Lean Body Mass Calculator

It's your total body weight minus fat mass — so it includes skeletal muscle, but also bone, organs, skin, and body water. That's a much bigger category than muscle alone, which is why lean body mass is always noticeably higher than skeletal muscle mass on its own. This calculator estimates it from the Boer, James, or Hume regression equations, or directly from a measured body fat percentage if you enter one.

Boer is the default here and is generally the best-behaved of the three across a normal weight range, since it was derived to normalize body fluid volumes. James is the only one of the three with a squared weight-to-height term, which makes it diverge most sharply — and behave unreliably — in short heavy or tall light people. Hume is the oldest and still used in some pharmacokinetic dosing, but tends to return lower values than Boer. None of the three was validated as more accurate than the others for general use; if you're tracking change over time, the important thing is picking one and sticking with it.

Each was fitted to a different cohort, in a different decade, against a different reference method — Boer used body fluid volumes, Hume used total body water, James used a mixed dataset — and none of them measured the specific person entering numbers into this calculator. At typical adult heights and weights the three commonly disagree by up to around 9%. That spread is a real limitation of the method, not a calculation error, so an absolute lean-mass figure carries meaningful uncertainty even though comparing two readings from the same equation over time is more trustworthy.

If you have a measured body fat percentage — from a DXA scan or another reliable method — entering it bypasses the three regression equations entirely. Lean mass is then calculated directly as weight × (1 − body fat percentage), which is arithmetic rather than estimation, and is a better answer than any of the three formulas can give from height and weight alone.

FFMI (fat-free mass index) is lean body mass divided by height squared — the lean-tissue equivalent of BMI — with a small adjustment so people of different heights compare on one scale. The reference bands are sex-specific because they come from separate population percentiles for men and women: judged against the male scale, most women would incorrectly read as 'below average,' when a woman at FFMI 17 is actually athletic. This calculator applies the correct band for your selected sex rather than one shared scale.