Lean Body Mass Calculator
Estimated lean body mass and fat mass (Boer formula)
Frequently Asked Questions
Lean body mass — total body weight minus fat mass — is estimated here using the Boer formula, one of several established equations for this calculation, and understanding it alongside body fat percentage gives a more complete picture of body composition than either weight or BMI alone.
Why lean mass and fat mass together tell a fuller story than total weight
Total body weight is a single number that doesn't distinguish between muscle, bone, organs, water, and fat — two people of identical total weight and height can have very different proportions of lean mass versus fat mass, which is exactly the distinction BMI can't capture, since it's calculated purely from weight and height. The Boer formula estimates lean body mass from height and weight using sex-specific coefficients (developed and validated against direct body-composition measurement methods), and subtracting the estimated lean mass from total weight gives estimated fat mass as the remainder — two complementary figures derived from the same underlying calculation rather than two independent measurements.
A worked example
Two people with identical total body weight can have meaningfully different estimated lean body mass and fat mass breakdowns if their height differs, since the Boer formula's height component affects the estimate independently of total weight — this is exactly the kind of nuance that a single "weight" or "BMI" number can't reveal on its own.
How this connects to your other health calculations
This calculator's fat-mass estimate is a formula-based approximation, distinct from the circumference-based estimate in the Body Fat Calculator — the two methods can produce somewhat different results for the same person, since they're derived through different estimation approaches, and neither should be treated as more definitively "correct" than a direct body-composition measurement method like DEXA.
Common mistakes
Treating lean body mass and muscle mass as interchangeable terms is a common imprecision — lean body mass includes bone, organs, and body water in addition to muscle tissue, so it's a broader category than muscle mass alone, and a lean-mass estimate shouldn't be read as a direct muscle-mass figure.