About the lean body mass calculation
Lean body mass is everything except fat: muscle, bone, organs, connective tissue and water. It is the number that actually drives your metabolic rate, and preserving it is the entire point of eating enough protein and lifting while dieting.
Formula
Boer (1984):
Men: LBM = 0.407×weight(kg) + 0.267×height(cm) − 19.2
Women: LBM = 0.252×weight(kg) + 0.473×height(cm) − 48.3Worked example
A man 175 cm and 70 kg: (0.407×70) + (0.267×175) − 19.2 = 28.49 + 46.73 − 19.2 = 56.0 kg lean.
Frequently asked questions
How is this different from body fat percentage?
They are two views of the same split. Lean mass is the absolute weight of non-fat tissue in kg; body fat percentage is the proportion that is fat. Lean mass is the more useful of the two to track while dieting, because it should stay flat while total weight falls.
What is FFMI and why does it matter?
Fat-Free Mass Index normalises lean mass for height, exactly as BMI does for total weight. It lets you compare muscularity between people of different heights. Values near 25 for men are around the ceiling reported in drug-free populations.
Can I increase lean mass while losing fat?
Yes, though slowly, and mainly if you are new to training, returning after a break, or carrying substantial excess fat. It requires adequate protein and progressive resistance training. Experienced lifters in a deficit should aim to maintain rather than gain.
Sources
- Boer P — Estimated lean body mass as an index for normalization (1984)