Lean Body Mass & FFMI Calculator
Enter weight, height and body fat percentage to get your lean body mass in kg/lb and your FFMI — the muscle-per-height score.
What is lean body mass?
Lean body mass (LBM) is everything except fat: muscle, bone, organs and water. If you weigh 80 kg at 20% body fat, LBM = 80 × 0.80 = 64 kg. It is the number protein and calorie needs really scale with, which is why the protein calculator can work from it.
FFMI chart: natural ranges for men and women
FFMI = fat-free mass (kg) ÷ height (m)², usually normalized to 1.8 m. Reference ranges from body-composition studies: men — average about 18–20, well-trained 21–23, exceptional 24–25; women — average about 15–16, well-trained 17–19, exceptional 20–21. Values above about 25 (men) are rare without pharmacological help, which is why FFMI is used in research on drug use in sport.
Lean body mass formulas (Boer, James, Hume)
When you do not know your body fat, LBM can be estimated from height and weight. Boer (1984): men 0.407×weight + 0.267×height(cm) − 19.2; women 0.252×weight + 0.473×height − 48.3. James and Hume are similar regression formulas from other populations. Estimates from body fat percentage are more accurate; use the formulas only as a fallback.
Lean body mass vs skeletal muscle mass
Skeletal muscle is only part of LBM — roughly 40–50% in adults; the rest is water, bone and organs. So an LBM of 60 kg does not mean 60 kg of muscle. Smart scales that report “muscle mass” estimate it from impedance and can vary several kilograms day to day.
Sources
- Kouri EM et al. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med 1995 (PubMed 7496846) — https://pubmed.ncbi.nlm.nih.gov/7496846/
- Boer P. Estimated lean body mass as an index for normalization of body fluid volumes. Am J Physiol 1984 (PubMed 6496691) — https://pubmed.ncbi.nlm.nih.gov/6496691/
- ISSN Position Stand: protein, 2017 — https://pubmed.ncbi.nlm.nih.gov/28642676/
This calculator is educational and not medical advice. If you have a health condition, are pregnant, or are under 18, ask a clinician before changing your diet.
What the result tells you
Your lean body mass and fat-free mass index — a way to judge muscularity relative to height, adjusted for body fat.
How to use it
- Enter an accurate body-fat percentage — the result depends on it.
- Use the same body-fat method each time so the trend is comparable.
- Compare FFMI over months of training, not week to week.
Recommendations
- Normalized FFMI adjusts for height, so tall and short lifters compare fairly.
- Drug-free trained lifters usually top out around FFMI 25.
- Lean mass includes muscle, bone, and water — not muscle alone.
Health & wellbeing
FFMI is a rough index, not a measure of health or worth. Don't chase an extreme number.
Troubleshooting
Result seems high or low? It hinges on your body-fat input — an inaccurate percentage throws off both LBM and FFMI.
How it’s calculated
Lean mass is your weight minus fat, then FFMI expresses it relative to height, with a height normalisation.
Lean mass(kg) = weight × (1 − body fat% ÷ 100). FFMI = lean mass ÷ height(m)². Normalized FFMI = FFMI + 6.1 × (1.80 − height m).
Source: Kouri EM, et al., Clinical Journal of Sport Medicine, 1995 (FFMI normalisation).
Common questions
What is a good FFMI?
Around 20 for men and 16–17 for women signals good muscle development for a non-athlete; higher is better for strength sports.
Can FFMI be used for women?
Yes, with the lower reference ranges above.
Does FFMI change with weight loss?
It falls if you lose muscle in a deficit; keeping protein high and training preserves it.