What is lean body mass?

Everything in the body that is not fat: muscle, bone, organs, water and connective tissue. It is not the same as muscle mass, which is a subset of it, and the difference is large — muscle is roughly half of lean mass in a typical adult.

Why do the three equations disagree?

They were fitted to different populations at different times, using different reference methods. None is more "correct" than the others; they are three attempts at the same estimate. The spread between them is a reasonable indication of the uncertainty involved.

Can these tell me how much muscle I have gained?

No. All three predict lean mass from height, weight and sex only, so nothing you do in training changes the answer except by changing your body weight. If you gain 2 kg, the equation credits a fixed share of it to lean mass regardless of what that 2 kg actually was.

What should I use instead if I know my body fat percentage?

Fat-free mass computed directly from that measurement: body weight × (1 − body fat percentage / 100). That responds to real changes in composition, which these equations cannot.

Why does the James equation behave strangely at high weights?

Its correction term subtracts a multiple of the squared weight-to-height ratio, and past about 150 kg that term grows faster than the linear weight term. The result is a curve that peaks and then falls. It is a known limitation of fitting a quadratic outside the range it was derived on.

Lean body mass calculator

Three published equations that predict lean mass from height, weight and sex. All three are shown together, because they disagree and the disagreement is informative.

Result

These are estimates from population-level equations, not a measurement of you and not medical advice. Individual results vary widely. Talk to a doctor or a registered dietitian before making significant changes to how you eat or train.

How this is calculated

Boerdefault

              men:   LBM = 0.407 × W + 0.267 × H − 19.2
women: LBM = 0.252 × W + 0.473 × H − 48.3
(W in kg, H in cm)

Source: Boer P., "Estimated lean body mass as an index for normalization of body fluid volumes in humans", American Journal of Physiology 247(4 Pt 2), 1984, pp. F632–F636.
Assumes: That lean mass can be predicted from stature and total mass alone, with no information about training status.
Accuracy: The most consistent of the three across the range of body sizes this site sees, and the reason it is the default. It was derived for normalising body fluid volumes rather than for fitness purposes.

James

              men:   LBM = 1.10 × W − 128 × (W / H)²
women: LBM = 1.07 × W − 148 × (W / H)²

Source: James WPT, "Research on Obesity", HMSO, London, 1976.
Assumes: The same, with a quadratic correction term for the weight-to-height ratio.
Accuracy: Reasonable in the middle of the range, but the squared term makes it turn over at high body masses: past roughly 150 kg it predicts less lean mass for more total mass, which is not physiologically meaningful. Prefer Boer outside the normal range.

Hume

              men:   LBM = 0.32810 × W + 0.33929 × H − 29.5336
women: LBM = 0.29569 × W + 0.41813 × H − 43.2933

Source: Hume R., "Prediction of lean body mass from height and weight", Journal of Clinical Pathology 19(4), 1966, pp. 389–391.
Assumes: The same as Boer, with different coefficients from an older cohort.
Accuracy: The oldest of the three and generally the lowest reading. Still widely used in clinical dosing contexts, which is where it came from.

Which formula should you use

This page defaults to Boer. The most consistent of the three across the range of body sizes this site sees, and the reason it is the default. It was derived for normalising body fluid volumes rather than for fitness purposes.

James — Reasonable in the middle of the range, but the squared term makes it turn over at high body masses: past roughly 150 kg it predicts less lean mass for more total mass, which is not physiologically meaningful. Prefer Boer outside the normal range.
Hume — The oldest of the three and generally the lowest reading. Still widely used in clinical dosing contexts, which is where it came from.

Lean body mass by weight and height (Boer)

The Boer equation predicts lean mass from height, weight and sex, so it cannot respond to training status: two people of the same size get the same figure. Rows are men; women read lower at the same dimensions.

Lean body mass by weight and height (Boer)
Weight	165 cm	175 cm	185 cm
55 kg	47.2 kg	49.9 kg	52.6 kg
65 kg	51.3 kg	54 kg	56.6 kg
75 kg	55.4 kg	58.1 kg	60.7 kg
85 kg	59.5 kg	62.1 kg	64.8 kg
95 kg	63.5 kg	66.2 kg	68.9 kg

Questions

What is lean body mass?: Everything in the body that is not fat: muscle, bone, organs, water and connective tissue. It is not the same as muscle mass, which is a subset of it, and the difference is large — muscle is roughly half of lean mass in a typical adult.
Why do the three equations disagree?: They were fitted to different populations at different times, using different reference methods. None is more "correct" than the others; they are three attempts at the same estimate. The spread between them is a reasonable indication of the uncertainty involved.
Can these tell me how much muscle I have gained?: No. All three predict lean mass from height, weight and sex only, so nothing you do in training changes the answer except by changing your body weight. If you gain 2 kg, the equation credits a fixed share of it to lean mass regardless of what that 2 kg actually was.
What should I use instead if I know my body fat percentage?: Fat-free mass computed directly from that measurement: body weight × (1 − body fat percentage / 100). That responds to real changes in composition, which these equations cannot.
Why does the James equation behave strangely at high weights?: Its correction term subtracts a multiple of the squared weight-to-height ratio, and past about 150 kg that term grows faster than the linear weight term. The result is a curve that peaks and then falls. It is a known limitation of fitting a quadratic outside the range it was derived on.

Calculators

Guides

Last reviewed August 20, 2026. Category: Body composition.