Lean Body Mass Calculator
Find out how much of your weight is muscle, bone, organs and water. Compare the Boer, James, Hume and Janmahasatian formulas in one view, then turn your lean mass into a fat-free mass index, a protein target and a muscle-protection plan for weight loss.
Written and fact-checked by Manzoor Ahmad, Pharm-DLast reviewed September 28, 2026
Enter your sex, age, height and weight to see your lean body mass. Everything is calculated in your browser and nothing is saved.
Your lean body mass
—
Lean share of your weight
Everything that is not fat.
Fat-free mass index
Lean mass adjusted for height.
Formula consensus
Each dot is one published formula. The shaded band is where they agree.
Muscle guard for weight loss
Planning to lose weight with diet, a GLP-1 medicine like semaglutide or tirzepatide, or both? Enter how much you want to lose to see how much lean mass is at stake.
What is lean body mass?
Lean body mass (LBM) is your total body weight minus your body fat. It includes skeletal muscle, bone, organs, skin, blood and the water held in all of them. For most adults it makes up roughly 60 to 90 percent of body weight, with men usually at the higher end.
If you know your body fat percentage, the math is simple: LBM = weight × (1 − body fat % ÷ 100). A 180 lb person at 20 percent body fat carries 144 lb of lean mass and 36 lb of fat. If you do not know your body fat, validated formulas such as Boer, James and Hume estimate LBM from sex, height and weight. This lean body mass calculator runs every one of them at once so you can see where they agree.
How to use the lean body mass calculator
- Choose your units and enter sex, age, height and weight. Weigh yourself in the morning after using the bathroom and before eating, because food and water can swing weight by a few pounds across a day.
- Add body fat percentage if you have it. A DEXA scan is the best source, calipers and bioimpedance scales are rougher. With a body fat number the calculator switches its headline result to the direct method and uses the formulas as a cross-check.
- Read the formula consensus strip. Tight agreement means your estimate is solid. A wide spread usually means your build sits outside the populations the formulas were built on, which is common at very high or very low body fat.
- Use the numbers. Your FFMI tells you how muscular you are for your height, the Katch-McArdle figure gives resting calories based on lean tissue, and the protein target is sized to your lean mass rather than your total weight.
- Plan weight loss with the muscle guard. Enter a weight-loss goal to see how much lean tissue could go with it, then use the strategies below to keep that number low.
Lean body mass formulas: Boer vs James vs Hume vs Janmahasatian
Each formula was built from a different group of people for a different purpose, which is why they rarely match to the decimal. Weight (W) is in kilograms and height (H) in centimeters unless noted.
Boer formula (1984)
MenLBM = 0.407 W + 0.267 H − 19.2
WomenLBM = 0.252 W + 0.473 H − 48.3
Developed to normalize body fluid volumes, Boer is the calculator’s default for adults because it stays well-behaved across a wide range of body sizes, including higher BMIs.
James formula (1976)
MenLBM = 1.1 W − 128 (W ÷ H)²
WomenLBM = 1.07 W − 148 (W ÷ H)²
James is still common in clinical references, but it has a mathematical flaw worth knowing. Because weight is squared in the second term, the formula peaks and then falls as weight rises. Work the calculus and that peak lands at a BMI of about 43 for men and about 36 for women. Past that point, James says a heavier person has less lean mass, which is physically impossible. This calculator flags James and leaves it out of the consensus band once your BMI reaches 35.
Hume formula (1966)
MenLBM = 0.32810 W + 0.33929 H − 29.5336
WomenLBM = 0.29569 W + 0.41813 H − 43.2933
Hume derived his equations from total body water in a small group of middle-aged and older adults, so it tends to run conservative. It is a sensible reference if you are over 40 and do not want an estimate that flatters you.
Janmahasatian formula (2005)
Men, BMI in kg/m²LBM = 9270 × W ÷ (6680 + 216 × BMI)
Women, BMI in kg/m²LBM = 9270 × W ÷ (8780 + 244 × BMI)
This is the formula pharmacists and anesthetists know best. It was built against DEXA-measured fat-free mass specifically for drug dosing, and it holds up well in obesity. Most consumer lean mass calculators skip it, which is a shame, because it is often the most realistic estimate for people carrying extra weight.
Peters formula for children (2011)
Ages 14 and undereLBM = 3.8 × (0.0215 × W0.6469 × H0.7236)
Adult formulas misbehave in children, so for ages 14 and under the calculator switches to the Peters equation, which estimates extracellular volume first and scales it to lean mass.
| Formula | Built for | Best used when | Watch out for |
|---|---|---|---|
| Body fat % method | Any measured body fat | You have a DEXA or other reliable body fat reading | Only as good as the body fat number you enter |
| Boer | Body fluid normalization | General adult default, including higher BMI | Can overestimate in very lean, very short people |
| James | UK obesity research | BMI below about 35 | Breaks down and eventually reverses at high BMI |
| Hume | Total body water studies | Adults over 40 wanting a conservative figure | May undershoot in young, muscular adults |
| Janmahasatian | Drug dosing, DEXA validated | Obesity and clinical settings | Less familiar, rarely shown on fitness sites |
| Peters | Pediatric anesthesia | Children 14 and under | Not for adults |
Swipe the table sideways on a phone.
Lean body mass vs fat-free mass vs muscle mass
These three terms get used interchangeably, but they are not the same thing, and the difference matters when you read a scan or a study.
Fat-free mass (FFM)Everything except all fat, including the small amount of essential fat inside cell membranes and nerves. DEXA reports this.
Lean body mass (LBM)Fat-free mass plus that small essential fat. In practice it is within a few percent of FFM, and most formulas use the terms loosely.
Skeletal muscle massOnly the muscles you train. It is a portion of lean mass, not all of it. Organs, bone and water make up the rest.
Why it mattersWhen a study says a diet or drug caused lean mass loss, part of that is water and organ tissue, not purely muscle. The number still deserves attention, but it is not the same as losing that much muscle.
Fat-free mass index (FFMI) chart
Lean mass on its own does not tell you how muscular you are, because taller people naturally carry more of it. FFMI fixes that by dividing lean mass in kilograms by height in meters squared, the same way BMI treats total weight. The normalized version adjusts to a 1.8 m reference height so short and tall people can compare fairly: normalized FFMI = FFMI + 6.1 × (1.8 − height in m).
| Description | Men (FFMI) | Women (FFMI) |
|---|---|---|
| Below average | Under 17 | Under 14 |
| Average | 17 to 19.9 | 14 to 16.9 |
| Above average | 20 to 21.9 | 17 to 18.9 |
| Excellent | 22 to 22.9 | 19 to 21.9 |
| Superior | 23 to 25 | 22 and above |
| Rarely reached without anabolic drugs | Above 25 | Not well defined |
Bands are practical reference ranges, not a medical classification. The 25 ceiling for men comes from a 1995 study of trained athletes by Kouri and colleagues. Formula-based lean mass can overstate FFMI in people with higher body fat, so a DEXA-based body fat entry gives the fairest FFMI.
A pharmacist’s view: why lean body mass matters beyond the gym
Most people meet lean body mass through fitness apps. I meet it through dosing. Many medicines, especially water-soluble ones that do not travel into fat tissue, distribute mainly through lean tissue. Dose them on total body weight in a person with obesity and you can overshoot; dose them on lean weight and the exposure lines up better with what the body can actually handle. That is why the Janmahasatian formula exists and why it is included here.
GLP-1 medicines and lean mass
This is the question I hear most in 2026. Semaglutide and tirzepatide produce large weight losses, and body composition sub-studies of their trials found that lean tissue made up roughly a quarter to about 40 percent of the weight lost. That range is not unique to these drugs; diet-only weight loss also costs lean mass. There is encouraging news too. In the 12-month SEMALEAN study of semaglutide 2.4 mg, lean mass dipped early and then held steady, while handgrip strength actually improved. The muscle guard panel in the calculator uses the 25 and 40 percent ends of that range so you can see the stakes in your own numbers.
What protects lean mass while you lose weight
- Resistance training two to four times a week. This is the single strongest signal to your body to keep muscle during a calorie deficit.
- Protein sized to lean mass. A range of about 1.6 to 2.2 grams per kilogram of lean mass is widely used in sports nutrition, which is how the calculator’s protein target is built. Spread it over three or four meals.
- A moderate pace. Losing about 0.5 to 1 percent of body weight per week gives muscle a better chance than crash dieting.
- Enough sleep. Short sleep during a diet shifts more of the loss toward lean tissue.
Supplements: what the evidence supports and what to skip
After years of reviewing supplement labels, my short list is short on purpose. Creatine monohydrate at 3 to 5 grams a day is the most studied supplement for building and keeping lean mass alongside training. It also pulls water into muscle, so expect a small scale increase that is lean tissue, not fat. Protein powder is simply convenient food. Vitamin D helps only if you are low, so test before you take high doses. Avoid SARMs and so-called legal steroids sold online; the FDA has warned that these products can contain unapproved drugs linked to liver injury, heart problems and hormone disruption.
Two safety notes
If you have chronic kidney disease, talk with your doctor before raising protein, because the right intake depends on your kidney function. And if you are losing weight on a GLP-1 medicine and feel weaker, more tired or less stable on your feet, mention it to your prescriber. Those are signs worth acting on early.
How accurate is a lean body mass calculator?
Formula-based lean body mass is a well-informed estimate, not a measurement. The formulas were built on populations and assume average proportions of muscle, bone and water. They tend to underestimate lean mass in muscular people and overestimate it in people with higher body fat. If accuracy matters, measure body fat directly and enter it.
| Method | What it measures | Accuracy | Cost and access |
|---|---|---|---|
| DEXA scan | Fat, lean and bone mass by region | High, the clinical reference in most settings | Moderate cost, clinic or specialty center |
| Bod Pod | Body volume and density | High when protocol is followed | Moderate, limited locations |
| Bioimpedance (BIA) | Electrical resistance through water | Moderate, sensitive to hydration | Low, home scales and gym devices |
| Skinfold calipers | Fat under the skin at set sites | Moderate, depends on the tester | Very low |
| Height and weight formulas | Population-based estimate | Fair for average builds | Free, this calculator |
For the best home routine, measure the same way each time, at the same time of day, and track the trend over weeks rather than any single reading. If you want to estimate body fat from tape measurements first, the body fat percentage calculator gives you a number you can bring back here for a more personal lean mass result.
Why lean body mass beats BMI for tracking progress
BMI only knows your height and weight. It cannot tell a strength athlete from someone with the same weight and much more fat, and it cannot see muscle loss hiding behind a falling scale. Lean body mass can. When you diet, the goal is to watch weight fall while lean mass holds steady. When you train, the goal is to see lean mass rise even if the scale barely moves.
BMI is still useful as a quick population screen, and you can check yours with the BMI calculator. Put the two side by side and you get a far clearer picture than either number alone. Soldiers and recruits dealing with waist-based standards can also run the Army body fat calculator, which estimates body fat from a single navel measurement and feeds straight into the lean mass math here.
Lean body mass calculator FAQ
Lean body mass is your total body weight minus body fat. It includes muscle, bone, organs, skin, blood and body water. For most adults it is roughly 60 to 90 percent of body weight, with men usually higher than women.
If you know your body fat percentage, multiply your weight by one minus the body fat fraction. For example, 180 lb at 20 percent body fat equals 144 lb of lean mass. Without body fat, use a formula such as Boer: for men 0.407 times weight in kg plus 0.267 times height in cm minus 19.2, and for women 0.252 times weight plus 0.473 times height minus 48.3.
A measured body fat percentage from DEXA beats every formula. Among height and weight formulas, Boer is a reliable general default, Janmahasatian performs well in obesity and is used for drug dosing, Hume runs conservative for older adults, and James should be avoided at a BMI of 35 or higher because it breaks down.
The James formula squares weight in its second term, so its output peaks and then falls as weight rises. The peak occurs near a BMI of 43 for men and 36 for women, after which it predicts less lean mass for a heavier person, which is not physically possible.
There is no single ideal, but healthy adult men often fall around 75 to 90 percent lean and women around 65 to 80 percent lean, which matches roughly 10 to 25 percent body fat for men and 20 to 35 percent for women. Athletes may sit higher.
For men, a normalized fat-free mass index of 17 to 19.9 is average, 20 to 22.9 is above average to excellent, and values above 25 are rarely reached without anabolic drugs. For women, 14 to 16.9 is average and 17 to 21.9 is above average to excellent. These are practical reference bands, not medical categories.
No. Skeletal muscle is only part of lean body mass. Lean mass also includes bone, organs, skin, blood and water, so a change in lean mass does not always mean the same change in muscle.
Body composition sub-studies of semaglutide and tirzepatide trials found that lean tissue made up roughly a quarter to about 40 percent of total weight lost. Diet-only weight loss also costs lean mass. Resistance training and adequate protein help keep the lean share of loss lower.
A range of about 1.6 to 2.2 grams of protein per kilogram of lean body mass per day is widely used in sports nutrition for building or keeping muscle. People with kidney disease should ask their doctor before increasing protein.
Lift weights or do other resistance training two to four times a week with gradually increasing load, eat enough protein spread across meals, sleep seven to nine hours, and avoid aggressive calorie cuts. Creatine monohydrate at 3 to 5 grams a day is the most studied supplement for supporting lean mass gains.
Yes, for ages 14 and under the calculator switches to the Peters formula, which was developed for children. Adult formulas and FFMI bands are not appropriate for kids, and results should be discussed with a pediatrician.
Formula-based calculators give population-level estimates and can be off by several kilograms for muscular people or people with high body fat. Entering a body fat percentage from DEXA, a Bod Pod or skilled caliper testing gives a much more personal result.
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About the author
Manzoor Ahmad, Pharm-D
Pharmacist, health content writer and founder of DexoCalc
Manzoor is a Doctor of Pharmacy with more than nine years of experience writing about supplements, weight management and medication safety. He runs a community pharmacy where questions about GLP-1 medicines, protein and muscle loss come up every week. He built this calculator from the original published equations and checks every formula against worked examples before release.
Sources and references
- Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. American Journal of Physiology. 1984;247(4 Pt 2):F632-F636.
- James WPT. Research on Obesity: a report of the DHSS/MRC Group. London: Her Majesty’s Stationery Office; 1976.
- Hume R. Prediction of lean body mass from height and weight. Journal of Clinical Pathology. 1966;19(4):389-391.
- Janmahasatian S, Duffull SB, Ash S, et al. Quantification of lean bodyweight. Clinical Pharmacokinetics. 2005;44(10):1051-1065.
- Peters AM, Snelling HLR, Glass DM, Bird NJ. Estimation of lean body mass in children. British Journal of Anaesthesia. 2011;106(5):719-723.
- Kouri EM, Pope HG, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clinical Journal of Sport Medicine. 1995;5(4):223-228.
- Katch FI, McArdle WD. Nutrition, Weight Control, and Exercise. Resting energy equation: 370 + 21.6 × lean mass in kg.
- Neeland IJ, Linge J, Birkenfeld AL. Changes in lean body mass with glucagon-like peptide-1-based therapies and mitigation strategies. Diabetes, Obesity and Metabolism. 2024.
- Impact of semaglutide on fat mass, lean mass and muscle function in patients with obesity: the SEMALEAN study. PubMed Central.
- Preservation of lean soft tissue during weight loss induced by GLP-1 and GLP-1/GIP receptor agonists: a case series. PubMed Central.
This lean body mass calculator is for education and general wellness. Formula results are estimates and can differ from DEXA or other measured values. It is not a diagnosis and not a substitute for advice from your doctor, pharmacist or dietitian, especially if you are pregnant, under 18, have kidney disease, or take medicines that affect weight.