Lean body mass is everything in the body that is not fat — muscle, bone, organs, water and connective tissue. It is a more physiologically meaningful reference point than total body weight for many purposes, since fat and lean tissue behave very differently metabolically.
This is the Boer formula, one of several regression equations derived from population data linking body weight and height to measured lean mass. Others in common use include the Hume and James formulas, which agree closely for most people but diverge somewhat at the extremes of body size.
Why lean mass matters more than total weight in some contexts
- Metabolic rate. Lean tissue is far more metabolically active than fat tissue, so two people of identical weight but different body composition can have meaningfully different resting energy expenditure.
- Medication dosing. Some medications are dosed relative to lean body mass rather than total weight, since fat tissue does not distribute or metabolise certain drugs the same way lean tissue does — though this is a clinical decision made by a prescriber with direct measurement, not something to self-calculate.
- Tracking training progress. Someone gaining muscle while losing fat can see total body weight stay flat or even rise, while lean mass and fat mass move in opposite directions underneath that number.
The limits of a population-average formula
These formulas were derived from average population data and applied backward as a general estimate — they were not built by measuring any specific individual’s actual lean mass. They tend to be reasonably accurate for people close to average build, and systematically less accurate at either end of the spectrum: very lean, heavily muscled individuals often have their lean mass underestimated, since the formula has no way to know that a given height and weight combination is unusually muscular rather than unusually fat.
Direct measurement is more accurate
DEXA scanning, bioelectrical impedance analysis and hydrostatic weighing all measure body composition more directly than a height-and-weight formula can. This calculator is a convenient estimate for tracking rough trends without access to that equipment, not a substitute for it.