Lean Body Mass Calculator
Lean body mass is everything you are made of except stored fat. Three published formulas estimate it from height and weight, and they routinely disagree by two to four kilograms for the same person. This shows all three at once, together with the gap between them — because that gap, not any one of the numbers, is the precision you actually have.
Height and weight
All three formulas were fitted separately to male and female samples, and each has its own pair of constants.
Centimetres, without shoes. Boer and Hume lean on height heavily, so a 2 cm error is worth about half a kilogram.
Kilograms. Weigh yourself at the same time of day for a figure you can compare week to week.
If you have had it measured — a DEXA scan, hydrostatic weighing, a caliper protocol you trust — that figure beats all three estimates, because it is a measurement rather than a regression.
Boer, Hume and James disagree by 4.0 kg on the same body. That gap is the precision you actually have; a single number here would be a decimal place nobody has earned.
- Spread between the formulas
- 4.0 kg
- Mean of the three
- 55.9 kg
- Boer (1984)
- 56.8 kg lean, 21.1% fat
- Hume (1966)
- 53.5 kg lean, 25.7% fat
- James (1976)
- 57.5 kg lean, 20.1% fat
| Formula | Lean | Fat | Implied body fat |
|---|---|---|---|
| Boer (1984) | 56.8 kg | 15.2 kg | 21.1% |
| Hume (1966) | 53.5 kg | 18.5 kg | 25.7% |
| James (1976) | 57.5 kg | 14.5 kg | 20.1% |
- None of the three formulas can see your body. They were fitted to samples of a few hundred people each, with different reference methods, and they are reading your height and weight and nothing else.
- The disagreement is the useful output. If you need one number — for a weight-based drug dose, or a protein target — take the mean and carry the spread with it, rather than quoting a formula’s answer to a tenth of a kilogram.
- A measured body fat percentage beats all three. It is the only figure here that starts from something someone actually observed about you.
This is general information, not medical advice
This page does arithmetic and explains where the arithmetic came from. It is general educational information about three published estimating equations. It is not medical advice, and it must not be used to work out a drug dose.
Each formula was fitted to a particular group of people. Hume (1966) derived his constants from a clinical sample referenced against total body water. James (1976)published his as part of the DHSS/MRC obesity group’s report in the United Kingdom. Boer (1984) fitted his against body fluid volumes in adults, for the purpose of normalising those volumes. All three samples were adults of ordinary build, numbering in the hundreds, and none of them included you. If you are much heavier or lighter than average, very muscular, an amputee, pregnant, or living with a condition that shifts body water, all three are extrapolating.
Weight-based dosing is a clinical decision made by a prescriber with access to your records, not a calculation to be run on a phone. If a figure from this page and a figure from a clinician disagree, the clinician’s is the one that counts.
The three formulas, and the fourth
All three take weight in kilograms and height in centimetres and return kilograms of lean mass. Imperial input is converted at the boundary and the answer converted back, so the two unit systems always describe the same body.
Boer — men: 0.407·W + 0.267·H − 19.2 • women: 0.252·W + 0.473·H − 48.3
James — men: 1.1·W − 128·(W ÷ H)² • women: 1.07·W − 148·(W ÷ H)²
Hume — men: 0.32810·W + 0.33929·H − 29.5336 • women: 0.29569·W + 0.41813·H − 43.2933
Direct — lean mass = W × (1 − body fat ÷ 100)- W
- Body weight in kilograms
- H
- Height in centimetres
- Direct
- The only line here that is arithmetic on a measurement rather than a regression on someone else’s sample
Notice the shape of the middle one. Boer and Hume are linear in both weight and height; James subtracts a term that grows with the square of weight divided by height. That is what makes it behave differently as bodies get larger, and it is why James collapses at very high BMI — at 175 cm and 220 kg it assigns 39.7 kg of lean mass, implying 82% body fat. This page refuses to print a figure like that rather than clamping it, because a clamped number looks like an estimate and an extrapolation is not one.
The spread and the mean are computed from the rounded estimates, not the raw ones, so that a reader who subtracts the smallest figure on screen from the largest gets exactly the number labelled spread.
A man of 175 cm and 72 kg, three ways
Boer: 0.407 × 72 = 29.30, plus 0.267 × 175 = 46.73, minus 19.2 gives 56.8 kg. That leaves 15.2 kg of fat, or 21.1%.
James: 1.1 × 72 = 79.2. The correction term is 72 ÷ 175 = 0.4114, squared is 0.1693, times 128 is 21.67. So 79.2 − 21.67 = 57.5 kg, leaving 14.5 kg of fat, or 20.1%.
Hume: 0.32810 × 72 = 23.62, plus 0.33929 × 175 = 59.38, minus 29.5336 gives 53.5 kg. That leaves 18.5 kg of fat, or 25.7%.
Three formulas, one body, answers spanning 53.5 to 57.5 kg. The spread is 4.0 kg, and the implied body fat runs from 20.1% to 25.7% — a range wide enough to move this person across two categories on any body composition chart. The mean is 55.9 kg, but quoting 55.9 kg without the 4.0 kg beside it would be the exact mistake this page exists to avoid.
If the same man has had a DEXA scan reading 18% body fat, the answer is simply 72 × 0.82 = 59.0 kg — above all three estimates, and the only figure on the page that started from an observation of his actual body. The formulas then serve as a sanity check on the scan rather than the other way round.
The disagreement is the information
Most lean body mass calculators ask you to pick a formula from a dropdown, then show you its answer to one decimal place. That framing is backwards twice over. It implies the formulas are alternatives you could have a reason to choose between, and it implies the chosen one is accurate to a tenth of a kilogram. You usually have no basis for the choice, and none of them is that accurate.
Showing all three converts a hidden uncertainty into a visible one. When Boer, James and Hume land within a kilogram of each other — which happens for people close to the middle of the samples they were fitted to — the estimate is reasonably well determined and you can act on it. When they scatter across four or five kilograms, that is the formulas telling you your build is somewhere they do not agree about, and the right response is to get a measurement rather than to pick whichever number you prefer.
The spread is not a confidence interval and should not be read as one. Three regressions agreeing does not make them right — they could be wrong together, and for unusual builds they often are. What the spread rules out is false precision. It is a floor on the uncertainty, not a ceiling.
Why anyone needs this number
Lean body mass is not a fitness vanity metric. It exists because two important quantities scale with lean tissue rather than with total weight.
- Drug dosing.Many drugs distribute mainly through lean tissue and body water, not fat. Scaling such a dose to total body weight systematically over-doses people carrying more fat, which is why lean body mass and its relatives appear in anaesthetic, chemotherapy and antimicrobial dosing guidance. This is a prescriber’s calculation with a prescriber’s safety margins, not one to run yourself.
- Protein targets. Protein recommendations are usually quoted per kilogram, and which kilogram matters. At 1.6 g/kg, the man in the worked example needs 115 g a day scaled to his 72 kg total weight, or 89 g a day scaled to his 55.9 kg mean lean mass. That 26 g gap is not a rounding difference — it is a quarter of the target, and it exists entirely because total weight includes fat that does not need feeding. Our protein intake calculator works the target itself.
- Reading a change in weight. Losing four kilograms means something quite different depending on whether lean mass held steady or fell with it. A lean mass estimate at two points in time, taken the same way, says more than either weight alone — with the caveat that a 4 kg spread between formulas means a 1 kg change is well inside the noise.
If you would rather start from a body composition measurement than a regression, the body fat calculator estimates the percentage from a tape measure, and you can feed the result back into the direct mode above.
What this calculator assumes
- You are an adult of roughly ordinary build. All three formulas were fitted to adults, and all three degrade at the extremes of height and weight — James fastest.
- You are not pregnant, and you are not carrying unusual amounts of body water. Every one of these equations is, at bottom, a statement about body composition inferred from body water or fluid volumes.
- Height is measured without shoes and weight without heavy clothing. Boer and Hume both weight height heavily, so a 2 cm error moves their answers by about half a kilogram.
- A measured body fat percentage, if you supply one, came from an actual measurement. A bathroom scale’s impedance reading varies with hydration and is not one.
- Imperial input is converted with the exact definitions — 1 inch = 2.54 cm and 1 pound = 0.45359237 kg — before the formulas are applied, and the answers converted back to pounds.
- An estimate that implies an impossible body is refused rather than shown. That is a deliberate choice: a clamped number reads like an answer, and outside the fitting range there is not one.
Lean body mass FAQ
What is lean body mass?
Everything in your body that is not stored fat: muscle, bone, organs, skin, connective tissue and body water. It is sometimes called fat-free mass, though strictly fat-free mass excludes the small amount of essential fat inside cell membranes and organs that lean body mass includes. For a person weighing 72 kg with 16% body fat, lean body mass is about 60 kg. It is the part of your weight that does the metabolic work, which is why doses and targets are often scaled to it rather than to total weight.
Which lean body mass formula is most accurate?
There is no settled answer, which is exactly why this page shows all three. Boer, James and Hume were fitted to different samples with different reference methods across two decades, and comparison studies disagree about which tracks best — the ranking changes with sex, build and body size. James is known to under-read at high BMI because it subtracts a term that grows with the square of weight over height. Rather than pick a winner, use the spread: if the three agree closely, the estimate is reasonably well determined for your build; if they scatter, it is not.
Why do the three formulas give different answers?
Because each is a regression fitted to a different group of a few hundred people, using a different reference method, in a different decade. Hume published in 1966 against total body water, James in 1976 for the DHSS/MRC obesity group, and Boer in 1984 against body fluid volumes. None of them saw you. Two to four kilograms of disagreement between them for the same height and weight is normal and is the honest error bar on any single one of them. A calculator showing one number to a tenth of a kilogram is inventing a precision that no formula here has.
What is lean body mass used for?
Two things mainly. First, drug dosing: several medicines — some anaesthetics, some chemotherapy agents, some antibiotics — distribute mainly through lean tissue rather than fat, so scaling a dose to total weight over-doses a heavier patient and scaling to lean mass does not. Second, protein targets: a common recommendation of 1.6 g of protein per kilogram gives 115 g a day on a 72 kg total weight but 89 g a day on a 55.9 kg lean mass — a 26 g gap that comes entirely from whether the target is inflated by fat mass. Dosing decisions belong to a clinician, not to a web form.
Is knowing my body fat percentage better than these formulas?
Yes, decisively, if the figure came from an actual measurement. Lean mass is then weight × (1 − body fat), which is arithmetic on something someone observed about you rather than a regression on a stranger's sample. That is why this page takes a measured percentage as an input and reports it separately from the three estimates: it does not belong in their spread, because it is not one of their guesses. The caveat is what counts as measured — a DEXA scan or hydrostatic weighing, yes; a bathroom scale's impedance reading, not really, because it moves with hydration.
Should I use the average of the three formulas?
If you need a single number, the mean is a defensible choice, and it is shown here. But carry the spread with it. Reporting 55.9 kg alone is the same false precision the three formulas were meant to expose; reporting 55.9 kg with a 4 kg spread tells the reader what the figure is worth. If you have a measured body fat percentage, use that instead and treat the formulas as a sanity check on it.
Sources and review notes
- Boer, P. (1984). Estimated lean body mass as an index for normalization of body fluid volumes in humans. American Journal of Physiology 247(4) — the source of the Boer constants
- Hume, R. (1966). Prediction of lean body mass from height and weight. Journal of Clinical Pathology 19(4), 389–391 — the source of the Hume constants
- James, W. P. T. (1976). Research on Obesity: a report of the DHSS/MRC group. London: Her Majesty’s Stationery Office — the source of the James formula, which predates the online era and has no stable public link
- NIST Special Publication 811 — the exact inch and pound definitions used for the imperial conversion
These constants are between forty and sixty years old and will not change. What has changed is the range of body sizes they are routinely applied to, which is wider than any of the three samples covered — the main reason this page shows the disagreement rather than hiding it behind an average.