Lean body mass
Your lean body mass (muscle, bone, organs and water), from three formulas using height and weight. No calipers needed.
Your details
Your lean body mass (Boer)
9 st 8 lb
About 76% of your weight
- James formula9 st 11 lb
- Hume formula9 st 0 lb
- Everything else (mostly fat)3 st 0 lb
- Fat free mass index (Boer)19.2 kg/m²
- Measured body fatadd a body fat % from a scan or scales to compare
Lean mass is everything that is not fat: muscle, bone, organs and water. These formulas only know your height and weight, so they cannot see how much you train.
The working
The Boer formula
Peter Boer published this in 1984 and it is still the one most often used in hospitals:
- Men: 0.407 × kg + 0.267 × cm − 19.2
- Women: 0.252 × kg + 0.473 × cm − 48.3
For a man of 12 st 8 lb (79.8 kg) and 5 ft 10 in (177.8 cm): 0.407 × 79.8 + 0.267 × 177.8 − 19.2 = 60.8 kg, about 9 st 8 lb, or 76% of his weight.
If you have a scan
Measured against the formula
The formulas cannot see training, so a body fat figure from a scan or scales gives a second view. A man of 80 kg and 178 cm has a Boer lean mass of 60.9 kg. At a measured 20% body fat his lean mass is 64.0 kg, 3.1 kg more than the formula. We also show the fat free mass index (lean kg divided by height in metres squared, Kouri 1995): 19.2 from the formula and 20.2 from the scan.
Who uses it
Why lean mass matters
Anaesthetists use lean body mass to dose some drugs, because fat does not take them up the same way. Sports scientists use it to set protein and energy needs. For the rest of us it is a reminder that most of our weight is not fat at all.
Read with care
What this cannot tell you
- TrainingA rower and a non-exerciser of the same size get the same answer.
- FluidSwelling or dehydration changes real lean mass, but not the formula.
- ExtremesJames’ formula goes wrong at high weights, so trust Boer if they differ a lot.
Questions
Common questions
What is lean body mass?
Everything except fat: muscle, bone, organs, skin and water. It is usually 70% to 80% of a man’s weight and 60% to 75% of a woman’s.
Is lean body mass the same as muscle?
No. Skeletal muscle is roughly half of lean mass. The rest is bone, organs, blood and water.
How can I increase my lean mass?
Strength training on two or more days a week, as the UK Chief Medical Officers suggest, plus enough protein. Progress is slow, which is normal.
Sources
- Kouri EM et al. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med 1995
- Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. Am J Physiol 1984
- Hume R. Prediction of lean body mass from height and weight. J Clin Pathol 1966
Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.
If your weight is on your mind
Talk to your GP if your weight has changed a lot without trying, or if it is worrying you. If food or your body feels hard to think about, Beat has a free helpline on 0808 801 0677 (England, weekdays 3pm to 8pm).
Related
Related calculators
- Body fat calculatorBody fat from a tape measure, with the healthy range for your age.Open
- Resting metabolism from lean massResting energy from lean mass, by Katch-McArdle and Cunningham.Open
- Muscle to fat ratio calculatorYour weight split into lean and fat, and the ratio between them.Open
- Body density estimateThree skinfolds into body density and body fat (Jackson-Pollock).Open
- Ideal weight calculatorFour classic ideal weight formulas, next to the NHS range.Open
- Body composition goal plannerThe weight that matches a body fat goal, kept within healthy limits.Open