Lean Body Mass Calculator

What lean body mass counts

Lean body mass is everything in the body that is not fat: muscle, bone, organs, skin, connective tissue and the water inside all of them. Weigh 180 lb with 45 lb of fat and your lean mass is 135 lb. The number is useful because two people at the same weight can carry wildly different amounts of muscle, and the scale cannot tell them apart.

Lean body mass and fat-free mass are treated as the same thing on this page, which is how the equations below use them. Strictly, fat-free mass excludes the small amount of essential fat stored in bone marrow, the nervous system and organs, while lean body mass includes it. The difference is a few percent of body weight at most, well inside the error of any height and weight equation.

What lean mass is not: a target to maximize. Muscle, organs and water are healthy; doubling lean mass is not a goal, and the equations cannot separate a bodybuilder's muscle from an overweight person's excess tissue and water. They only split your weight into a fat part and a non-fat part using your height.

The Boer equation this page uses

The default result comes from the Boer equation, published in 1984 in the American Journal of Physiology. Boer fitted lean mass from weight and height to normalize body fluid volumes across people of different sizes, and the resulting pair of lines is still one of the most widely quoted estimates.

W is body weight in kilograms and H is height in centimetres, so imperial input has to be converted before any of the three runs. The men and women versions differ because the average body composition differs, mostly in the fat fraction and in how much of the weight sits in muscle.

The Boer form is a plain plane: a fixed slice of every kilogram plus a fixed slice of every centimetre, minus a constant. That means the estimate grows steadily with weight, which behaves sensibly across the normal range. The original Boer paper is short and worth reading if you want the sample it came from: 66 healthy adults.

Worked example: 80 kg at 180 cm

Take a man who weighs 80 kg and stands 180 cm tall.

• 0.407 x 80 = 32.56 kg from the weight term
• 0.267 x 180 = 48.06 kg from the height term
• 32.56 + 48.06 - 19.2 = 61.42 kg of lean mass

The remainder is fat: 80 - 61.42 = 18.58 kg, which works out to 23.2 percent body fat. Divide lean mass by height in metres squared and you get a fat-free mass index of 18.96, close to the median for young men in a Swiss reference sample.

Run the same body through the other two equations and the answer moves. James returns 62.72 kg and Hume returns 57.79 kg, a spread of 4.93 kg or about 10.9 lb. Nothing went wrong: the three equations were fitted to different samples with different measurement methods, and each carries its own bias.

The same arithmetic for a woman of 60 kg and 165 cm gives 44.87 kg of lean mass from Boer, 44.63 kg from James and 43.44 kg from Hume. Her fat mass is 15.13 kg, or 25.2 percent, and her fat-free mass index is 16.48.

Pounds and feet: 180 lb at 5 ft 10 in

Imperial input needs two conversions, and the second one catches people out because height arrives as feet plus inches rather than a single number.

180 x 0.45359237 = 81.6466 kg for the weight. For the height, 5 ft 10 in is 70 in, and 70 x 2.54 = 177.8 cm. Feed those in: 0.407 x 81.6466 = 33.23, and 0.267 x 177.8 = 47.47. Add them and subtract the constant, and lean mass is 61.50 kg.

Converting back, 61.50 / 0.45359237 = 135.59 lb of lean mass, leaving 44.41 lb of fat and an implied body fat percentage of 24.7. The calculator does both conversions for you and reports the answer in whichever unit you typed, but it always runs the equation in metric internally.

One warning on feet and inches: 5 ft 10 in is not 5.10 ft. Written as a decimal that is 5.833 ft, or 177.8 cm. Entering 5.1 ft gives 155.4 cm and understates lean mass by roughly 6 kg, which is a bigger error than the gap between the three equations.

Where the three formulas disagree

Boer, James and Hume are all regressions on height and weight, so they agree closely on average bodies and drift apart at the edges. The calculator shows all three so the spread is visible rather than hidden behind one number.

The last row is the reason to look past a single equation. James subtracts a term that grows with the square of weight divided by height, so at extreme weight it collapses: 39.71 kg of lean mass on a 220 kg body implies 82 percent fat, which is not a believable reading for most people at that size.

Boer and Hume stay linear and therefore keep rising with weight, which overstates lean mass in heavy people, since some of the extra weight is fat and water. In the normal range the three land within about 5 kg of each other, and that is the honest precision of any height and weight estimate.

If you know your body fat percentage

A measured body fat percentage beats all three equations, because it uses information the equations do not have. The arithmetic is one line: lean mass equals weight times one minus body fat divided by 100.

For 180 lb at 22 percent body fat: 180 x 0.78 = 140.4 lb of lean mass and 39.6 lb of fat. Compare that with the 135.59 lb the Boer equation guessed for the same height and weight. The 4.8 lb gap is the equation's error, not a change in the body.

The catch is measurement quality. A DEXA scan or a properly run hydrostatic weighing gives a number worth using. A smart scale using bioelectrical impedance can swing several points with hydration, recent exercise, or even the time of day, and a pair of calipers depends heavily on who is holding them. A single impedance reading is roughly as trustworthy as the equations, so do not treat a 1 percent change as real.

If you have a body fat number you trust, enter it and the calculator will use it in place of Boer for the lean mass line while still showing the three equations for comparison.

Normalizing for height: FFMI

Lean mass alone cannot compare a 5 ft 4 in person with a 6 ft 4 in person, since the taller body carries more of everything. Dividing lean mass by height in metres squared gives the fat-free mass index, or FFMI, which is the lean counterpart of BMI.

For the 80 kg, 180 cm man: 61.42 / 1.8 squared = 61.42 / 3.24 = 18.96. The 60 kg, 165 cm woman comes out at 16.48. In a Swiss sample of 5,635 adults, the median FFMI for 18 to 34 year olds was 18.9 in men and 15.4 in women, with a median fat mass index of 4.0 and 5.5 respectively. Later work on race and ethnic differences shows the same index running higher in African American adults and lower in Asian adults at the same age.

FFMI is a comparison tool, not a diagnosis. Values well above the median usually mean more muscle, more fluid, or an estimate that has drifted, and the equations cannot tell you which. A trained lifter with a genuine FFMI near 22 and a person carrying 10 kg of edema can post the same number.

What the number is good for, and where it fails

The practical use is tracking direction. If your weight is flat but your estimated lean mass is falling, more of the loss is muscle than the scale suggests, and the same weight at the end of a diet is not the same body. Re-run the estimate every four to six weeks with the same method, and read the trend rather than any single figure.

The equations assume a standard adult skeleton and a typical fat distribution. They are poor fits for people with a missing limb, for anyone under about 18 whose body is still changing, for pregnancy, and for high body weights where the linear forms overstate lean mass. Boer and Hume cannot see whether the weight is muscle, fat or swelling, and no height and weight formula can.

Hydration matters more than most people expect. A 2 percent swing in body water moves a 70 kg person by 1.4 kg of estimated lean mass with no change in muscle at all, so weigh yourself under similar conditions each time.

None of this is medical advice. If you are tracking lean mass around an illness, surgery, or a medication that changes body composition, take the number to a clinician who can measure rather than estimate. The CDC adult BMI calculator is the right companion check, since BMI and FFMI together say more than either one alone.