FFMI Calculator guide
Enter height, weight, and body fat % to get your FFMI, the height-adjusted score from Kouri's 1995 study, and your fat-free and fat mass.
BMI for people who lift
BMI can't tell muscle from fat. A 5'10", 200 lb lifter at 12% body fat and a 5'10", 200 lb office worker at 32% both score 28.7, "overweight." Fat-free mass index (FFMI) fixes that by removing fat from the equation. It divides only your fat-free mass by height squared, so it measures how much lean tissue you carry for your frame.
That makes it the go-to number for tracking muscle gain over years and for the internet's favorite argument: how much muscle is realistic without drugs.
The formula, and the height correction
Step 1: fat-free mass (kg) = weight (kg) x (1 - body fat % / 100).
Step 2: FFMI = fat-free mass (kg) / height (m)².
Step 3: normalized FFMI = FFMI + 6.3 x (1.8 - height in m).
The third step comes from Kouri, Pope, Katz, and Oliva, published in the Clinical Journal of Sport Medicine in 1995. Taller people naturally score slightly lower FFMI at the same level of development, so the authors added a small correction that adjusts everyone to what they'd score at 1.8 m (about 5'11"). Their abstract gives the factor as 6.3; many calculators use 6.1. The difference between the two is less than 0.1 FFMI point for almost everyone, so don't lose sleep over it.
Worked example
A lifter, 5'10" (1.778 m), 180 lb (81.6 kg), 15% body fat.
Fat-free mass = 81.6 x 0.85 = 69.4 kg (153 lb). FFMI = 69.4 / 1.778² = 69.4 / 3.161 = 21.95. Normalized = 21.95 + 6.3 x (1.8 - 1.778) = 21.95 + 0.14 = 22.1.
A shorter, heavier example: 5'6" (1.676 m), 170 lb, 12% body fat. Fat-free mass = 67.9 kg. FFMI = 24.15. Normalized = 24.15 + 6.3 x 0.124 = 24.9. The correction adds nearly a full point because he's well under 1.8 m.
The "natural limit" of 25
Kouri's team measured 157 male athletes: 83 who admitted to using anabolic steroids and 74 who didn't. The drug-free group topped out at a normalized FFMI of about 25. Many steroid users were above that.
Read that carefully. It's the highest value seen in one sample of drug-free men from the early 1990s, not a law of biology or a drug test. A small number of drug-free people with good genetics and years of training exceed 25, and body fat measurement error alone can move a score by a point. Scoring 26 is not proof of anything; scoring 22 is not a sign you're failing. Plenty of men who train consistently for years land in the low 20s.
The study included only men, so there is no equivalent benchmark for women. Women's FFMI runs a few points lower on average, and you should compare against your own history.
Common mistakes
Garbage body fat in, garbage FFMI out. The whole index hangs on your body fat %. A bathroom bioimpedance scale can be off by 5 points or more, which shifts FFMI by roughly 1-1.5. Use the same method every time, ideally DEXA or careful skinfolds, and look at the trend.
Underestimating body fat. Most people guess themselves leaner than they are. If you think you're 10% but you're really 16%, your FFMI is inflated by more than a point.
Measuring while bloated or dehydrated. Water and glycogen count as fat-free mass. Weigh in the morning, same conditions each time.
Using FFMI as a health marker. A high FFMI doesn't mean healthy, and a modest one doesn't mean unhealthy. It's a muscularity measure, nothing more.
Chasing the number with drugs. Anabolic steroids and SARMs carry real risks to the heart, liver, fertility, and hormones. If you're thinking about them, talk to a doctor first.
How we calculate: sources
Frequently asked questions
How do you calculate FFMI?
Fat-free mass (kg) = weight x (1 - body fat %). FFMI = fat-free mass / height in meters squared. Normalized FFMI adds 6.3 x (1.8 - height in m) to adjust to a 1.8 m person.
What is a good FFMI?
It depends on sex, training age, and genetics. Plenty of men who train for years land in the low 20s. Compare against your own trend more than against others.
Is an FFMI of 25 the natural limit?
In Kouri's 1995 study of 157 male athletes, drug-free lifters topped out around a normalized FFMI of 25. It's a rough ceiling from one sample, not proof: some drug-free people exceed it.
What's the difference between FFMI and normalized FFMI?
Taller people score slightly lower FFMI at the same development. Normalized FFMI corrects for that, adjusting everyone to 1.8 m (about 5'11"). Shorter people get a small boost, taller people a small cut.
Why do some calculators use 6.1 instead of 6.3?
Kouri's abstract uses 6.3 x (1.8 - height). Many websites use 6.1. The gap is under 0.1 point for almost every height, so it doesn't change any conclusion.
How accurate is FFMI?
Only as accurate as your body fat %. A 5-point body fat error shifts FFMI by roughly 1-1.5 points. Use the same measuring method each time, ideally DEXA or careful skinfolds.
Does FFMI work for women?
The formula works, but Kouri's study included only men, so the 25 benchmark doesn't apply. Women's FFMI typically runs a few points lower.
Is my data uploaded?
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