Free tool
Lean body mass calculator
Know your body fat %? Get an instant number. Don't know it? Estimate it anyway from height, weight and sex.
Need a body-fat estimate? See paid report options · Compare methods
Lean body mass
—
More free tools
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 and weight using sex-specific equations. It was developed as an estimate for normalizing body-fluid volumes, not to measure an individual's physique. Boer (1984).
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 result is a direct measurement. Different methods estimate body compartments differently; a scan also has method-specific limits. Keep the method and inputs with any result you compare over time.
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. The Boer formula estimates lean body mass from height and weight using sex-specific equations. It is a population-derived estimate and cannot distinguish people with the same height and weight but different body composition.
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 is a prediction equation, not a direct measurement. Its result may differ from an individual's body composition; use it as an estimate and retain the method when comparing values.
Need more context on body fat?
Compare measurement methods and understand what an image-based estimate can tell you.
Read the measurement guide