Lean Body Mass Calculator

About the Lean Body Mass Calculator

Lean body mass (LBM) is everything in your body that is not fat: muscle, bone, organs, skin and water. It cannot be weighed directly, so this calculator estimates it from your sex, height and weight with four published equations — Boer, James, Hume and Janmahasatian — and shows them side by side, in kilograms or pounds and as a share of your weight. If you know your body-fat percentage from a scan or a body-composition scale, enter it and the headline uses it instead.

The equations were fitted to groups of people, so they give the average for someone of your sex and size, not a measurement of you. The page also flags the best-known weakness: the James equation stops rising at a BMI of about 43 for men and 36 for women, and beyond that gives less lean mass for more weight.

Without a pass, the free preview shows your lean body mass in full, with every warning and caution, the first rows of the equation table and the first step of the working; its share of your weight, the other equations, the BMI, Copy result and the CSV come with a Pro pass.

Common uses

• Seeing how much of your weight is lean tissue, and how that changes as you lose fat or gain muscle.
• Comparing the classic equations used in research and pharmacology with each other for one person.
• Checking whether the James equation still makes sense at a given BMI.
• Turning a body-fat percentage from a scan or scale into kilograms of lean mass.

The four equations

W is the weight in kilograms and H the height in centimetres; results are in kilograms.

• 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. Hume derived them from total body water measured in 29 men and 27 women.
• Janmahasatian: men 9270 W ÷ (6680 + 216 × BMI); women 9270 W ÷ (8780 + 244 × BMI), where BMI = W ÷ (H ÷ 100)². It predicts fat-free mass and was built from DXA measurements in adults with BMIs from 17 to 70.
• From a measured body fat: W × (1 − body fat % ÷ 100), the fat-free mass.

Why the James equation fails at a high BMI

The James equation subtracts a term that grows with the square of your weight. Worked through, its result peaks where BMI = 10 000 × 1.1 ÷ 256 ≈ 43.0 for men and 10 000 × 1.07 ÷ 296 ≈ 36.1 for women (published analyses describe the plateau at about 43 and 37). Past that point a heavier person gets a smaller lean mass, and the result falls to zero at about twice that BMI. The calculator marks the James result above a BMI of 35 and tells you to ignore it once it is past its peak.

This matters outside this page too: the Schnider model that target-controlled infusion systems use for the anaesthetic propofol calculates lean body mass with the James equation, and most such systems do not accept higher BMIs for exactly this reason.

Which equation is most accurate?

It depends on who you are. A comparison with whole-body DXA scans in 1,558 adults aged 60 to 65 found Janmahasatian’s equation among the best overall and the best for women (average difference −0.9 kg); for men several equations, Boer among them, did about equally well. Hume’s equation had the smallest average difference overall, but its errors changed with body size. The same study saw individual results several kilograms either side of the scan for every equation.

So read the range across the equations as the honest answer, and use a measured body-fat percentage when you have one.

Lean body mass, fat-free mass and muscle

Strictly, fat-free mass is your weight minus all fat, while some definitions of lean body mass keep a little essential fat; in practice the two are used for the same thing, and Janmahasatian’s equation and a body-fat measurement both give fat-free mass. Neither is the same as muscle: lean mass also includes bone, organs, skin, blood and water. For a muscle-size measure that allows for height, see the FFMI calculator.

Sources

• Boer P, Estimated lean body mass as an index for normalization of body fluid volumes in humans, American Journal of Physiology.
• James WPT, Research on Obesity, a report of the DHSS/MRC group (HMSO).
• Hume R, Prediction of lean body mass from height and weight, Journal of Clinical Pathology.
• Janmahasatian S et al., Quantification of lean bodyweight, Clinical Pharmacokinetics.
• Coefficients as tabulated by De Cassai A et al., Regional anesthesia in obese patients, and Puri T, Blake GM, Comparison of ten predictive equations for estimating lean body mass with dual-energy X-ray absorptiometry in older patients (also the accuracy figures).
• The James plateau: Oldan JD et al., Comparison of standardized uptake value and standardized uptake lean body mass metrics; infusion pumps: Bidkar PU et al., Target-controlled infusion — past, present, and future.

Limitations

• For adults: the equations were derived in adults and are not meant for children or during pregnancy.
• They predict the average for your sex, height and weight. Very muscular people, older adults and people with an unusual build can be many kilograms away from the estimate.
• The calculator accepts weights of 25–300 kg and heights of 120–230 cm, and warns when the two together give a BMI below 15 or above 70, where none of the equations can be relied on.
• Not for drug doses: some medicines are dosed by lean body weight, and those doses are worked out by the prescriber with their own checks, never from this page.

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