Lean Body Mass Calculator

What lean body mass includes

Lean body mass is total body mass minus fat mass: skeletal muscle, bone, organs, connective tissue and the water in all of them. It is not muscle mass — muscle is roughly half of it in a typical adult. People chasing “muscle gained” often quote LBM changes, which overstates the result because water shifts land in the same number.

Why three equations, and why they disagree

Boer (1984) was fitted on bioelectrical impedance data, James (1976) on body density, Hume (1966) on total body water. Each was derived from a different population with a different reference method, so each drifts differently as you move away from average builds. Disagreement of two or three kilograms between them is normal; a much larger spread usually means your height and weight combination sits outside the range they were fitted on.

Katch-McArdle needs lean mass, which is the point

Most resting-metabolism formulas use total body weight, so two people of the same weight get the same answer regardless of composition. Katch-McArdle uses lean mass instead (370 + 21.6 × LBM), which is why it tends to be more accurate for very lean or very heavy people — and why it needs a lean-mass estimate first.

When to stop using an equation

If the number matters — clinical dosing, competitive sport, a medical condition — use a measurement rather than an estimate. DXA, air displacement plethysmography and hydrostatic weighing all measure composition directly. An equation built from height and weight cannot know whether the extra kilogram is muscle or fat, which is precisely the question being asked.

Frequently Asked Questions