

• Recalculate your LBM every few weeks rather than daily to see meaningful trends, not day-to-day fluctuation
• Pair resistance training with a moderate calorie deficit to help preserve lean mass while losing fat — see our calorie deficit calculator NHS
• Use the Boer formula as your primary reference, since it is the most widely cited
• Compare your LBM trend alongside your BMI progress tracker for a fuller picture
• If precision matters — for athletic performance or clinical reasons — ask your GP about referral for a DEXA scan or bioelectrical impedance analysis

Sources & References

• Boer P (1984) — Estimated lean body mass as an index for normalization of body fluid volumes in humans. American Journal of Physiology.
• James WPT (1976) — Research on Obesity: A Report of the DHSS/MRC Group.
• Hume R (1966) — Prediction of lean body mass from height and weight. Journal of Clinical Pathology.
• NHS Live Well — Healthy Weight: nhs.uk/live-well/healthy-weight
• NICE Guideline CG43 — Obesity: Identification, Assessment and Management (2023): nice.org.uk/guidance/cg43

Frequently Asked Questions — Lean Body Mass Calculator

Conclusion: Understanding Your Lean Body Mass

Your lean body mass gives you a useful way to understand what your body weight is actually made of, beyond the single number on the scales. The free lean body mass calculator above gives you results from all three widely used formulas — Boer, James and Hume — side by side, plus an estimated fat mass and body fat percentage, so you get a fuller picture than any single figure could provide.

Remember that these formulas are estimates derived from population data, not precise clinical measurements, and are most useful for tracking trends over time rather than as a one-off diagnostic figure. For a broader look at your overall weight status, pair this tool with our ideal body weight calculator, healthy weight range calculator, or the main BMI Calculator NHS, and speak to your GP for personalised advice.