Lean Body Mass Calculator (Boer, James, Hume)

1. Choose sex and units. Pick male or female, then metric or imperial. The coefficients differ by sex, so this choice changes the result.
2. Enter weight and height. Type the figures in the units you chose. Pounds and inches are converted to kilograms and centimeters before the formula runs.
3. Pick a formula, or compare all three. Boer is the default. The results panel always lists Boer, James and Hume side by side so you can see how far apart they fall.

Lean body mass, or LBM, is everything in the body that is not fat: muscle, bone, organs, connective tissue and the water they hold. It is also called fat-free mass. Body weight is simply lean body mass plus fat mass.

Lean body mass is not measured directly by a tape or a scale. Methods such as DEXA scanning, underwater weighing or air-displacement can measure it in a clinic. The formulas on this page instead predict it from sex, weight and height, which is why every result is an estimate.

Clinicians use lean body mass to scale things that track lean tissue rather than total weight, such as some drug doses and body-fluid volumes. Boer first proposed his equation in 1984 for exactly that purpose, to normalize body-fluid volumes.

Because the number depends on the formula, this tool shows all three estimates at once. Treat the figure as a guide to body composition, not a precise measurement.

The calculator converts your inputs to kilograms and centimeters, then applies the formula you selected. Each formula has one set of coefficients for men and one for women.

In every formula, W is weight in kilograms and H is height in centimeters. Note that James uses a non-linear weight-to-height-squared term, while Boer and Hume are straight-line equations.

1. Convert units. Pounds become kilograms (x 0.45359237) and inches become centimeters (x 2.54).
2. Apply the formula. The coefficients for your sex are placed into the equation you chose.
3. Find fat mass. Body fat mass is weight minus lean body mass, and the lean share is lean body mass divided by weight.
4. Compare methods. All three formulas run on the same inputs so you can read the spread between them.

Take a man who weighs 80 kg and is 180 cm tall, using the default Boer formula.

1. 0.407 x 80 = 32.56.
2. 0.267 x 180 = 48.06.
3. 32.56 + 48.06 - 19.2 = 61.42 kg of lean body mass.
4. Body fat mass = 80 - 61.42 = 18.58 kg, and lean mass is 76.8 percent of body weight.

The same man comes out at 62.72 kg under James and 57.79 kg under Hume. That is a spread of about 4.9 kg between the three methods, from the same height and weight. A woman of 65 kg and 165 cm gets 46.13 kg of lean mass and 18.88 kg of fat under Boer.

Lean Body Mass by Formula

Values are lean body mass in kilograms. The gap between columns shows why no single formula is the one true answer.

Only three inputs feed these formulas, so the result moves in predictable ways.

Body Weight

Heavier people have more lean tissue on average, so every formula rises with weight. In Boer, each extra kilogram of body weight adds about 0.41 kg of estimated lean mass for men and 0.25 kg for women.

Height

Taller people carry more lean tissue too. In Boer, each extra centimeter adds about 0.27 kg for men and 0.47 kg for women.

Sex

Men and women have different coefficients because men carry more lean mass on average at the same height and weight. Switching the sex selector changes the whole equation, not just a constant.

The Formula You Pick

James uses a weight-over-height-squared term, so it pulls away from Boer and Hume at the extremes of body size. For a tape-measure view of fat instead, see the body fat calculator.

What It Cannot See

None of these formulas know your actual muscle, how much you train, or your fat distribution. Two people with the same height, weight and sex get the same estimate even if their bodies differ.

The three equations were built by different researchers for different reasons, which is why they disagree.

Boer published his equation to normalize body-fluid volumes and it is the one most calculators use by default. Hume is the oldest of the three. James, developed through United Kingdom obesity research, uses a curved term that behaves differently for very heavy or very light bodies.

How Far Apart Are They?

For the 80 kg, 180 cm man above, the estimates run from 57.79 kg (Hume) to 62.72 kg (James), a spread near 5 kg. That gap is the honest margin of this kind of estimate. No formula here is a measurement, so the spread is the best guide to the uncertainty.

Tracking Body Composition

If your weight is steady but you are training, lean mass and fat mass can shift in opposite directions. Pairing this with the body fat calculator gives a fuller picture than the scale alone.

Setting Protein or Energy Targets

Some nutrition plans scale protein to lean mass rather than total weight. The lean figure here can feed a tool such as the BMR calculator, which offers a lean-mass energy estimate.

Putting a BMI in Context

A high BMI can come from muscle rather than fat. A lean-mass estimate and a fat-mass estimate help explain which one is driving the number.

Comparing with a Weight Range

The ideal weight calculator gives healthy weight ranges for your height, which sit alongside a lean-mass estimate rather than replacing it.

Background Reading

For how body fat relates to health by age, see our guide to a healthy body fat percentage by age.

1. Treating the Estimate as a Measurement

These are prediction formulas, not a DEXA scan. The right way to read them is as a range, which is why all three are shown.

2. Mixing Units

Typing pounds while the tool is set to kilograms more than doubles the weight term. Check the units box first.

3. Using It for a Child

All three formulas were built and validated in adults. This tool is for adults only and does not include a pediatric equation.

4. Expecting One True Number

Boer, James and Hume can differ by several kilograms for the same person. There is no single correct value, only estimates.

5. Trusting It at the Extremes

Very muscular, very lean or very large bodies sit outside the range these equations handle well, so the estimate is least reliable there.

6. Reading It as a Diagnosis

Lean body mass is one descriptive number. It does not diagnose anything. Clinical decisions need a clinician and, often, a measured value.

The arithmetic follows the published equations exactly, and the example figures reproduce the worked examples in our verified data. The limits are in what a height-weight formula can know about one body.

This page is not a diagnosis. The several-kilogram spread between formulas is the honest accuracy caveat: use any single figure as an estimate and confirm with a clinician for clinical use.

Lean body mass is your body weight minus fat mass: muscle, bone, organs, connective tissue and body water. It is also called fat-free mass.

In everyday use they mean the same thing. Strictly, fat-free mass excludes all fat, while lean body mass keeps a small amount of essential fat in cell membranes and organs. The formulas here treat them as interchangeable.

Boer (1984) is the most widely used and is the default here. James and Hume are shown too. None is a measurement, so comparing all three is more useful than picking one.

They were built by different researchers on different data, and James uses a curved weight-to-height term. For an 80 kg, 180 cm man the estimates span about 5 kg, from 57.8 to 62.7 kg.

Convert first. Multiply pounds by 0.45359237 to get kilograms and inches by 2.54 to get centimeters, then apply the formula. This tool does the conversion for you in imperial mode.

Body fat mass is body weight minus lean body mass. If Boer estimates 61.4 kg of lean mass for an 80 kg man, his fat mass is about 18.6 kg.

The arithmetic is exact, but the formulas are population estimates validated in adults. They are least accurate for very muscular, very lean or very large bodies, and they are not a substitute for a DEXA scan.

No. All three formulas were derived and validated in adults. This tool is for adults only and does not include a pediatric equation.

No. Muscle is part of lean body mass, but lean mass also includes bone, organs, connective tissue and water. Muscle is only a portion of the total.

No. The math runs in your browser. The Save button keeps results only in your own browser storage.

• Lean body mass: Boer, James and Hume formulas (Wikipedia, citing the originating papers; coefficients re-read 2026-10-04).
• Lean Body Weight-Tailored Iodinated Contrast Injection: Boer versus James Formula (Caruso and others, BioMed Research International, 2018; peer-reviewed cross-check of Boer and James).
• Estimated lean body mass as an index for normalization of body fluid volumes in humans (Boer P, American Journal of Physiology, 1984; originating paper for the Boer formula).
• Prediction of lean body mass from height and weight (Hume R, Journal of Clinical Pathology, 1966; originating paper for the Hume formula).
• James formula origin: DHSS/MRC Group research on obesity (Waterlow JC, James WPT, Research on Obesity, HMSO, 1976; restated with the James coefficients).