What lean body mass includes
Lean body mass is everything in your body except stored fat. That covers skeletal muscle, but also organs, bone, skin, connective tissue and the water held in all of them. The terms lean body mass and fat-free mass are often used as if they were the same, and in everyday use they are close enough. Neither one means muscle alone. If you weigh 75 kg and a formula says 58.1 kg of that is lean, the rest, 17.0 kg, is the implied fat. Searches for a muscle mass calculator usually want a different number, and this page does not pretend to give it. Resting energy formulas such as the ones on the BMR calculator use lean mass as an input in some cases, which is one reason people look for it.
Compare Boer, James and Hume
The calculator works from height and weight with three published regression equations, each with separate coefficients for males and females. Weight W is in kilograms and height H in centimeters. None of them is the right answer by default, so all three are shown together and you choose which one drives the headline figure.
Boer
For males the equation is 0.407 W + 0.267 H − 19.2, and for females 0.252 W + 0.473 H − 48.3. It was published in 1984 from a study of normal-weight and obese adults, with a lean mass reference measured by potassium counting (Boer, 1984). The full coefficient table of the original was not retrieved for this page, so the coefficients above follow the widely published form.
James
The James equation is 1.1 W − 128 (W ÷ H)² for males and 1.07 W − 148 (W ÷ H)² for females. It comes from a 1976 monograph on energy and body weight, which was cited here but not read directly. Because the squared term grows with weight, the result can fall as a very heavy person gets heavier: at 140 kg and 160 cm the male version gives 56.0 kg, which is not a believable lean mass.
Hume
For males Hume gives 0.3281 W + 0.33929 H − 29.5336, and for females 0.29569 W + 0.41813 H − 43.2933. It was derived in 1966 from body water and potassium measurements in a small group of adults (Hume, 1966), so it reflects an older and narrower sample than the other two.
Use an entered body fat percentage
If you already have a body-fat percentage from a scale, calipers, a tape measurement or a scan, switch to the second method. The arithmetic is simple: lean mass = weight × (1 − body fat ÷ 100), and fat mass is whatever remains. This is not a new measurement. It simply turns one estimate into another, so it inherits every error in the percentage you typed. You can produce a percentage on this site with the body fat percentage calculator, and then enter it here yourself; the two pages never pass numbers to each other.
Worked example: 75 kg and 175 cm
For a male of 75 kg and 175 cm, Boer gives 0.407 × 75 + 0.267 × 175 − 19.2 = 58.05 kg. James gives 1.1 × 75 − 128 × (75 ÷ 175)² = 58.99 kg, and Hume gives 54.45 kg. For a female with the same height and weight the three results are 53.37, 53.07 and 52.06 kg. The three male results span 4.54 kg, which is a useful sense of the uncertainty. With an entered body fat of 20%, the same 75 kg person has 60 kg of lean mass and 15 kg of fat, a figure that sits between the regressions only by coincidence.
Why estimates differ from muscle mass
A regression equation looks at height and weight and predicts the lean mass of an average person of that size. It cannot see whether two people of the same size carry different amounts of muscle, bone or water. A trained lifter and a sedentary person of identical height and weight get identical results. The equations were also fitted in particular groups, in particular decades, with particular measurement methods, and they extrapolate poorly beyond them. Scans that measure tissue directly will disagree with any formula by several kilograms. For protein planning, the protein reference calculator works from body weight rather than lean mass, and there is no need to treat a lean mass estimate as a target.
Related height and weight tools
If you only need a height-and-weight screening number, the BMI calculator uses the CDC adult categories. The ideal body weight calculator compares three historical equations with the BMI-based range, and neither is a substitute for a body-composition estimate.
Units, limits and sources
Inputs may be metric or US. The calculator converts exactly, using 1 inch = 2.54 cm and 1 pound = 0.45359237 kg, and the formulas always run in kilograms and centimeters. Results are shown to one decimal place, though nothing is rounded before the arithmetic is done. The tool is for adults aged 19 to 80. If a formula returns zero, a negative value or more than your whole body weight, that row reads "Outside this formula's useful domain" and is never clamped to something that looks plausible. This is a statement about the equation, not about your health. Sources were last checked on 8 November 2026. This is an educational estimate. It is not a medical measurement, and it must not be used to calculate drug doses.
Lean body mass questions
Is lean body mass the same as muscle?
No. It includes muscle, but also organs, bone, water and other non-fat tissue, so it is always larger than muscle mass alone.
Can I calculate it from body fat?
Yes. Multiply your weight by one minus the body-fat fraction. The answer depends entirely on the percentage you enter, so it is a restatement of that estimate rather than a measurement.
Which formula should I trust?
They are population models with different origins, so compare them instead of picking the one that gives the answer you hoped for. Choose one and use it consistently if you track changes.
Why is one result unavailable?
That equation returned a value that cannot be a real lean mass for your inputs, usually at extreme weights or heights. It reflects the limits of the formula, not a problem with you.
Can this set medication doses?
No. The calculator has no dosing function, and a regression estimate is not suitable for deciding how much of a drug to take. Ask the prescriber or a pharmacist.