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Lean body mass calculator
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Lean body mass
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What lean body mass actually is
Lean body mass (LBM) is everything you weigh that isn't fat — muscle, bone, organs, water, connective tissue. It's the raw number underneath FFMI, before height gets factored in.
- With a body fat % you know — LBM = weight × (1 − body fat %). Simple, direct.
- Without one — the Boer formula estimates LBM from height, weight, and sex alone, using population-level regression data. Less precise than a real body fat number, but useful as a starting point.
LBM vs FFMI
Lean body mass is a raw weight — 68kg of lean mass means something different on a 160cm frame than a 190cm one. FFMI takes your LBM and adjusts it for height, so it's the number to use when comparing muscularity across different heights rather than just tracking your own raw mass over time.
Neither of these is a medical measurement. The Boer formula especially is a population-level estimate — actual scanning methods (DEXA, for example) will always be more accurate for any one individual.
Questions
What is lean body mass?
Lean body mass (LBM) is everything your body weighs that isn't fat — muscle, bone, organs, water, and connective tissue. It's the same core idea FFMI is built on.
Can I calculate lean body mass without knowing my body fat %?
Yes, using formulas like the Boer formula, which estimates LBM from height, weight, and sex alone. It's less precise than using a real body fat percentage, but useful when you don't have one.
What's the difference between lean body mass and FFMI?
Lean body mass is a raw weight figure (e.g. 68kg of lean mass). FFMI takes that number and adjusts it for height, so it can be compared meaningfully across people of different heights.
How accurate is the Boer formula?
It's a population-level estimate, not a direct measurement — actual body composition testing (DEXA, for example) will always be more accurate for an individual. Treat it as a reasonable starting estimate, not a precise figure.
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