Lean body mass is everything in your body that is not fat: muscle, bone, organs, skin and the water inside them. People often use the term as shorthand for muscle, and that is wrong. A well-hydrated person and a dehydrated one can have different lean mass on the same day, because water sits inside it.
This calculator gives three estimates from just height, weight and sex. Enter your numbers in pounds and inches or kilograms and centimetres. The three results usually land within a few kilograms of each other, and the spread is part of the answer.
Why three formulas
Three equations are common in the literature, and none has been shown to be the one to trust. They were fitted on different people for different purposes, so we show all of them.
Hume (1966) came first. His paper estimated lean mass from total body water, measured with antipyrine, in 29 men and 27 women and fitted equations to height and weight. Correlation was 0.96 for the men and 0.83 for the women.
James (1976) comes from a government report on obesity and is one of the most widely applied formulas in clinical practice, according to a 2018 comparison paper. It uses a squared weight-to-height term, so it treats a heavy body differently from a tall one.
Boer (1984) was built to scale blood volume and extracellular fluid volume. Boer studied 66 healthy people for blood volume and 54 for extracellular fluid, and concluded that estimated lean mass was a suitable way to normalize fluid volumes. His formula is the simplest of the three.
The formulas in plain words
All use weight W in kilograms and height H in centimetres. The result is in kilograms.
Boer, men: 0.407 x W + 0.267 x H - 19.2. Women: 0.252 x W + 0.473 x H - 48.3.
James, men: 1.10 x W - 128 x (W / H)^2. Women: 1.07 x W - 148 x (W / H)^2.
Hume, men: 0.32810 x W + 0.33929 x H - 29.5336. Women: 0.29569 x W + 0.41813 x H - 43.2933.
For US units the calculator converts pounds to kilograms (divide by 2.2046) and inches to centimetres (multiply by 2.54) first.
Worked example
A man weighs 80 kg (176 lb) and is 180 cm (about 5 ft 11 in) tall.
Boer: 0.407 x 80 = 32.56; 0.267 x 180 = 48.06; 32.56 + 48.06 - 19.2 = 61.4 kg.
James: 80 / 180 = 0.4444, squared is 0.1975, times 128 is 25.28; 1.10 x 80 = 88; 88 - 25.28 = 62.7 kg.
Hume: 0.3281 x 80 = 26.25; 0.33929 x 180 = 61.07; 26.25 + 61.07 - 29.53 = 57.8 kg.
That is a range of 57.8 to 62.7 kg, or about 127 to 138 lb. A woman at 60 kg and 165 cm gets 44.9 (Boer), 44.6 (James) and 43.4 kg (Hume), a narrower spread of 1.5 kg.
To turn lean mass into a body fat estimate, subtract it from your weight and divide by weight. The man above would be at 23.2 percent using Boer's figure. Compare that with your tape-based figure on the body fat calculator. If the two disagree by more than four or five points, treat both with caution.
Where James breaks down
Because James subtracts a squared term, lean mass stops rising as weight climbs and eventually falls. By our arithmetic the peak is near a BMI of 43 for men and 36 for women. Above that, adding weight lowers the estimate, which makes no physical sense. A 2018 paper that compared Boer and James for contrast dosing in obese patients exists because of exactly this problem. If your BMI is above those values, the James result should be ignored.
How accurate is it
The three formulas use only height, weight and sex. They cannot see that one person is muscular and another carries more fat at the same weight. Two people of identical height and weight get identical output from every formula here, whatever their physiques. Equations like these predict group averages. For an individual, the error can be several kilograms.
Direct measurement methods, such as DEXA, give actual regional estimates, though at a cost and with their own variation between machines. The tape-based Navy method at least responds to your waist and neck, so it can differ for two people of the same size.
Who it does not suit
The equations were developed for adults. Hume's sample was small. Do not use them for children, pregnant women, or people with fluid retention, because lean mass in those cases includes water that changes the totals. Very muscular people will see the formulas underestimate. People with high body fat may see them overestimate or, for James, produce a result that behaves oddly.
Do not use the output to size a medication dose or to make a medical decision. The formulas are used in clinical research, but a clinician decides how, and with what safeguards.