Lean Body Mass Calculator

How to use this tool

Choose your sex, enter your height and weight, and the calculator gives lean body mass from four published equations side by side: Boer, James, Hume and Janmahasatian. The headline is their median, so one outlier cannot set the answer, and you can switch it to any single equation. Fat mass, the lean share of your weight and FFMI follow, with the working underneath.

The input people get wrong most often is the unit box beside weight, so check it says pounds or kilograms. If you have a body fat percentage from a scan or a careful caliper reading, type it in or use the body fat mode. The other modes cover FFMI, where the James equation breaks, and a lean mass table for your sex.

What the calculation is doing

All four equations take weight W in kilograms and height H in centimetres, with separate versions for men and women. Boer (1984) gives 0.407 W + 0.267 H − 19.2 for men and 0.252 W + 0.473 H − 48.3 for women. Hume (1966) gives 0.32810 W + 0.33929 H − 29.5336 for men and 0.29569 W + 0.41813 H − 43.2933 for women. Both are straight lines in weight, so every extra kilogram adds a fixed amount of lean mass.

James (1976) is quadratic in the ratio of weight to height: 1.10 W − 128 (W ÷ H)² for men and 1.07 W − 148 (W ÷ H)² for women. Janmahasatian (2005) works from BMI: 9270 W ÷ (6680 + 216 BMI) for men and 9270 W ÷ (8780 + 244 BMI) for women. It keeps rising with weight, more slowly each time, and has a ceiling.

From a body fat percentage, lean mass is W × (1 − body fat ÷ 100) and fat mass is the rest. FFMI is lean mass divided by height in metres squared, and the height adjusted value adds 6.1 × (1.8 − height in metres), so a person exactly 1.8 m tall gets the same figure either way.

What lean body mass is, and why equations exist

Lean body mass is everything in you that is not stored fat: skeletal muscle, but also bone, organs, skin, connective tissue and the water they hold. Muscle is the part that changes with training and much of the rest does not, so a lean mass figure is not a muscle score.

Measuring it takes a DXA (dual energy X ray) scan or similar, and most people never have one, yet several jobs need the figure. Dosing is the classic case: Janmahasatian’s paper is in Clinical Pharmacokinetics, and Smith’s 2021 letter investigates lean body mass for drug dosage in patients with obesity. Resting energy formulas such as Katch McArdle start from lean mass instead of weight, which suits people whose build differs a lot from average.

An equation swaps a measurement for an assumption: that someone of your height and weight is built like the people it was fitted to. Puri and Blake compared ten equations with DXA in 1,558 adults aged 60 to 65. For the four used here, the limits of agreement, the band holding 95 percent of individual differences, were 10.7 to 14.2 kg wide. This page gives general information, not medical advice, and any lean mass figure used for a medicine dose should come from your clinician.

Where the James equation breaks

At a fixed height, James is a hill: the squared term eventually outgrows the linear one, so the result peaks and then falls. Setting the slope to zero puts the peak at a BMI of 10,000 × 1.10 ÷ (2 × 128) = 42.97 for men and 10,000 × 1.07 ÷ (2 × 148) = 36.15 for women, whatever the height.

A man 5 ft 10 in tall peaks at 299 lb with 164.7 lb of lean mass. At 350 lb, BMI 50.2, James gives 160.0 lb while Janmahasatian gives 185.1. A woman 5 ft 4 in tall peaks near 211 lb at 112.7 lb. The result reaches zero at a BMI of 85.9 for men and 72.3 for women, and a 165 cm woman weighing 200 kg gets −3.4 kg.

Smith’s 2021 letter blames the height in the denominator for the parabolic curve, notes that a negative value is possible, prints −3 kg for that woman, and reports that the formulas diverge above a BMI of about 50 in men and 45 in women. A 2016 Radiology letter is titled “James Lean Body Weight Formula Is Not Appropriate for Determining CT Contrast Media Dose in Patients with High Body Mass Index”.

A man 5 ft 10 in tall, lean body mass in pounds at the weight that gives each BMI.

Boer and Hume never turn over, being straight lines in weight. Janmahasatian was proposed for this range: two of its authors, Green and Duffull, had published a 2002 letter titled “Caution when lean body weight is used as a size descriptor for obese subjects”. Its ceiling is 9270 ÷ 216 = 42.9 times height in metres squared for men and 9270 ÷ 244 = 38.0 for women, an FFMI far above the values in the next section.

FFMI and what the Kouri study showed

FFMI is lean mass in kilograms divided by height in metres squared, the same idea as BMI applied to lean tissue. The height adjusted version corrects for taller people scoring higher on the plain ratio. It is only as good as the lean mass fed in: five kilograms of error moves FFMI by about 1.6 at 5 ft 10 in.

Three reference points are built in. A 2015 consensus statement from the European Society for Clinical Nutrition and Metabolism treats an FFMI below 15 in women and below 17 in men as low. Schutz and colleagues measured 5,635 adults in Switzerland and reported medians of 18.9 for men and 15.4 for women aged 18 to 34. The third is the 25 from Kouri and colleagues.

Kouri, Pope, Katz and Oliva (Clinical Journal of Sport Medicine, 1995) studied male athletes who had and had not used anabolic steroids. Among 74 nonusers the highest height adjusted FFMI was about 25, and many steroid users went above it. That is all it showed: in one sample of men, no nonuser passed 25. It did not show that 25 is a natural limit for everyone. The authors called FFMI an initial screen, said steroid use cannot be diagnosed from it, and said 74 nonusers might not reveal the natural upper limit.

Body fat came from skinfold calipers, which the authors called far from ideal, and only six nonusers were above 20 percent body fat, so the adjustment may not suit heavier people. Women were not studied, so the tool shows no ceiling for women.

How far apart the four equations are

At ordinary weights the equations agree more than they differ, but not equally for everyone. For a 180 lb man 5 ft 10 in tall, Boer, James and Janmahasatian land within 3 lb of each other while Hume is 8.6 lb below Boer. For a 140 lb woman 5 ft 4 in tall the odd one is Janmahasatian, 7 to 11 lb under the other three. That gap is real uncertainty, which is why the tool reports a median and a spread. The tables give the median for a range of heights and weights.

Women: median lean body mass in pounds at each height and weight.

Men: median lean body mass in pounds at each height and weight.

A worked example you can check on paper

Take the default: a man 5 ft 10 in tall weighing 180 lb. In metric, W = 180 × 0.45359237 = 81.65 kg and H = 70 × 2.54 = 177.8 cm. Boer gives 0.407 × 81.65 + 0.267 × 177.8 − 19.2 = 33.23 + 47.47 − 19.2 = 61.50 kg. James gives 1.10 × 81.65 − 128 × (81.65 ÷ 177.8)² = 89.81 − 26.99 = 62.82 kg. Hume gives 0.32810 × 81.65 + 0.33929 × 177.8 − 29.5336 = 57.58 kg.

For Janmahasatian, BMI is 25.83, so 9270 × 81.65 ÷ (6680 + 216 × 25.83) = 61.74 kg. In pounds the four results are 135.6, 138.5, 126.9 and 136.1, and their median, the mean of the middle two, is 61.62 kg or 135.9 lb.

Fat mass is 81.65 − 61.62 = 20.02 kg, or 44.1 lb, so 75.5 percent of the weight is lean. FFMI is 61.62 ÷ 3.16 = 19.49, and the height adjusted value adds 6.1 × (1.8 − 1.778) to give 19.63.

Mistakes that catch people out

Reading lean mass as muscle. It includes bone, organs and water, and a few pounds of change in a week is mostly water. Judge trends over weeks.

Trusting James at high weight. Past a BMI of 36 for women and 43 for men its answer falls as weight rises. Use the Janmahasatian line or a measured figure there.

Picking one equation. The spread between them is the honest uncertainty: at BMI 20 to 30 it is typically about 8 percent of the median for men and 12 percent for women. Quote the range or the median.

Reading fat mass as a body fat measurement. In the equation modes fat mass is weight minus estimated lean mass, so it carries every error in that estimate.

Popular calculations

Each opens the calculator with its own address, and every value can be changed once it is open.

Women, lean mass from height and weight

Men, lean mass from height and weight

Metric heights and weights

Heavy builds, where James turns over

From a body fat percentage

FFMI

Lean mass tables

How this was checked

The engine was tested in Node against a second implementation written with different algebra, at 128 height and BMI pairs for each equation and sex, and the largest difference was 4 × 10 to the power of −14. The constants match four independent listings, including the ten equation table in Puri and Blake (2022), which prints the James constants under the name Hallynck.

One calculator page publishes a worked example, a man 5 ft 10 in and 160 lb: Boer 127.4, James 129.0 and Hume 120.4 lb. The engine gives 127.45, 128.98 and 120.38. A published example of 180 lb at 18 percent body fat gives 147.6 lb, which the tool reproduces, and a round trip through 60 weight and body fat pairs recovered every input.

Smith’s 2021 Table 1 lists the four equations at six weights for a 170 cm man and a 165 cm woman. Of its 48 printed cells, 38 match the engine within rounding, including the −3 kg cell. The ten that differ include a James value of 73 kg for the man, above the 68.3 kg James can ever give at that height. It is not clear whether those are misprints in the letter or in the retrieved copy, and nothing was tuned to them.

The James turnover found by searching the numbers matches the exact values from calculus, 42.968750 and 36.148649, to six decimal places. Janmahasatian rose at every step from BMI 8 to 150 at four heights per sex and stayed below its ceiling. The Kouri details come from published reviews that quote the paper, and the article keeps to the points they agree on. Every address listed above was opened and checked for a distinct title, the right answer, a chart that renders and clean exports.

Common questions

What is lean body mass?

It is everything in your body that is not stored fat: muscle, bone, organs, skin and the water they hold, so it is a bigger figure than muscle mass. For a man 5 ft 10 in tall weighing 180 lb the four equations here give 126.9 to 138.5 lb, with a median of 135.9 lb.

How do I calculate lean body mass?

Use an equation, or a body fat percentage if you have one. Boer for a man is 0.407 times weight in kilograms plus 0.267 times height in centimetres, minus 19.2. From body fat, multiply weight by one minus the percentage: 180 lb at 18 percent is 180 × 0.82, or 147.6 lb.

Which lean body mass formula is most accurate?

None is exact. Puri and Blake compared ten equations with DXA in 1,558 people aged 60 to 65, and Janmahasatian and Gallagher, which needs age, did best. Read the range, not one number.

Why does the James equation give less lean mass at a higher weight?

Its squared term overtakes the linear one past a BMI of 43.0 for men or 36.1 for women, and the result reaches zero at 85.9 and 72.3. That is the shape of the curve, not a fact about bodies.

What is a good FFMI?

European clinical nutrition guidance treats below 17 for men and below 15 for women as low. A Swiss study found medians of 18.9 for men and 15.4 for women aged 18 to 34. Kouri and colleagues found no nonuser above about 25 once height adjusted, in men only.

Is lean body mass the same as muscle mass?

No. Muscle responds to training, but lean mass also counts bone, organs, skin and water, so dehydration moves it without changing muscle.

Related Calculators and Reference

• Body Fat Calculator: estimates body fat from Navy or BMI and age methods, then reports lean mass and FFMI, where this page goes straight from height, weight and sex to lean mass.
• BMR Calculator: resting calories, including the Katch McArdle formula that starts from lean mass.
• Protein Intake Calculator: daily protein targets.
• Ideal Weight Calculator: target weight ranges by height.
• BMI Calculator: the index behind the James turnover and the Janmahasatian equation.
• Puri and Blake, British Journal of Radiology: ten lean body mass equations compared with DXA.