Lean Body Mass Calculator
Calculate your lean body mass from total weight and body fat.
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About
Lean Body Mass Calculator
Lean body mass is what is left when you take the fat off. Total body weight minus body fat weight: bones, muscle, blood, skin, organs, water and everything else. The percentage is rarely quoted on its own, but it usually falls between 60% and 90% of total weight, and men sit higher in that band than women.
Why anyone measures it
The everyday reason is that lean mass is the half of body composition worth protecting. Two people at 160 pounds can carry very different amounts of muscle, and the scale cannot tell them apart. When you lose weight, the split between fat lost and lean tissue lost is what decides whether you end up smaller and stronger or smaller and weaker.
The clinical reason is dosing. Many drugs distribute through lean tissue rather than fat, so a dose calculated on total body weight can badly overshoot in a heavier patient. Water-soluble anaesthetic agents are the standard example, and lean body mass is used routinely to set them. Several other medical calculations, including renal function and some chemotherapy protocols, normalise to lean mass or to body surface area for the same reason.
For fitness and everyday tracking, most people care more about body fat percentage than the lean figure itself, and the two are just different views of one measurement. Our body fat calculator and ideal weight calculator come at the same body from the other side.
The formulas for adults
Three formulas are in common use. Each was fitted to a different population and each returns a slightly different answer, which is why the calculator above prints all of them rather than picking one.
The Boer formula (1984)
Men: eLBM = 0.407W + 0.267H - 19.2
Women: eLBM = 0.252W + 0.473H - 48.3
The James formula (1976)
Men: eLBM = 1.1W - 128(W/H)²
Women: eLBM = 1.07W - 148(W/H)²
The Hume formula (1966)
Men: eLBM = 0.32810W + 0.33929H - 29.5336
Women: eLBM = 0.29569W + 0.41813H - 43.2933
where:
- W is body weight in kilograms
- H is body height in centimetres
- eLBM is estimated lean body mass in kilograms
Take a man of 5 feet 10 inches at 160 pounds, which is 177.8 cm and 72.57 kg. Boer returns 57.8 kg, or 127.5 pounds, putting him at 80% lean and 20% fat. James returns 58.5 kg, 129.0 pounds, 81% lean. Hume returns 54.6 kg, 120.4 pounds, 75% lean. That is a spread of about 9 pounds and 6 percentage points of body fat for one identical body.
Boer and Hume are straight lines: both add a fixed amount for every kilogram of weight and every centimetre of height. James behaves differently because it subtracts a term that grows with the square of weight over height. At ordinary weights James sits near the others, and as weight climbs its curve bends back down, which reflects the assumption that additional weight past a point is mostly fat. Push it far enough and the James estimate starts falling as weight rises, which is the formula telling you it has left the data it was built on.
The formula for children
None of the three works on a small body. Peters and colleagues published a formula in 2011 in the British Journal of Anaesthesia for exactly this gap, and the authors limit it to children of roughly 13 to 14 years and under.
It runs in two steps, through estimated extracellular volume:
eECV = 0.0215 x W^0.6469 x H^0.7236
eLBM = 3.8 x eECV
Extracellular volume is the fluid outside the cells, and it scales with lean tissue in a way that survives the changing proportions of a growing body better than a linear height-and-weight formula does. Set the age question above to yes and the calculator switches to Peters and drops the adult three, because printing all four for a child would suggest a comparison that does not exist.
Lean body mass and fat free mass are not the same thing
The two terms get used interchangeably and the difference rarely matters, but they are not identical.
Lean body mass is the combined mass of bones, muscles, water, ligaments, tendons and internal organs. Organs contain essential fat, and that fat is counted inside lean body mass. The subcutaneous fat surrounding those organs is not.
Fat free mass excludes all fat, essential fat included. Subtract essential fat from lean body mass and you get fat free mass. The gap is around 2% to 3% in men and 5% to 12% in women, because women carry more essential fat, largely in breast tissue, hips and the reproductive system.
Essential fat is not spare. It cushions organs, insulates, carries fat-soluble vitamins and supports hormone production. The floor sits near 3% for men and 12% for women, and going below it causes real damage rather than a better physique. This is the reason a woman at 20% body fat and a man at 20% body fat are in very different positions.
How accurate is an estimate from height and weight?
These formulas take two measurements and predict a third. They cannot see how much muscle you carry, so a trained lifter and an untrained person of the same height and weight receive the same answer, and it is wrong for at least one of them. The error is largest at both extremes: very muscular bodies, very high body fat, and anyone far from the average build the equations were fitted to.
Direct measurement is a different exercise. A DEXA scan splits the body into fat, lean tissue and bone mineral, and is the practical reference standard. Hydrostatic weighing and air displacement plethysmography are comparably accurate and less available. Bioimpedance scales are cheap and swing with hydration, meal timing and skin temperature, so a single reading means little while the direction over months means something. Skinfold callipers are accurate in trained hands and unreliable in untrained ones.
Use the estimate as a baseline you re-measure the same way each time. Consistency of method matters more than the accuracy of any single reading, because what you actually want to know is whether the number is moving.
Holding on to lean mass while losing weight
Weight loss always costs some lean tissue. How much depends on choices rather than luck.
Protein does most of the work. Around 1.6 grams per kilogram of body weight a day is the level at which the benefit for retaining muscle levels off, and higher intakes are useful in a deficit rather than out of one. Resistance training gives the body a reason to keep the muscle it has, and two or three sessions a week is enough to change the split substantially.
Rate matters too. Losing more than about 1% of body weight a week pushes an increasing share of the loss onto lean tissue, and a very aggressive deficit takes muscle regardless of what you eat. Sleep is the quiet variable: short sleep during a deficit shifts the loss towards lean tissue even when protein and training stay the same.
The reason to bother is that lean mass drives resting metabolic rate. Losing muscle lowers the number of calories you burn doing nothing, which makes the next stage of any deficit harder and regain easier.
What the number is not
Lean body mass is not a fitness score and not a target. There is no ideal figure to reach, and comparing yours against someone else's tells you very little, because it scales with height, frame and sex before anything you have any control over. What is worth watching is your own number over time: rising while your weight is steady means you are gaining muscle, and falling while you lose weight means the diet is taking more than it should.
References
Boer P. "Estimated lean body mass as an index for normalization of body fluid volumes in man." American Journal of Physiology 1984; 247: F632-5.
James W. "Research on obesity: a report of the DHSS/MRC group." HM Stationery Office, 1976.
Hume R. "Prediction of lean body mass from height and weight." Journal of Clinical Pathology 1966; 19(4): 389-91.
Peters AM, Snelling HLR, Glass DM, Bird NJ. "Estimation of lean body mass in children." British Journal of Anaesthesia 2011; 106(5): 719-23.
Common questions
Frequently asked questions
Total body weight minus body fat weight. It covers bones, muscle, blood, skin, water and internal organs, along with the essential fat inside those organs. It usually falls between 60% and 90% of total body weight, and men typically sit higher in that band than women.
There is no single target, because the figure scales with height, frame and sex before anything you control. Most people land between 60% and 90%, with men higher. Your own number over time is the useful measure: rising while weight holds steady means muscle gained, falling during weight loss means the diet is taking too much.
Boer is the most commonly cited for adults and sits between the other two at ordinary body sizes. For a 5 foot 10 inch man at 160 pounds, Boer gives 127.5 lbs, James 129.0 and Hume 120.4, a spread of about 9 pounds. Treat the range as the answer rather than any one figure, and use the same formula every time you re-measure.
Lean body mass includes the essential fat inside your organs. Fat free mass excludes all fat, essential fat included, so it is the smaller number. The gap is roughly 2% to 3% in men and 5% to 12% in women, because women carry more essential fat.
Boer, James and Hume were fitted to adults and return unreliable figures for a small body. Peters and colleagues published a formula in 2011 built specifically for children, working through estimated extracellular volume, and limited it to about ages 13 to 14 and under. Showing all four for a child would imply a comparison that does not exist.
Subtract lean body mass from total weight to get fat mass, then divide by total weight. A 160 pound man with 127.5 pounds of lean mass carries 32.5 pounds of fat, which is 20%. The calculator prints both figures for every formula.
They take height and weight and predict a third number, so they cannot see how much muscle you carry. A trained lifter and an untrained person of identical height and weight get the same answer. Error is largest at the extremes of muscularity and body fat. A DEXA scan is the practical reference standard if you need a real measurement.
Eat around 1.6 grams of protein per kilogram of body weight a day, lift weights two or three times a week, and keep the loss near 1% of body weight a week or slower. Sleep matters more than most people expect: short sleep during a deficit shifts more of the loss onto lean tissue even when protein and training stay the same.
Many drugs, particularly water-soluble ones, distribute through lean tissue rather than fat. Dosing on total body weight in a heavier patient can overshoot badly, because the extra weight is largely fat the drug never reaches. Water-soluble anaesthetic agents are the standard case.
Yes. Body water is a large share of it, which is why lean mass estimates from bioimpedance scales move with hydration, meal timing and even skin temperature. The formulas here use only height and weight, so they do not swing day to day, but they also cannot react to a real change in your composition.