Body composition · 6 min read
Lean Body Mass and FFMI: How to Calculate Both
Your weight is two numbers pretending to be one: fat mass and everything else. Splitting them takes a single line of arithmetic once you have a body fat estimate, and it unlocks a better protein target, a better BMR formula, and a fair way to judge how much muscle you actually carry.
Lean body mass in one line
Lean body mass (LBM), also called fat-free mass, is everything that is not fat: muscle, bone, organs, connective tissue and the water in them. Once you have a body fat percentage, the calculation is trivial.
Lean body mass (kg) = weight − fat mass
Worked example. An 82 kg man at 18% body fat: fat mass = 82 × 0.18 = 14.8 kg, so lean body mass = 82 − 14.8 = 67.2 kg. If he diets to 74 kg at 12% body fat, his fat mass is 8.9 kg and his lean mass is 65.1 kg. He lost 5.9 kg of fat and 2.1 kg of lean mass, and that second number is what tells him whether the diet was well run.
Why lean mass is the more useful number
Protein targets. Protein requirements track lean mass, not total weight. For someone carrying a lot of fat, a target set per kg of body weight overshoots badly. Setting it per kg of lean mass avoids that: reviews of protein intake in energy-restricted, resistance-trained athletes support roughly 2.3 to 3.1 g per kg of fat-free mass during a cut.4 See protein per kg by goal for the full breakdown.
A better BMR estimate. Height-weight formulas such as Mifflin-St Jeor cannot tell muscle from fat, so they underestimate BMR in muscular people and overestimate it in people with high body fat. The Katch-McArdle equation uses lean mass directly and sidesteps the problem.3
For the 82 kg man above: 370 + (21.6 × 67.2) = 1,822 kcal. Multiply by an activity factor to get TDEE; the mechanics are worked through in how to calculate your TDEE by hand.
Judging a diet. Weight loss is only good news if most of it is fat. Tracking lean mass alongside weight is how you find out. If lean mass is falling fast, the deficit is too aggressive, protein is too low, or resistance training is missing: a meta-analysis found that an energy deficit impairs lean mass gains during resistance training even when strength is maintained.5
FFMI: lean mass adjusted for height
Sixty-five kilograms of lean mass means something very different on a 1.65 m frame than on a 1.92 m frame. Fat-free mass index normalises for height exactly the way BMI does, but using only the lean part of your body.
A commonly applied adjustment normalises everyone to a 1.83 m reference height, which reduces the residual height bias in taller and shorter lifters:
Worked example. The same man, 1.78 m with 67.2 kg of lean mass: FFMI = 67.2 ÷ 1.78² = 67.2 ÷ 3.168 = 21.2. Normalised: 21.2 + 6.1 × (1.83 − 1.78) = 21.5. That is a visibly fit, well-trained physique: several years of consistent lifting, not a beginner.
How to read your FFMI
| FFMI (men) | Interpretation |
|---|---|
| Below 18 | Below average muscle mass for an adult male |
| 18–20 | Average; typical of an untrained adult |
| 20–22 | Visibly fit; a year or more of consistent training |
| 22–25 | Very muscular; years of training, often with favourable genetics |
| Above 25 | Rare in drug-free lifters |
Women read the same scale roughly 1.5 to 2 points lower. Population percentile data across a broad age range gives context for both sexes.2
The 25 ceiling, and what it really means
The figure comes from a 1995 study that computed FFMI in steroid users and non-users and found drug-free lifters clustered below about 25, while users routinely exceeded it.1 It is a genuinely interesting observation and it is routinely over-read. It was one sample, of one population, using one body fat method, and a handful of documented natural athletes sit above it. Treat 25 as a rough marker of where drug-free physiques become uncommon, not as a physiological wall.
The error you inherit
Every number on this page is downstream of your body fat estimate, so it inherits that estimate's error. A body fat reading that is off by 4 percentage points moves lean mass by more than 3 kg on an 82 kg frame, and FFMI by about a point. Use one method consistently, track the direction of travel over months, and do not treat a one-decimal FFMI as precise. The measurement methods guide covers how much error each method carries.
The body fat calculator gives you the percentage. From there lean mass is a single multiplication, and the formulas below do the rest.
Get your body fat percentage first →Frequently asked questions
How do I calculate lean body mass?
Multiply your weight by your body fat percentage to get fat mass, then subtract that from your weight. An 82 kg person at 18% body fat has 14.8 kg of fat and 67.2 kg of lean mass. You need a body fat estimate first, from a tape measure, calipers or a scan.
What is a good FFMI?
For men, roughly 18 to 20 is average, 20 to 22 is visibly fit, and 22 to 25 indicates years of serious training. Women read the same scale about 1.5 to 2 points lower. The number is only as good as the body fat estimate behind it.
Is FFMI better than BMI?
For anyone who trains, yes, because it separates muscle from fat and does not penalise you for being muscular. It requires a body fat estimate, though, so it carries measurement error that BMI does not.
Should I set protein by body weight or lean mass?
Lean mass is more precise, particularly at higher body fat levels where a per-kilogram-of-bodyweight target can overshoot considerably. If you have no body fat estimate, using total body weight is a reasonable fallback for people in a normal body fat range.
References
- Kouri EM, Pope HG, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med. 1995;5(4):223-228. doi:10.1097/00042752-199510000-00003
- Schutz Y, Kyle UUG, Pichard C. Fat-free mass index and fat mass index percentiles in Caucasians aged 18-98 y. Int J Obes. 2002;26(7):953-960. doi:10.1038/sj.ijo.0802037
- Katch FI, McArdle WD. Prediction of body density from simple anthropometric measurements in college-age men and women. Hum Biol. 1973;45(3):445-455.
- Helms ER, Zinn C, Rowlands DS, Brown SR. A systematic review of dietary protein during caloric restriction in resistance trained lean athletes. Int J Sport Nutr Exerc Metab. 2014;24(2):127-138. doi:10.1123/ijsnem.2013-0054
- Murphy C, Koehler K. Energy deficiency impairs resistance training gains in lean mass but not strength: a meta-analysis and meta-regression. Scand J Med Sci Sports. 2022;32(1):125-137. doi:10.1111/sms.14075