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LBM

Lean Body Mass Calculator — Boer Formula

Estimate the portion of your body weight that isn't fat — muscle, bone, organs, and water — using the well-validated Boer formula.

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Disclaimer: This calculator provides a general estimate for fitness-tracking and educational purposes only and is not medical advice. Body composition formulas cannot match a direct scan (DEXA, BIA, or calipers) for individual accuracy. Consult a doctor, registered dietitian, or trainer before making decisions based on this result.

How Lean Body Mass Is Calculated

Lean body mass (LBM) is your total body weight minus your estimated fat mass — everything that isn't fat, including muscle, bone, organs, and body water. This calculator uses the Boer formula, a regression equation built from body composition studies that estimates LBM directly from weight, height, and sex without needing calipers or a scan.

Men: LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2

Women: LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3

Once LBM is known, fat mass follows automatically: Fat Mass = Total Weight − LBM, and fat mass as a percentage of total weight is (Fat Mass ÷ Weight) × 100.

Why Track Lean Body Mass

Scale weight alone can't tell you whether a change came from fat, muscle, or water — LBM adds context. During a fat-loss phase, the goal is usually to see total weight drop while lean mass stays roughly flat, which is a strong sign the loss is coming from fat rather than muscle. During a muscle-building phase, a rising LBM alongside stable body fat percentage suggests the extra weight is productive.

Limits of a Formula-Based Estimate

Boer and similar formulas are population averages — they work well on average across large groups but can be off for individuals with unusually high muscle mass, very low or high body fat, or atypical body proportions. Use the trend over weeks and months rather than fixating on any single reading.

Frequently Asked Questions

No — lean body mass includes muscle, but also bone, organs, water, and connective tissue. It's essentially your total body weight minus your fat mass. To isolate muscle mass specifically you'd need a body composition scan such as DEXA, which separates fat, bone, and skeletal muscle individually.
The Boer formula was built from regression analysis of body composition data and captures the statistical relationship between those three easily measured variables and total lean mass across a general population. It won't be as precise as a direct scan for any one individual, but it's a fast, equipment-free way to track your trend over time.
Many people use LBM to set a more personalized protein target, since muscle-building and muscle-preserving research is often expressed per kilogram of lean mass rather than total body weight. It's also a useful number to track over months during a diet — the goal during fat loss is usually to keep lean mass roughly stable while total weight and fat mass drop.
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