Lean Body Mass Calculator

Lean body mass is your weight minus your fat. It is the muscle, bone, organs, skin and water that keep you alive and moving, and it is the number behind protein targets set per kilogram of lean mass, the fat-free mass index and some resting metabolic rate equations.

You can get it two ways. A body fat measurement gives it directly. Without one, prediction formulas estimate it from your sex, height and weight. This lean mass calculator runs four formulas used in medical imaging and drug dosing research, shows who each one was built on and where it breaks down, and switches to the direct route as soon as you enter a body fat percentage.

How to calculate lean body mass

There are two ways to calculate lean body mass, and the calculator uses the one that fits what you enter.

1. From a body fat percentage. Lean body mass = weight × (1 − body fat ÷ 100). A 200 lb man at 20 percent body fat has 200 × 0.80 = 160 lb of lean mass. This is the better route when your body fat number comes from a decent measurement, because it describes you rather than an average person of your size.
2. From height and weight. A prediction formula estimates lean mass from sex, height and weight. Each formula was fitted to a group of adults whose lean mass was measured another way, such as total body water or a DXA scan, so it returns the lean mass of a typical person with your measurements.

To run a formula by hand, convert to metric first: pounds × 0.4536 gives kilograms and inches × 2.54 gives centimeters. For a man who is 5 ft 10 in (177.8 cm) and 180 lb (81.6 kg), the Boer formula gives 0.407 × 81.6 + 0.267 × 177.8 − 19.2 = 61.5 kg, which is 135.6 lb.

Strictly, some formulas predict lean body weight and others predict fat-free mass. The two differ only by a small amount of essential fat, and Janmahasatian et al. (2005) describe fat-free mass as a close approximation of lean body weight, so this page uses one number for both.

The four lean body mass formulas, side by side

Each formula below has a version for men and one for women. Weight is in kilograms and height in centimeters. We checked every coefficient against a published reprint: Caruso et al. (2018) in BioMed Research International for Boer and James, and the table in Puri and Blake (2022) in the British Journal of Radiology for all four.

The Hume coefficients are printed here as Puri and Blake give them, rounded to two decimals in the constant. The abstract of Hume's 1966 paper confirms the sample of 29 men and 27 women and the correlations; we could not open the full paper.

Why the headline uses Boer

The calculator leads with Boer for three reasons. It is a straight line in weight and height, so it never turns down for a heavier person. In a study of 55 CT patients with a BMI above 35, Caruso et al. recommended it over James for dosing contrast by lean body weight. And it is the estimate our FFMI calculator uses, so the two pages agree.

Boer is not the closest to a DXA scan in every group. In the Puri and Blake data, Janmahasatian matched women far better, reading 0.9 kg high on average against 4.5 kg for Boer. If you are a woman, treat the Janmahasatian row as a fair second opinion: for a woman who is 5 ft 5 in and 140 lb, it comes out 11.3 lb lower than Boer.

Why the James formula breaks down at high BMI

The James formula comes from a 1976 UK government report on obesity research. That report described body fat as a straight line rising with BMI: 1.281 × BMI − 10.13 percent for men and 1.48 × BMI − 7.0 percent for women, as Puri and Blake print it. Take that line away from 100 percent, multiply by weight, and you get the lean mass form most calculators use, which Puri and Blake credit to Hallynck et al.: 1.10 × weight − 128 × (weight ÷ height)² for men and 1.07 × weight − 148 × (weight ÷ height)² for women.

The squared term is the problem. As weight rises, the part being subtracted grows faster than the first part, so the estimate climbs, levels off and then falls. Setting its slope to zero puts the peak at a BMI of 43.0 for men and 36.1 for women. Caruso et al. and Puri and Blake, who cite a 2016 letter by Nyman in Radiology, round the plateau to about 43 for men and 37 for women. Past it, a heavier person gets less lean mass than a lighter person of the same height. That runs the wrong way: in MRI data from 468 adults, skeletal muscle kept rising with body weight, only more slowly as weight climbed (Janssen et al. 2000).

Here is what that looks like for a woman who is 5 ft 5 in (165 cm), as her weight rises.

At 300 lb the James formula gives her 99.3 lb of lean mass, less than the 105.1 lb it gives her at 150 lb, while Boer and Janmahasatian keep rising. The calculator still shows the James number past the peak so you can see the problem, but it marks the row and leaves it out of the range.

How accurate is a lean body mass estimate?

Less accurate than the decimal point suggests. The largest head-to-head test we found is Puri and Blake (2022), who ran ten formulas on 1,558 British adults aged 60 to 65 (747 men and 811 women) from the MRC National Survey of Health and Development and compared each result with a whole-body DXA scan.

Three things stand out. First, every formula read high on average except Hume in men, so most people in this age group carried less lean mass than the formulas predicted. Second, the spread for individuals was wide: the 95 percent limits of agreement, the band that holds 19 in 20 people, ran 11 to 14 kg wide for every formula. A formula gives everyone of the same sex, height and weight the same number, while their scans can differ by several kilograms either way. Third, Hume's small average miss hides an error that changed with body size: it read low for people with more lean mass and high for people with less, which is why Puri and Blake ruled it out.

Keep the limits of this study in mind. These were British adults aged 60 to 65 measured on one brand of scanner, and Puri and Blake point out that most of the formulas were developed in white populations. Younger adults carry more muscle for their weight, since skeletal muscle as a share of body weight starts falling in the third decade (Janssen et al. 2000), so the formulas may read closer for them. For anyone who lifts regularly they read low. That is why a measured body fat percentage beats any formula.

Worked examples: four formulas for nine people

Nine people run through the calculator. Seven leave body fat blank, so the headline is Boer. Two enter a measured body fat percentage, so the headline switches to the direct route while the formula columns stay the same.

Two patterns stand out. For women at ordinary weights, Janmahasatian sits about 11 lb below the other three formulas. For heavier builds, James falls behind once BMI passes its peak: the 5 ft 10 in, 330 lb man gets 163.0 lb from James against 196.6 lb from Boer. And the 15 percent body fat man carries 17.4 lb more lean mass than Boer predicts for his size, which is the gap a formula cannot see.

Lean body mass vs muscle mass: what a muscle mass calculator measures

A muscle mass calculator and a lean body mass calculator answer different questions. Lean body mass counts every kilogram that is not fat. Skeletal muscle, the muscle attached to your bones that strength training builds, is only one part of it. Bone, organs, skin and body water make up the rest.

Janssen and colleagues measured skeletal muscle with whole-body MRI in 468 men and women aged 18 to 88. Men averaged 33.0 kg of skeletal muscle and women 21.0 kg, which is 38.4 and 30.6 percent of body weight. Muscle as a share of body weight started to fall in the third decade, while the absolute amount held until the end of the fifth decade.

If you want a skeletal muscle number, Lee et al. (2000) fitted MRI measurements in 244 adults with a BMI under 30 to weight, height, age and sex: skeletal muscle (kg) = 0.244 × weight (kg) + 7.8 × height (m) + 6.6 for men − 0.098 × age − 3.3, with a typical error of 2.8 kg. The paper subtracts 1.2 kg for Asian adults and adds 1.4 kg for African American adults. For a 30-year-old man who is 5 ft 10 in and 180 lb, that gives about 34.2 kg (75 lb) of skeletal muscle, 56 percent of his Boer lean mass. For a 30-year-old woman who is 5 ft 5 in and 140 lb, it gives 22.1 kg (49 lb), 48 percent of hers.

So skeletal muscle is roughly half of lean body mass in these two examples. This calculator does not print a muscle number of its own, because the Lee equation needs your age and covers only a BMI under 30. If a smart scale gives you a muscle percentage, check its manual: some scales report skeletal muscle and others count all lean tissue, which is a much larger share of your weight.

How to measure lean body mass more accurately

A formula gives you the average for your size. To get your own number, you need a body fat measurement, and the methods differ in cost and error.

• DXA scan. A low-dose X-ray scan that measures fat, lean tissue and bone separately. It is the reference method in most of the studies on this page. Puri and Blake point out that it is not a true gold standard, but its low radiation dose, low cost and wide availability make it the usual benchmark.
• Bioimpedance. Smart scales and handheld devices send a small current through your body and predict lean mass from the resistance. They are quick and cheap, but hydration and each device's built-in equations move the result.
• Skinfold calipers and tape measures. These estimate body fat from the fat under your skin or from body circumferences. The body fat calculator runs both, and its result unlocks the direct route above.

Whichever method you use, repeat it under the same conditions, ideally first thing in the morning, and follow the trend across several readings rather than trusting one.

What to do with your lean body mass number

Set protein. Most protein targets use body weight, but some scale to lean mass. For lifters dieting down for a contest, Helms, Aragon and Fitschen (2014) recommend 2.3 to 3.1 g of protein per kg of lean body mass. For the 5 ft 10 in man at 15 percent body fat, with 69.4 kg of lean mass, that is about 160 to 215 g a day. The protein calculator applies a lean-mass range when you pick fat loss and enter your body fat.

Compare it with your height. Lean mass in kilograms divided by height in meters squared is the fat-free mass index. The result card shows it as the lean mass index, and the FFMI calculator places it against population and athlete reference values.

Estimate resting calories. Some resting metabolic rate equations use lean mass instead of total weight. The BMR calculator includes one of them, Katch-McArdle, next to Mifflin-St Jeor.

Track what you lose. When you diet, two body fat measurements taken the same way show whether the weight that came off was fat or lean. Keeping lean mass through a diet comes down to eating enough protein and lifting the whole way through. If you train at home, our comparison of the best dumbbell workout apps looks at which ones teach form and add load over time.

When to ignore this calculator

These formulas describe healthy adults of average build. In the cases below, skip the number or ask a clinician.

• Children and teenagers. The calculator is for adults 18 and over. Body composition changes from year to year while children grow. Ask a pediatrician.
• Pregnancy and the months after birth. Pregnancy adds weight and body water that no lean mass formula was built to handle. Your obstetric care team is the right source.
• Eating disorders or recent recovery. Body composition numbers can work against recovery. Work with a clinician or dietitian who specializes in eating disorders.
• Swelling or fluid retention. Heart, kidney and liver disease can add water, and every method here counts water as lean mass, so lean mass looks higher than it is. Ask your care team.
• Drug or contrast dosing. Hospitals use some of these formulas to dose medicines and imaging contrast. This page is not for that. Dosing decisions belong to your care team.
• Very muscular builds. Every formula assumes an average build and reads low for people who carry more muscle than most. Use the body fat route instead.

Related reading

References

1. Boer P. "Estimated lean body mass as an index for normalization of body fluid volumes in humans." Am J Physiol. 1984;247(4 Pt 2):F632-F636. doi:10.1152/ajprenal.1984.247.4.F632
2. Hallynck TH, Soep HH, Thomis JA, Boelaert J, Daneels R, Dettli L. "Should clearance be normalised to body surface or to lean body mass?" Br J Clin Pharmacol. 1981;11(5):523-526. doi:10.1111/j.1365-2125.1981.tb01163.x. Credited by Puri and Blake (2022) with the lean mass form of the James equation; the 1976 UK report it comes from (DHSS/MRC Group on Obesity Research, Research on Obesity, HMSO) is not online.
3. Hume R. "Prediction of lean body mass from height and weight." J Clin Pathol. 1966;19(4):389-391. doi:10.1136/jcp.19.4.389, PubMed 5929341
4. Janmahasatian S, Duffull SB, Ash S, Ward LC, Byrne NM, Green B. "Quantification of lean bodyweight." Clin Pharmacokinet. 2005;44(10):1051-1065. doi:10.2165/00003088-200544100-00004
5. Puri T, Blake GM. "Comparison of ten predictive equations for estimating lean body mass with dual-energy X-ray absorptiometry in older patients." Br J Radiol. 2022;95(1133):20210378. doi:10.1259/bjr.20210378, PMC10993957
6. Caruso D, De Santis D, Rivosecchi F, et al. "Lean body weight-tailored iodinated contrast injection in obese patient: Boer versus James formula." Biomed Res Int. 2018;2018:8521893. doi:10.1155/2018/8521893, PMC6110034
7. Nyman U. "James lean body weight formula is not appropriate for determining CT contrast media dose in patients with high body mass index." Radiology. 2016;278(3):956-957. doi:10.1148/radiol.2016152031
8. Janssen I, Heymsfield SB, Wang ZM, Ross R. "Skeletal muscle mass and distribution in 468 men and women aged 18-88 yr." J Appl Physiol. 2000;89(1):81-88. doi:10.1152/jappl.2000.89.1.81
9. Lee RC, Wang Z, Heo M, Ross R, Janssen I, Heymsfield SB. "Total-body skeletal muscle mass: development and cross-validation of anthropometric prediction models." Am J Clin Nutr. 2000;72(3):796-803. doi:10.1093/ajcn/72.3.796
10. Helms ER, Aragon AA, Fitschen PJ. "Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation." J Int Soc Sports Nutr. 2014;11:20. doi:10.1186/1550-2783-11-20

How this calculator differs from typical lean body mass calculators

Most online lean body mass calculators run one formula, usually Boer, and print a single number to the decimal as if it were a measurement. Some list several formulas without saying which people each one was built on, and few warn that the James formula turns down and hands heavier people less lean mass once BMI passes the high 30s or low 40s.

This calculator runs four published formulas side by side, names the one it uses for the headline, shows the range the four span for your size, and drops James from that range once your BMI passes the point where it stops rising. Every coefficient was checked against a published reprint, the accuracy figures come from a DXA comparison of 1,558 adults, and a measured body fat percentage switches the result to a direct calculation and says so on the card. It is built for adults only, it treats lean body mass and fat-free mass as one number, and it shows no number until height, weight and body fat sit inside realistic adult ranges.