What lean body mass is
Lean body mass (LBM), also called fat-free mass, is the total weight of everything in your body that isn't fat — muscle, bone, organs, connective tissue and the water they hold. For most people it makes up 60–90% of body weight.
How it's calculated
True LBM requires a body-composition scan, but it can be estimated accurately from height, weight and sex. This calculator shows two validated equations — the Boer formula (widely used) and the Hume formula — and derives your implied fat mass from the difference.
Why it matters
LBM drives your metabolism: muscle and organs burn far more calories at rest than fat, so two people of the same weight can have very different calorie needs. It's also used clinically to dose certain medications and to set protein targets. When you diet, the goal is to lose fat while protecting LBM.
Building and protecting lean mass
Resistance training plus enough protein is the proven recipe. Use our protein calculator to set a target, and the body fat calculator to track composition alongside this estimate.
Helpful tools
Track lean mass and body fat trends at home.
View options →Support muscle with a convenient protein top-up.
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Frequently asked questions
What is a good lean body mass?
There's no single 'good' number — it depends on height, frame and sex. What matters more is keeping it stable or rising while you lose fat, which signals you're preserving muscle.
How is lean body mass different from body fat?
Lean body mass is everything that isn't fat; body fat is the fat portion. Together they make up your total weight, so knowing one gives you the other.
Which formula is more accurate, Boer or Hume?
Both are well validated for adults and usually agree closely. Boer is the more commonly cited; for a precise figure, a DEXA or bioimpedance scan is best.
Can I increase my lean body mass?
Yes — progressive resistance training combined with adequate protein and a small calorie surplus builds muscle, the main component of lean mass you can change.