What lean body mass actually measures
Lean body mass is everything you weigh that is not fat: skeleton, muscle, organs, blood and the water sitting inside all of it. It is not the same thing as muscle mass, which is only one part of the lean total, and it is not a fitness score either. Two people at an identical lean body mass can have very different amounts of muscle between them, one carrying more of that lean weight as organs and skeleton, the other as trained muscle tissue.
The number matters because it changes what a bathroom scale cannot tell you on its own. A rising weight could mean fat, muscle or both, and lean body mass is the figure that lets you separate the two, either by estimating it from a formula or by working it out directly once you know your body fat percentage. It is also the figure clinicians reach for when a drug dose needs to track metabolically active tissue rather than total body weight, since fat barely changes how many drugs are processed.
The three formulas, printed in full
Boer, James and Hume each built their equation from a different study, at a different time, using different reference measurements, so treat the three as independent opinions rather than three attempts at the same sum.
Boer published its formula in 1984 by refitting body-composition data with a simple linear equation, and it remains the most widely validated of the three in clinical use today. James dates to 1976 and takes a different shape entirely, squaring the ratio of weight to height rather than adding a straight height term, which makes it behave differently at unusual body sizes. Hume is the oldest, from 1966, built the same way as Boer but with its own constants from an earlier dataset.
Why the three disagree, and when to be cautious
Run the same height, weight and sex through all three and expect a spread of one to two kilograms between the lowest and highest, sometimes more. That gap is not an error in the maths; it reflects three genuinely different studies rather than one formula done three ways. Reading all three side by side, as this page does, tells you more than picking one and trusting it blindly.
The James formula is worth a specific caution. Because it squares the weight-to-height ratio rather than adding a height term, it can behave oddly once body mass index climbs well past 35: the estimate can plateau or even fall as weight keeps rising, which is the opposite of what should happen. Boer stays the steadiest of the three across a wider range of body sizes, so at a high BMI it is the one to lean on if the three formulas start pulling apart sharply.
All three were built from studies of adults with an ordinary build. None of them was fitted to children, and a separate equation from Peters exists for exactly that case; it is not built into this page, since a children's formula needs different safeguards around growth stage that a general calculator cannot apply safely. If you are calculating for anyone under eighteen, a paediatric reference or a doctor is the right next stop rather than any of the three formulas above.
The direct method: working from a known body fat percentage
If you already have a body fat percentage from callipers, a scan or the Body Fat Calculator, this page switches its headline result to a direct calculation instead of the three formulas: lean body mass equals weight multiplied by one minus body fat percentage divided by a hundred. An 80 kilogram person measured at 20% body fat has a lean body mass of 80 × (1 − 0.20) = 64 kilograms, worked out from an actual measurement rather than a population average.
That is why the direct method takes priority the moment you supply it. Boer, James and Hume all guess your body composition from height, weight and sex alone, three variables that say nothing about how much of any individual actually carries as fat versus everything else. A real body fat reading replaces that guess with your own number, so the calculator prefers it and shows the three formulas underneath purely for comparison once it is switched on.
Reading fat mass and lean mass as a percentage
Alongside the headline figure, this page works out two more numbers from the same weight: fat mass, simply your weight minus lean body mass, and lean body mass expressed as a percentage of your total weight. The percentage tends to be the steadier number to track over weeks, since it moves less with normal day-to-day water shifts than a fat mass figure quoted in kilograms does on its own.
A typical adult carries somewhere around two-thirds to four-fifths of total body weight as lean mass, with the exact figure shifting by sex, age and training history rather than sitting at one universal target. Watching that percentage rise slowly over a training block, alongside a roughly stable or falling fat mass, is a better sign of real progress than the bathroom scale total on its own, since the scale cannot tell you which of the two actually moved.
What this page is not for
None of the figures here are a target to chase for its own sake, and none of them is a medical measurement. They are estimates or, once you supply a body fat percentage, a single calculation from a measurement taken elsewhere; either way, they describe composition, not health or fitness on their own. Someone with a high lean body mass and a poor diet is not automatically healthier than someone with a lower one and good habits, and chasing the number itself misses what it was built to describe.
Pair this page with the BMI Calculator if you want a second, faster reference point, or the Calorie Deficit Calculator if the goal is to change your composition over time rather than just measure where it stands today.
Questions people ask
What is the lean body mass for a 180cm, 80kg man?
Boer gives 61.4 kg, James gives 62.7 kg and Hume gives 57.8 kg, an average of 60.6 kg, about 76% of total body weight. The three-kilogram spread between the lowest and highest formula is typical; none of the three is more correct than the others by default.
What is the lean body mass for a 165cm, 65kg woman?
Boer gives 46.1 kg, James gives 46.6 kg and Hume gives 44.9 kg, an average of 45.9 kg, around 71% of total body weight. Again, treat the three figures as a small range rather than hunting for one exact answer.
Is lean body mass the same as muscle mass?
No. Lean body mass includes bone, organs, blood and body water alongside muscle, so it is always a larger figure than muscle mass on its own. A rise in lean body mass over a training block is a reasonable sign that at least some of it is muscle, but the number cannot separate the two by itself.
Why does entering my body fat percentage change the result so much?
Because it replaces three formulas guessing your composition from height, weight and sex with an actual measurement of your body. Boer, James and Hume have no way of knowing whether your weight leans towards more muscle or more fat than the average person of your height; a real body fat reading does, so the direct method usually lands closer to the truth for anyone whose build sits away from average.
Can I use this for a child or teenager?
No. All three formulas here were built from adult data, and applying them to a growing body gives an unreliable answer. A separate equation from Peters exists for children, built with growth-stage safeguards this page does not include; ask a paediatrician or GP for a proper reading instead.
Why do the three formulas disagree so much for some people?
Each was fitted to a different study, at a different time, so a one to two kilogram spread between the lowest and highest is normal rather than a sign anything is wrong. The gap tends to widen further at unusually high or low body mass index, where the James formula in particular can behave oddly; Boer stays the more dependable of the three at those extremes.
Does a higher lean body mass mean I burn more calories?
Broadly yes: lean tissue is more metabolically active than fat, so someone with a higher lean body mass at the same total weight typically has a higher resting metabolic rate. The BMR Calculator works that resting rate out directly rather than inferring it from this page alone.
These figures are estimates from published equations, or a direct calculation from a body fat percentage you supply; none of them is a medical or diagnostic measurement. They were built for healthy adults and are not suitable for children, and anyone with an unusually muscular, unusually lean, or medically unusual build should treat the result as a rough guide rather than an exact one.