Lean Body Mass Calculator

Lean body mass (LBM) is your total weight minus fat mass, so it covers muscle, bone, organs, skin, and water. Body fat is the rest. This calculator estimates body fat percentage from the average of the available valid formulas, so LBM and body fat add up to 100%.

No single formula wins for every body type, but Boer (1984) is widely cited for healthy adults and Hume (1966) tends to read a bit lower. James (1976) can become invalid at higher body weights. The calculator averages only the available valid formulas instead of forcing an invalid estimate into the result.

Boer, James, and Hume each used different study populations, reference measurement methods, and statistical weights for body weight and height. For an average-build adult the three estimates often sit within a few pounds of each other, but they can diverge by 10 lb or more at higher body weights, which is why the average gives a more stable picture.

Clinicians use LBM to dose medications where total body weight would overdose patients with obesity, with anesthesia and certain chemotherapy agents being common examples. Athletes use it to track whether a training block is adding muscle or just total weight. It also gives a more accurate baseline than total body weight for estimating resting metabolic rate.

It is an estimate derived from population-level equations, not a direct tissue measurement, so it can be off by several percentage points if your proportions differ from the study populations. For precise body composition, DEXA scanning or hydrostatic weighing are the gold standards. This calculator is most useful for tracking directional change over time, not as a single absolute reading.