BCBetter Calculators

Lean Body Mass Calculator

Calculate your lean body mass and fat mass from your weight and body fat percentage.

kg
%
🧮

Enter your values and click Calculate

How It Works

The calculation uses two formulas in sequence. First, fat mass (kg) = total body weight × (body fat % ÷ 100). For example, an 80 kg person at 20% body fat has 80 × 0.20 = 16 kg of fat. Second, lean body mass = total body weight − fat mass, so 80 − 16 = 64 kg. This direct method is accurate when the body fat percentage input is reliable. DEXA scans and hydrostatic weighing are the gold standards, with error margins under 2%. Bioelectrical impedance devices are convenient but can vary ±3–5% depending on hydration. Visual estimates carry the most uncertainty. The formula itself contains no rounding — results are as accurate as the body fat input you provide.

Examples

80 kg person at 20% body fat
A common starting point for a moderately lean adult.
Result: 64 kg lean body mass, 16 kg fat mass.
65 kg person at 28% body fat
A higher body fat percentage scenario.
Result: 46.8 kg lean body mass, 18.2 kg fat mass.
100 kg person at 15% body fat
A heavier but relatively lean athlete tracking body composition.
Result: 85 kg lean body mass, 15 kg fat mass.

Frequently Asked Questions

Why does lean body mass matter?
LBM is a better predictor of metabolic rate than total weight. It also helps set protein targets — most recommendations are 1.6–2.2g of protein per kg of lean body mass for active people.
How do I measure body fat percentage accurately?
DEXA scans are the gold standard. Hydrostatic weighing and Bod Pod are also accurate. Bioimpedance scales are convenient but can vary by ±3–5%. Skinfold calipers with a skilled technician are a good middle ground.
Can lean body mass include fat?
No. By definition, LBM = total mass − fat mass. It includes muscle, bone, water, organs, and connective tissue — everything except stored fat.

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