Lean Body Mass Calculator
Estimate muscle-and-more mass: everything in your body that isn't fat.
The Lean Body Mass Calculator estimates your fat-free mass — muscle, bone, organs, blood and water — using the Boer formula, a well-validated equation based on weight, height and sex. Subtract that from total weight and you have an estimated fat mass.
Lean mass matters for several reasons: it is metabolically active tissue, protein requirements and some medication doses are calculated per kilogram of lean mass rather than total weight, and tracking it over time reveals whether weight changes are fat loss or muscle loss — the difference between a good diet and a crash diet.
The estimate assumes average hydration and bone density; the formula is population-derived, so treat it as a useful approximation and track trends rather than single readings. Educational tool only — runs entirely in your browser.
Lean body mass
58.9 kg
Boer formula
Estimated fat mass
16.1 kg
≈ 21.5% body fat
In pounds
129.7 lb
lean mass
LBM = everything that is not fat: muscle, bone, water, organs. Protein needs and some drug dosages are often calculated per kg of LBM — useful when body weight overestimates due to fat.
How to use the Lean Body Mass Calculator
- Choose your sex.
- Enter your weight in kilograms and height in centimeters.
- Read your estimated lean body mass.
- Note the derived fat mass and body-fat percentage.
- Re-measure monthly to track whether changes are muscle or fat.
Frequently asked questions
What is the Boer formula?
For men: LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2. For women: LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3. It was developed from a large adult sample and is widely used in research settings.
Why calculate protein from lean mass?
Fat tissue has minimal protein needs, so dosing by total weight overestimates for heavier individuals. Per-kg-of-LBM guidance (roughly 2–3 g per kg of lean mass for athletes) is more accurate.
Is lean mass the same as muscle mass?
No — lean mass includes organs, bone, blood and water as well as muscle. Muscle is the component most people can meaningfully change through training; the formula cannot separate muscle from the rest.
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