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 by subtracting the average LBM from your total weight and dividing by total weight, so LBM and body fat always add up to 100%.

No single formula wins for every body type, but Boer (1984) is widely cited as the most reliable for healthy adults and is often preferred for clinical drug dosing. Hume (1966) tends to read a bit lower. James (1976) is unreliable in people with obesity, so treat it with caution there. The calculator also shows the average of all three, which smooths out individual formula bias.

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.