FFMI Calculator

What FFMI measures

Fat-free mass index compares the weight you carry that is not fat against your height. The formula is FFMI = lean mass in kilograms / height in meters squared, and lean mass comes from body weight times (1 - body fat percentage / 100).

Take a man at 180 lb, 5 ft 11 in, with 15% body fat. His weight is 81.6 kg and his lean mass is 81.6 x 0.85 = 69.4 kg. His height is 1.803 meters, and squaring that gives 3.252. Divide and you get 69.4 / 3.252 = 21.3, so his FFMI is 21.3 kg per square meter.

BMI does the same division with total weight, which is why two men can share a BMI and still carry different amounts of muscle. FFMI is an attempt to isolate the lean part, and it is the number bodybuilding coaches quote when they argue about who is carrying more muscle at the same height.

The calculator takes pounds and feet or kilograms and centimeters, and reports lean mass, fat mass, raw FFMI, normalized FFMI, BMI and how your number sits against published reference groups.

Why height gets squared

Muscle, bone and fluid scale with body volume, and volume scales with the cube of height, so a strict physical comparison would divide lean mass by height cubed. Nobody does that, because the result is a small number that behaves badly at the edges. Dividing by height squared is the compromise BMI already made, and it keeps FFMI on the same familiar scale as BMI.

The compromise leaves a bias that the original FFMI paper documented. Kouri and colleagues measured the shortest nonuser in their sample at 1.59 m and one of the tallest at 1.93 m. The short man scored a raw FFMI of 23.5 and the tall man 25.4, yet on inspection the short man looked more muscular. Raw FFMI hands a bonus to tall lifters, because a taller body carries more lean mass without looking denser.

That matters when you compare yourself with someone four inches taller, or with a chart built from athletes of a different height. The raw number is still useful for tracking yourself over time, since your height does not change.

The height correction, step by step

Kouri's group fit a regression through the most muscular nonusers, the ones scoring 22 or higher, and came out with a slope of 6.1 kg per cubic meter. That slope drives the normalized version: normalized FFMI = FFMI + 6.1 x (1.8 - height in meters). The idea is to express everyone as if they were 1.8 m tall.

Run it on a 5 ft 6 in lifter who weighs 150 lb at 12% body fat. His lean mass is 59.9 kg, his height is 1.6764 m, and 59.9 / 2.810 = 21.3. The correction is 6.1 x (1.8 - 1.6764) = 0.754, so his normalized FFMI is 21.3 + 0.754 = 22.1.

The man at 5 ft 11 in gets almost nothing: 6.1 x (1.8 - 1.8034) = -0.021, which leaves him at 21.3. Two lifters who look different in the mirror now sit 0.8 points apart in the direction the eye expects, which is the whole point of the adjustment.

The calculator shows both numbers, because the raw value is what appears in most charts and the normalized value is what the 1995 study used for its cutoff.

Reference numbers worth knowing

Two studies cover most of the useful ground. Schutz, Kyle and Pichard measured 5,635 Swiss adults and reported median FFMI values of 18.9 for men and 15.4 for women in the 18 to 34 age group (International Journal of Obesity, 2002, PubMed record). Kouri and colleagues measured 157 male athletes, 83 of them steroid users, and published the cutoff that made FFMI famous (Clinical Journal of Sport Medicine, 1995, PubMed record).

A reading of 21.3 sits well above the population median for men and just under the mean of a group of dedicated natural athletes. That is a useful frame: most people who lift seriously land in the low twenties, and the high twenties is a small club with a specific membership history.

Body fat percentage is the weak link

FFMI is only as good as the body fat number you feed it, and body fat is the hardest thing on the page to measure. At 180 lb and 5 ft 11 in, every 3 points of body fat moves FFMI by about 0.75, so the five rows below cover a spread of 3 points.

Different methods disagree by more than that. Calipers depend on the person holding them, bioimpedance scales depend on hydration, and DEXA is a snapshot from one machine on one day. None of them is wrong enough to make FFMI useless, but all of them are loose enough that a 0.5 point difference between two people means nothing.

Pick one method and use it the same way each time. A trend measured with the same tape, at the same time of day, tells you more than a single reading from a more expensive machine.

FFMI and BMI side by side

Two men at 5 ft 11 in and 180 lb both carry a BMI of 25.1, since 703 x 180 / 5,041 = 25.1. One has 15% body fat and the other 25%. The first carries 153.0 lb of lean mass, the second 135.0 lb, and their FFMI values are 21.3 and 18.8.

That gap is the reason FFMI exists, and it is also the reason BMI has not gone anywhere. BMI needs a scale and a tape measure, costs nothing, and has decades of health outcome data behind it. The CDC describes BMI as a screening measure that should be read alongside blood pressure, cholesterol and a physical exam (CDC, 2025), which is a fair summary of what it can and cannot do.

FFMI needs a body fat estimate, which costs either money or a bit of skill with calipers. Use it when composition matters more than weight status, and use BMI when you want the cheap population-level screen.

What it takes to reach 25

The number that gets quoted most is 25, and it is worth seeing what it asks for. A man at 5 ft 11 in needs a raw FFMI of 25.02 to reach a normalized 25.0, since the correction for his height is a wash. Multiply by his height squared and he needs 81.4 kg of lean mass, or 179.4 lb. At 12% body fat that is a body weight of 203.9 lb.

Read the table as a scale check, not a goal. A 190 lb man at 6 ft 2 in with 12% body fat carries 167.2 lb of lean mass and reads about 20.4, which looks athletic in most gyms. The 225 lb row describes a physique few people reach, and not in a year. If a program promises you 25 by next summer, this table is the reality check.

What FFMI cannot tell you

FFMI does not measure strength, fitness or health. It cannot tell a trained lifter from someone carrying extra fluid, and it cannot tell you whether a physique is achievable for your frame, your training history or your age.

Lean mass includes everything that is not fat: muscle, bone, organs, glycogen and water. Creatine pulls water into muscle, a salty meal shifts fluid, and edema from an injury or a medical condition adds weight that the formula reads as lean. That is why a single high reading is a question, not an answer.

The 25 cutoff came from 74 nonusers in one 1995 sample of male athletes, and the authors called their findings preliminary. The largest reference data set behind the medians is a Swiss cohort of mostly white adults, so the numbers travel imperfectly to other populations. The 25 line was also derived from men, which is why a woman with a median-level 15.4 should not read it as a target.

Treat the output as one measurement among several, and talk to a clinician about anything that looks like a sudden change in lean mass or fluid.