Lean Body Mass Calculator

Lean body mass is everything in the body that is not fat: muscle, bone, organs, connective tissue and the water in all of it. This page estimates it with the Boer formula, published in 1984. For men: LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2. For women: LBM = 0.252 × weight + 0.473 × height − 48.3. Fat mass is then your weight minus that figure, and the lean share is fat-free mass divided by total weight. Only the first line is an estimate — everything after it is arithmetic performed on that estimate, so an error in the first line propagates into both of the others. The formula was developed in a clinical context, as a way of normalizing body fluid volumes between people of different sizes, rather than as a fitness metric. That is worth remembering when reading it as one.

Look at the inputs: weight, height, sex. Nothing else. That means two people with the same height, weight and sex always receive the same lean body mass from this formula, regardless of how much muscle either of them actually carries. A trained athlete and a sedentary person who happen to share those three numbers get identical results, and at least one of those results is wrong. Like every prediction equation in this family, Boer is a regression fitted to a population. It returns the lean mass typical of someone with your dimensions, not yours. The gap between typical and actual is exactly what anyone tracking body composition wants to know, and this formula is structurally unable to report it. The estimate is least reliable at the extremes: very muscular people, very high body fat, and anyone with significant fluid retention or loss.

The two are routinely confused, and the difference matters. Lean mass includes bone, organs, connective tissue and body water alongside skeletal muscle. Muscle is only a portion of it, so reading this figure as "how much muscle I have" overstates the case considerably — and the overstatement is large, not marginal. Water is the part that moves fastest. Hydration status, sodium intake, carbohydrate stores and, for some people, the menstrual cycle all shift total body water by amounts large enough to register on a scale, and in any method that measures rather than predicts, all of that lands inside "lean mass". In this formula, which knows only your dimensions, none of it appears at all — which is a different problem, not a smaller one.

Fat-free mass is still worth thinking about, because it is what most of the resting metabolism runs on. Organs and muscle account for the great majority of the energy burned at rest, while fat tissue is comparatively inert. That is a large part of why two people at the same body weight can have measurably different resting energy expenditure. It is also the quantity that a number of clinical calculations are normalized to instead of total body weight, which is why formulas like this were written in the first place. Those are clinical applications performed by clinicians; nothing on this page should be used to carry out that kind of calculation yourself. For most people the practical point is directional rather than numerical: during weight loss, preserving fat-free mass is what you want, and a bathroom scale alone cannot tell you whether you are managing it.

To know your lean mass rather than predict it, you need a method that measures body composition: DEXA, air-displacement plethysmography or hydrostatic weighing, or — with more error and far more convenience — bioelectrical impedance and skinfold calipers. Impedance in particular is sensitive to hydration, recent food and recent exercise, so it needs consistent conditions to mean anything. Whichever you choose, measure the same way each time and read the trend rather than the single value. The Boer formula was derived in adults and does not apply to children, teenagers or pregnancy. And this page estimates a quantity and explains how much to trust it — it does not tell you what your lean mass should be, what to eat, or how to train. Those decisions belong with a physician or a registered dietitian.