Formula
Use W in kg and H in cm. Boer: male 0.407W + 0.267H − 19.2; female 0.252W + 0.473H − 48.3. James: male 1.10W − 128(W/H)²; female 1.07W − 148(W/H)². Hume: male 0.32810W + 0.33929H − 29.5336; female 0.29569W + 0.41813H − 43.2933.
Lean body mass refers broadly to the body’s non-fat component, including water, organs, and other tissues. It is not the same as skeletal muscle mass. These three equations use height, weight, and historical sex-specific coefficients to estimate lean mass. Showing them together makes model disagreement visible. A difference between equations does not mean one is accurate for a particular person, and an average of the outputs is not offered as a validated improvement.
Worked example
For a male formula at 80 kg and 180 cm, Boer gives 61.42 kg, James gives about 62.716 kg, and Hume gives 57.7866 kg. These correspond to approximately 76.8%, 78.4%, and 72.2% of body weight. Results are rounded to one decimal place after calculation.
How to use this calculator
- Choose the male or female coefficient set from the historical formulas.
- Enter actual weight in kg or lb and height in cm or feet and inches.
- Select Calculate to compare all three estimates in kilograms, pounds, and percentage of actual weight.
- Read any applicability warning and remember that these are predictions, not body-composition measurements.
When not to use this
Do not use these equations as a substitute for a body-composition scan or an individualized assessment. They may perform poorly in children, pregnancy, edema, amputations, unusually muscular people, and at extremes of body size. Their original populations do not represent every age, ancestry, or body composition. The binary sex coefficients are limitations of the historical models. The tool cannot determine whether a model is suitable for an individual or select an appropriate measurement method.
Reference ranges
| Measure | Context |
|---|---|
| Lean mass in kg or lb | Estimated mass, not directly measured tissue |
| Lean mass percentage | Estimated lean mass ÷ actual weight × 100 |
| Healthy target interval | No universal interval is supplied |
All output descriptions are reference only. The James equation includes a negative squared term, so its estimate eventually decreases as weight increases at a fixed height. This mathematical turning point occurs around BMI 43.0 for the male equation and 36.1 for the female equation. The calculator flags that limitation without assigning a diagnosis. Any non-positive prediction or prediction above actual weight is withheld as outside plausible physical bounds. Valid-looking outputs can still be inaccurate; a range check is not validation against a real measurement.
References
- Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. Am J Physiol. 1984;247:F632–F636.
- James WPT, compiler. Research on obesity: a report of the DHSS/MRC Group. London: HMSO; 1976.
- Hume R. Prediction of lean body mass from height and weight. J Clin Pathol. 1966;19:389–391.
Frequently asked questions
Does lean mass mean muscle mass?
No. Lean mass includes non-fat tissues and body water. These equations do not isolate skeletal muscle.
Why are there three different answers?
The models use different coefficients and were developed from different observations. Their disagreement illustrates estimation uncertainty.
Can I enter pounds and inches?
Yes. Inputs are converted to kg and cm before all three equations are evaluated, and outputs include both weight units.
Why can James become unreliable at high weight?
Its negative squared weight-to-height term eventually dominates. A warning appears beyond the mathematical turning point.
Is the lean percentage a health score?
No. It is the formula estimate divided by actual weight. This site does not provide a universal target or interpret it as a health score.
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