Body weight alone does not tell you what that weight is made of. Lean body mass (LBM) separates out everything that is not stored fat. Doctors use it to dose some medications, coaches use it to set protein and calorie targets, and anyone losing weight can use it to check that the loss is mostly fat. This calculator gives three published estimates from height and weight, plus a direct figure if you know your body fat percentage.
How to use the lean body mass calculator
- Choose US or metric units and select your sex.
- Enter your height and weight.
- Optionally add a body fat percentage from a DEXA scan, skinfold calipers or a smart scale.
- Compare the Boer, James and Hume results, the implied body fat for each, and the measured-fat figure if you entered one.
Lean body mass formulas
All three equations use weight W in kilograms and height H in centimeters.
Boer (1984)
Women: 0.252W + 0.473H − 48.3
James (1976)
Women: 1.07W − 148(W/H)²
Hume (1966)
Women: 0.29569W + 0.41813H − 43.2933
From measured body fat
Worked example
A man who is 180 cm tall and weighs 85 kg:
Boer: 0.407 × 85 + 0.267 × 180 − 19.2 = 34.6 + 48.1 − 19.2 = 63.5 kg
James: 1.1 × 85 − 128 × (85 ÷ 180)² = 93.5 − 28.5 = 65.0 kg
Hume: 0.3281 × 85 + 0.33929 × 180 − 29.5336 = 59.4 kg
Boer's estimate implies 21.5 kg of fat mass, or about 25% body fat.
The spread of about 5.5 kg between Hume and James is typical and shows why a single number should be read as an estimate.
Why the formulas disagree
Each equation was fitted to a different group of people with a different reference method. Hume’s data came from measurements of total body water; James’s formula was built for a UK government report and has a quadratic term that eventually turns downward, so at a BMI around 35 and above it starts to underestimate lean mass badly. Boer’s equations are often preferred in pharmacology because they behave sensibly in larger patients.
None of them account for training history. A muscular athlete and a sedentary person with the same height and weight get the same answer, even though their real lean mass could differ by 10 kg or more. If you know your body fat percentage from a reliable method, the direct calculation is usually more accurate for you personally.
Using your lean body mass
- Protein: some coaches set protein per kilogram of lean mass rather than total weight; the protein intake calculator covers both approaches.
- Metabolism: lean mass drives most resting energy use. Enter your body fat in the BMR calculator to get the Katch–McArdle estimate.
- Progress: when you lose weight, re-measure body fat and compare lean mass before and after. Losing mostly fat while lean mass holds steady is the goal.
To estimate body fat from tape measurements, try the body fat calculator. For drug dosing that uses total body size, see the body surface area calculator.
Informational only — not medical advice. Medication doses that depend on lean body mass must be set by a clinician or pharmacist.
Frequently asked questions
What counts as lean body mass?
Lean body mass is everything except stored fat: muscle, bone, organs, skin, blood and body water. It includes a small amount of essential fat inside organs and nerves, which is why it is slightly larger than strict fat-free mass.
Which lean body mass formula should I use?
Boer's formula is the primary result here because it performed well in later validation work and stays sensible across a wide range of body sizes. Hume's tends to give lower values. James's formula is fine at average weights but underestimates lean mass at high BMI.
Why are the formula results different from my smart scale?
The formulas only know your height, weight and sex, so they assume average body composition. Bioimpedance scales estimate body fat from electrical resistance, which shifts with hydration. Both carry errors of several percentage points.
How is lean body mass used?
Clinicians use it to dose some drugs and anesthetics, dietitians use it to set protein targets, and the Katch–McArdle equation uses it to estimate resting metabolic rate. Tracking it over time also shows whether weight loss is coming from fat or muscle.