Lean Body Mass Calculator

Boer, James, and Hume each predict lean mass from height, weight, and sex. We average the three; fat mass = weight − lean mass, and body-fat % follows from that.

Boer P (1984); James WPT (1976); Hume R (1966).

Height-and-weight equations assume an average build and are least accurate for very lean, very muscular, or very heavy people; a direct body-fat measurement beats them.

Clinical dosing decisions — use a measured lean-mass method (e.g. DEXA) where precision matters medically.

Lean body mass is everything in your body that isn't fat — muscle, bone, organs, and water. Subtract it from your total weight and what's left is fat mass, which gives you an estimated body-fat percentage.

We use three published equations — Boer, James, and Hume — that predict lean mass from your height, weight, and sex, then average them. Averaging smooths out the quirks of any single formula.

Each was fitted to a different population, so they weight height and weight slightly differently. The gaps are usually small; the average is a sensible single figure to work from.

They assume an average build, so they're least accurate for very muscular, very lean, or very heavy people. A direct measurement — DEXA, a good body-fat estimate, or Katch-McArdle from measured body fat — will be more precise.

It feeds several useful calculations: the Katch-McArdle equation uses it for a body-fat-aware metabolic rate, and protein targets are often set per kilogram of lean mass rather than total weight.