Lean Body Mass Calculator (Boer)

Lean body mass (LBM) is an estimate of everything in your body except fat: muscle, bone, organs, connective tissue, and water. This calculator uses the Boer 1984 equations, which estimate lean mass from sex, height, and weight, and then derives the two numbers most people actually track: fat mass and body fat percentage.

The calculation is fully transparent: enter your sex, height in centimeters, and weight in kilograms, and you get lean body mass, fat mass, and body fat percentage in one step. The formulas were published by Boer in the American Journal of Physiology as an index for normalizing body fluid volumes and have since become one of the commonly cited height-and-weight lean mass equations. No tape measures, calipers, or scales beyond weight are required.

What the calculator shows

The primary result is lean body mass (Boer) in kilograms. Two supporting readouts follow:

• Fat mass — entered weight minus estimated lean mass, in kilograms.
• Body fat — fat mass divided by weight, shown as a percentage.

For a 175 cm, 70 kg man the result is 56.0 kg of lean mass, 14.0 kg of fat mass, and 20.0% body fat. Because the equation is linear in weight and height, changing one input changes the estimate proportionally — there is no magic threshold in the math.

How the formula works

The Boer equations are different for men and women because average body composition differs between the sexes. Both take weight in kilograms and height in centimeters. The calculator then subtracts the lean mass estimate from the entered weight to get fat mass, and divides fat mass by weight to get the body fat percentage.

If the estimated lean mass exceeds the entered weight — which can happen at extreme combinations such as a very tall height with a very low weight — the formula is outside its usable range. In that case fat mass is clamped to zero and a note explains why, rather than showing a negative fat mass.

Formula

For weight W in kg and height H in cm:

The derived values are:

Worked example

Take a man who is 175 cm tall and weighs 70 kg. His lean body mass is:

The calculator rounds this to 56.0 kg. Fat mass is:

Rounded to 14.0 kg. Body fat percentage is:

Rounded to 20.0%. For a 165 cm, 60 kg woman, the equations give 0.252 × 60 + 0.473 × 165 − 48.3 = 44.865 kg of lean mass, 15.1 kg of fat mass, and 25.2% body fat.

Interpreting the results

Boer’s paper estimated lean body mass as part of normalizing body fluid volumes in a healthy research population. That origin matters: the equation was fitted to people with typical body compositions, so it works best for similar adults and is a rough approximation elsewhere. As the StatPearls “Anthropometric Measurement” chapter explains, body-composition estimates vary with age, sex, and ethnicity, and methods like skinfold calipers, bioelectrical impedance, or DXA scans measure or estimate body composition more directly than a height-and-weight equation. The IARC handbook on body fatness makes a similar point: BMI and simple anthropometric measures are indirect, and direct measures of fatness are needed for many research and clinical questions.

Practical implications:

• Trends matter more than single readings. The estimate responds linearly to weight changes, so it is reasonable for tracking direction, but the absolute percentage should not be treated as a measurement.
• Athletic and older bodies break the assumptions. Large amounts of muscle inflate lean mass beyond what the equation expects, and age-related muscle loss is not captured at all.
• It is not a health verdict. Body fat percentage from any estimate is one input among many; the BMI calculator puts weight in a height-relative context, the body fat estimate calculator uses tape-measure input instead of height-and-weight only, and the BMR calculator turns the same inputs into energy estimates. If you track protein for training, the protein reference calculator shows how lean mass relates to protein needs.

Tips

• Weigh yourself at a consistent time of day, ideally in the morning, so hydration and meals do not distort the input weight.
• Use the same height each time. Height enters the equation with a large coefficient (0.267 for men), so a 2 cm measurement difference moves the estimate by about 0.5 kg.
• Compare percentage changes over weeks, not the single number itself. A change of a few percentage points between different methods or days is usually noise, not a real body-composition change.

Common mistakes

• Treating the estimate as a measured value. A height-and-weight equation cannot see muscle, bone density, or water retention.
• Comparing this result directly with a bioelectrical impedance scale and expecting them to agree; each method has its own assumptions.
• Ignoring the out-of-range case. If lean mass comes back above your entered weight, the formula is not applicable to that body type, not proof of zero body fat.
• Confusing lean body mass with ideal body weight. The ideal body weight calculator on this site answers a different question with a different formula.

Sources

• Boer P, Estimated lean body mass as an index for normalization of body fluid volumes in humans (American Journal of Physiology, 1984) — the source of the lean body mass equations used here.
• StatPearls, Anthropometric Measurement (NCBI Bookshelf) — body composition assessment methods and their variability and limits.
• IARC, Body fatness (IARC Handbooks of Cancer Prevention No. 16) — measuring body fatness and the limits of indirect measures such as BMI.