How it works
This calculator applies the Boer formula, a regression equation that predicts lean body mass from height, weight, and biological sex. It separates your total body weight into two components: lean mass (muscle, bone, organs, water) and fat mass. The formula was developed from body composition studies and is widely used in fitness, clinical nutrition, and research settings.
You enter your weight, height, and gender in either metric or US units. The calculator then estimates your lean body mass, and subtracts it from your total weight to derive fat mass and fat percentage.
The formula
LBM (kg) = 0.407 × weight (kg) + 0.267 × height (cm) − 19.2 (males) or LBM (kg) = 0.252 × weight (kg) + 0.473 × height (cm) − 48.8 (females)
Once LBM is calculated, fat mass is simply: Fat mass = Total weight − LBM
And fat percentage: Fat % = (Fat mass ÷ Total weight) × 100``
Worked example
Scenario: A 30-year-old male, 180 cm tall, weighing 82 kg.
Step 1: Apply the male Boer formula:
- LBM = (0.407 × 82) + (0.267 × 180) − 19.2
- LBM = 33.37 + 48.06 − 19.2
- LBM = 62.23 kg
Step 2: Calculate fat mass:
- Fat mass = 82 − 62.23 = 19.77 kg
Step 3: Calculate fat percentage:
- Fat % = (19.77 ÷ 82) × 100 = 24.1%
Interpretation: This person has about 62 kg of lean mass and carries roughly 24% body fat—a healthy range for an adult male.
US units example: If the same person is 5'11" (180 cm) and 181 lb (82 kg):
- The calculator converts to metric internally, then returns results in both lb and kg.
- LBM ≈ 137 lb; Fat mass ≈ 44 lb; Fat % ≈ 24%
Common mistakes
Underestimating height or overestimating weight will artificially lower your LBM estimate. Measure height without shoes, on a flat surface, and weigh yourself at the same time each day (morning, after bathroom, before eating).
Confusing LBM with "muscle mass" is common. Lean body mass includes all non-fat tissue—bone, organs, water, connective tissue, and muscle. Actual skeletal muscle is typically 30–40% of LBM.
Relying on one estimate without context: The Boer formula is a population average. Athletes, elderly individuals, and people with unusual body composition may fall outside the ±5–10% accuracy range. Use this as a screening tool, not a clinical diagnosis.
Note: This calculator provides an estimate, not professional medical or fitness advice. For personalized body composition assessment, training, or nutrition guidance, consult a physician, registered dietitian, or certified fitness professional.