Lean Body Mass Calculator
Estimate your lean body mass from weight and height using the Boer and James formulas, plus the fat mass and body-fat percentage that follow.
For a male at 80 kg and 180 cm, the Boer formula estimates 61.4 kg of lean mass — about 76.8% of body weight.
Lean body mass is everything in your body that is not fat — muscle, bone, organs, and water. For an 80 kg, 180 cm man the Boer formula gives about 61.4 kg of lean mass, leaving roughly 18.6 kg of fat (≈ 23.2% body fat). The James formula estimates a close 62.7 kg.
What lean body mass is
Lean body mass (LBM) is your total weight minus the mass of body fat. It bundles together muscle, bone, connective tissue, organs, and the water they hold. Because muscle is metabolically active while fat is largely storage, LBM tracks with strength, resting energy use, and general fitness far better than scale weight alone.
This calculator estimates LBM from just your sex, weight, and height using two established equations. The Boer formula is the common default and drives the headline result; the James formula is shown alongside as a cross-check. From the Boer estimate it also derives your fat mass (weight − LBM) and body-fat percentage.
Women use Boer 0.252 × kg + 0.473 × cm − 48.3, and James 1.07 × kg − 148 × (kg ÷ cm)². Weight is in kilograms and height in centimetres.
Worked example: male, 80 kg, 180 cm
Apply the Boer formula, then the James formula, then derive the fat figures:
- 1 Put weight in kg and height in cm. Metric input is used directly: 80 kg and 180 cm. In imperial, multiply pounds by 0.453592 and inches by 2.54 first.
- 2 Apply the Boer formula (men). 0.407 × 80 + 0.267 × 180 − 19.2 = 32.56 + 48.06 − 19.2 = 61.42 kg, so LBM ≈ 61.4 kg.
- 3 Cross-check with James (men). 1.1 × 80 − 128 × (80 ÷ 180)² = 88 − 128 × 0.1975 ≈ 62.7 kg — within about 1 kg of Boer.
- 4 Find fat mass. Fat mass = weight − LBM = 80 − 61.42 ≈ 18.6 kg.
- 5 Convert to body-fat percentage. 18.6 ÷ 80 × 100 ≈ 23.2%, the share of body weight that is fat.
Boer vs James at a glance
Both take weight in kg and height in cm. Boer is linear in height; James uses the weight-to-height ratio, so it drifts lower at higher body weights.
| Formula | Men | Women |
|---|---|---|
| Boer (1984) | 0.407·kg + 0.267·cm − 19.2 | 0.252·kg + 0.473·cm − 48.3 |
| James (1976) | 1.1·kg − 128·(kg ÷ cm)² | 1.07·kg − 148·(kg ÷ cm)² |
How to read the result
Boer and James usually agree within a kilogram or two for average builds, which is why showing both is a useful sanity check. They diverge at the extremes: because James subtracts a term that grows with the weight-to-height ratio, it estimates lower lean mass for heavier bodies, while Boer stays linear. Neither can tell muscle from water or account for an unusually light or heavy frame.
These are estimates from population formulas, not measurements of your body. A body-composition scan — DEXA, hydrostatic weighing, or a bioimpedance device — is more accurate. This tool is for general information, not medical advice; consult a qualified professional before making significant changes to training or diet.