Calculate Your Lean Body Mass
Enter your total body weight and body fat percentage to calculate your lean body mass.
Lean Body Mass Formula
LBM = total body weight - (total body weight × body fat percentage)
Or: LBM = total body weight × (1 - body fat percentage)
Example: 70 kg body weight, 15% body fat → LBM = 70 × (1 - 0.15) = 59.5 kg
Why Track Lean Body Mass?
- More accurate protein needs: Protein targets are based on LBM, not total body weight. See Protein Calculator.
- Better progress tracking: Weight loss should be fat loss, not muscle loss. Tracking LBM ensures you are preserving muscle.
- Metabolic rate: LBM is the primary determinant of resting metabolic rate (RMR). Higher LBM = higher metabolism.
- Body recomposition: You can gain muscle and lose fat simultaneously (especially as a beginner), even if total weight stays the same.
How to Measure Body Fat Percentage
To calculate LBM, you need an estimate of body fat percentage. Methods include:
- Body fat calipers (skinfold) — inexpensive, moderate accuracy
- Bioelectrical impedance (BIA scales) — convenient, but accuracy varies
- DEXA scan — most accurate, but expensive
- US Navy method — uses waist, neck, and height measurements. See Body Fat Calculator.
References
- Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. Am J Physiol. 1984;247(4 Pt 2):F632-6.
- Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-7.
Frequently asked questions
What is lean body mass?
Lean body mass (LBM) is your total body weight minus fat mass. LBM includes muscle, bone, organs, water, and connective tissue. LBM is often used interchangeably with "fat-free mass" (FFM).
How do you calculate lean body mass?
LBM = total body weight - fat mass. Fat mass = total body weight × (body fat percentage / 100). For example, if you weigh 70 kg and have 15% body fat: fat mass = 70 × 0.15 = 10.5 kg. LBM = 70 - 10.5 = 59.5 kg.
Why is lean body mass important?
LBM is a better indicator of metabolic health and fitness than total body weight. People with higher LBM burn more calories at rest (higher BMR). LBM is used to calculate protein needs and medication dosing. Preserving LBM during weight loss is critical for long-term success.