Lean body mass is everything in you that is not fat, the muscle, bone, organs, and water that actually burn calories and process drugs. The Boer formula estimates it from height, weight, and sex.
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The metabolically active you
Strip away stored fat and what remains is lean body mass, the tissues doing the daily work of being alive. Muscle makes up the largest share, joined by bone, organs, blood, and skin. This compartment burns the overwhelming majority of your resting calories, clears medications from your bloodstream, and determines how strong you are for your size. Tracking it over time reveals whether weight change is building you up or hollowing you out.
The Boer approach
James Boer published his equation in 1984, fitting lean mass against height and weight in measured subjects. For men the formula takes about 41 percent of weight and adds a height term, while the female version leans more heavily on height. The result lands close to what underwater weighing or DEXA scans report for average builds, which made it a favorite in anesthesiology where hydrophilic drug doses scale to lean mass rather than total weight.
Where estimates strain
Any formula built from height and weight stumbles at the extremes. Very muscular athletes read as heavier than their leanness suggests, while people with obesity carry lean mass the equations undercount. For those cases the adjusted body weight approach or an actual body composition measurement serves better. For everyone in the broad middle, the estimate is close enough to guide training targets and protein planning.
Putting the number to work
Protein targets often key off lean mass, commonly 1.6 to 2.2 grams per kilogram for muscle building phases. Resting metabolism estimates sharpen when lean mass replaces total weight in the math. And during weight loss, watching lean mass hold steady while the scale drops is the clearest sign the loss is coming from the right place.