Lean Body Mass Calculator

Lean body mass (LBM) is everything in your body except fat — muscles, bones, organs, water, and connective tissue. Knowing your LBM is essential for setting accurate protein and calorie targets, since metabolic rate and nutritional needs scale with lean mass rather than total weight. Athletes use it to track muscle gain and fat loss separately, avoiding the confusion of a scale that stays flat while body composition improves. Personal trainers, dietitians, and sports coaches rely on LBM to personalize supplement protocols and training loads. It also helps evaluate health risks — someone at the same total weight can have vastly different LBM and therefore very different health profiles. Calculating it requires only total weight and body fat percentage, making it accessible to anyone with a reliable body fat measurement.

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.