Lean Body Mass Calculator

What lean body mass is

Lean body mass (LBM) is everything in the body that is not stored fat: skeletal muscle, bone, organs, connective tissue, skin and the water they hold. It also includes a small amount of essential fat in the nervous system, bone marrow and organs, which is roughly 3 to 5 percent of body weight in men and 10 to 13 percent in women.

Lean mass matters because it drives resting energy use, strength and the way drug doses are scaled. Two people with the same weight can have very different lean mass, which is why scale weight alone is a poor guide to body composition. If you want a fat-focused view, use the body fat calculator.

How to use the calculator

Pick a unit system, enter your age, sex, weight and height, then add the circumference measurements: neck and waist for everyone, and hip as well for women. Use inches with US units and centimetres with metric units. The calculator returns:

• your estimated lean body mass in pounds or kilograms,
• the lean and fat shares of your weight as percentages,
• the lean mass index (LMI), measured in kg per square metre,
• and a percentile band for your LMI among adults of your sex and decade of age.

If the measurements do not make physical sense, for example a waist smaller than the neck or a result that implies zero body fat, the tool reports unrealistic measurements instead of a number. Re-measure and try again.

How to take the measurements

• Height: stand barefoot against a wall, look straight ahead and mark the top of your head with a flat object.
• Neck: wrap a soft tape just below the larynx (Adam's apple), sloping slightly down toward the front.
• Waist: measure around the abdomen at the level of the navel. Breathe out normally and keep the tape snug but not tight.
• Hip (women): measure around the widest part of the buttocks with your feet together.

Measure to the nearest half inch or centimetre, repeat each measurement twice and use the average. The estimate is sensitive to the waist in particular: in the example below, one extra centimetre at the waist raises estimated body fat by about three-quarters of a percentage point.

The formulas

The tool first estimates body fat percentage with the U.S. Navy circumference method, then keeps the lean share of your weight. The Navy equations use base-10 logarithms and come in an inch version and a centimetre version.

Inches (height, neck, waist and hip in inches):

men: body fat % = 86.01 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76

women: body fat % = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387

Centimetres:

men: body fat % = 495 ÷ (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450

women: body fat % = 495 ÷ (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)) − 450

Then the remaining quantities follow directly:

lean body mass = weight × (1 − body fat % ÷ 100)

LMI = lean body mass in kg ÷ (height in m)²

Worked examples

A man aged 40, 178 cm tall, 85 kg, neck 38 cm, waist 86 cm. The waist minus neck is 48 cm and log₁₀(48) = 1.6812. For the height, log₁₀(178) = 2.2504. The denominator is 1.0324 − 0.19077 × 1.6812 + 0.15456 × 2.2504 = 1.0595, so body fat is 495 ÷ 1.0595 − 450 = 17.2%. Lean body mass is 85 × (1 − 0.172) = 70.4 kg, which is 82.8% of his weight. His LMI is 70.4 ÷ 1.78² = 22.2 kg/m², a value above the 80th-percentile cut-off in the table below for men of his age.

A woman aged 28, 165 cm tall, 62 kg, neck 32 cm, waist 72 cm, hip 96 cm. Waist plus hip minus neck is 136 cm. The formula gives 26.4% body fat, so lean body mass is 62 × (1 − 0.264) = 45.6 kg (73.6% of her weight) and LMI is 45.6 ÷ 1.65² = 16.8 kg/m².

Reading the lean mass index

Lean mass index is to lean tissue what BMI is to total weight: it scales lean mass by height squared so that tall and short people can be compared. A higher LMI means more lean tissue relative to size. The table gives approximate reference values from DXA scans of Caucasian adults, grouped by age.

The percentile line in the results is an approximation, not an exact rank and not a table lookup. The calculator fits a bell-shaped curve through the 20th, 50th and 80th percentile cut-offs for your sex and age group, finds where your LMI falls on that curve and rounds to the nearest ten percent, so it never shows less than the 10th or more than the 90th percentile. The label names the decade of your age (ages under 20 are shown with the 20-29 band). The reference data describe Caucasian adults only, so treat the line as a rough guide, especially for other groups. For ages beyond the range of the data (80 and over) the tool shows a 0 percentile and the 70-79 band; do not read anything into that figure.

Lean mass, fat-free mass and muscle

These terms are often mixed up. Fat-free mass is body weight minus all extractable fat, essential fat included. Lean body mass keeps that essential fat inside it, so it is slightly larger. Muscle mass is only one component of both, next to bone, organs, ligaments and water. Saying "my lean mass went up 2 kg" does not mean 2 kg of muscle.

Accuracy and other methods

Tape-measure estimates are convenient but approximate. The standard error of the Navy method is about 3.5 percentage points of body fat, so for roughly two out of three people the true value lies within 3.5 points of the estimate and for about 95 percent it is within 7 points. Every error in body fat passes straight through to lean mass.

More accurate options include dual-energy X-ray absorptiometry (DXA), underwater weighing and air-displacement plethysmography. Bioelectrical impedance scales and skinfold callipers are cheaper but depend on hydration and technique. For military-style screening, the army body fat calculator applies a similar measurement approach.

Results are estimates for general information and are not a medical assessment. See our disclaimer.