Lean Body Mass Calculator
Everything you weigh that is not fat.
Calculate lean body mass with the Boer, James and Hume formulas. See your fat mass, lean mass percentage, and why LBM matters for dosing and dieting.
These formulas estimate from height and weight only; they cannot see how muscular you actually are. If you know your body fat percentage, weight × (1 − body fat ÷ 100) is more accurate than any of them.
Lean body mass (Boer)
56.0 kg
80.0% of body weight
How the Lean Body Mass Calculator works
Lean body mass is your weight minus your fat: muscle, bone, organs, blood and water. It matters more than scale weight for two reasons. It is what determines your resting metabolic rate, so preserving it is what keeps a diet working. And several drugs are dosed against lean mass rather than total weight, because fat tissue takes up far less of them.
Also known as: LBM calculator · fat free mass calculator · how much muscle do I have
What lean body mass includes
Lean body mass is everything that is not fat: muscle, bone, organs, connective tissue and the water within them. It is sometimes confused with muscle mass, which is only part of it, and the difference is substantial. Skeletal muscle accounts for roughly 40% to 50% of lean mass in most adults, with the rest being bone, organs and fluid.
Fat-free mass is a closely related term and is not quite identical. Lean body mass conventionally includes the small amount of essential fat within organs and the central nervous system; fat-free mass excludes all fat. The difference is a few percent and the terms are used interchangeably in most contexts.
The number matters because it is the metabolically active part of the body. Resting energy expenditure tracks lean mass far more closely than it tracks total weight, which is why two people of the same weight can have noticeably different calorie requirements.
The equations, and where they came from
The Boer formula, published in 1984, was derived from a sample of adults and is widely used for its simplicity. For men it is 0.407 times weight in kilograms plus 0.267 times height in centimetres minus 19.2; for women, 0.252 times weight plus 0.473 times height minus 48.3.
The James formula from 1976 uses weight and the square of the body mass index and tends to give slightly different results, particularly at the extremes of body composition.
The Hume formula, from 1966, was derived against total body water measurements and is still used in clinical settings. None of these is definitively better than the others across all body types, which is a reason to treat any single figure as approximate.
Where the equations break down
All of these formulas were derived from populations of average body composition, and they behave poorly outside it. In someone with a very high body fat percentage they overestimate lean mass, because the equations assume a proportion of the extra weight is lean tissue and in severe obesity a smaller share of it is.
They also underestimate in trained athletes, for the opposite reason. A muscular individual carries more lean mass than the height and weight inputs suggest, and the equation has no way to know.
Which means the figure is most reliable for people of unremarkable body composition and least reliable for exactly the people most likely to be interested in it. Anyone at either extreme should treat a formula-based estimate as a rough starting point rather than a measurement.
How it is measured properly
DEXA scanning is the practical reference standard, using two X-ray energies to distinguish bone, fat and lean tissue. It is accurate, it involves a small radiation dose, and it typically costs somewhere between £70 and £200 privately in the UK.
Hydrostatic weighing and air displacement plethysmography, the BOD POD, measure body density and infer composition from it. Both are accurate and neither is widely available.
Bioelectrical impedance, the technology in consumer scales, estimates composition from how a small current passes through the body. It is convenient and it is heavily affected by hydration, recent food, recent exercise and skin temperature, which is why the same scale can give readings several percent apart within a day.
What the number is used for
Protein requirements are more sensibly set against lean mass than against total weight, particularly for anyone carrying substantial body fat, since the fat tissue does not need feeding with protein.
Drug dosing in clinical settings sometimes uses lean body mass rather than total weight, because many drugs distribute into lean tissue rather than fat. That calculation belongs to a clinician rather than to a website.
For anyone changing their body composition, the useful application is tracking. Losing weight while holding lean mass steady is the goal of most sensible fat loss programmes, and a total weight figure cannot distinguish that from losing both. The trend in the estimate matters far more than any single reading.
Where to go next
The Lean Body Mass question rarely arrives on its own. These are the ones that usually come with it:
- Body Fat Calculator — Body fat percentage from tape measurements.
- FFMI Calculator — Fat-free mass index, muscularity adjusted for height.
- Ideal Weight Calculator — Target weight from four clinical formulas plus BMI range.
- BMI Calculator — Body mass index with category and healthy weight range.
Not medical advice. This calculator gives a general estimate, not medical advice. It cannot account for your individual health, medical history, or medication. Talk to a qualified healthcare professional before acting on any result.
Frequently asked questions
How is lean body mass calculated?
The Boer formula, the most widely used, is LBM = 0.407 × weight in kg + 0.267 × height in cm − 19.2 for men, and 0.252 × weight + 0.473 × height − 48.3 for women. It estimates from height and weight alone, so it cannot see how muscular you actually are.
Which lean body mass formula should I use?
Boer is the general-purpose default and the one most clinical references cite. James tends to read lower in heavier people. Hume was derived for drug dosing specifically. If you know your body fat percentage, skip all three, lean mass is simply weight × (1 − body fat ÷ 100), which is more accurate than any height-based estimate.
Why does lean body mass matter when losing weight?
Because lean tissue burns calories at rest and fat does not. Losing weight rapidly or without enough protein strips lean mass along with fat, which lowers your maintenance calories and makes the weight easier to regain. Resistance training and roughly 1.6–2.2 g of protein per kg of body weight are what protect it.
Is lean body mass the same as muscle mass?
No, and the difference is large. Lean body mass includes bone, organs, blood and the water in every tissue. Skeletal muscle is typically only about half of it. Someone with 60 kg of lean mass does not have 60 kg of muscle.
Related calculators
Body Fat Calculator
Body fat percentage from tape measurements.
OpenFFMI Calculator
Fat-free mass index, muscularity adjusted for height.
OpenIdeal Weight Calculator
Target weight from four clinical formulas plus BMI range.
OpenBMI Calculator
Body mass index with category and healthy weight range.
Open