FFMI Calculator

On paper FFMI is simple arithmetic. Weight multiplied by one minus body fat gives fat-free mass, and fat-free mass divided by height squared gives the index. The hard part is the inputs, and a photo on its own records neither your weight nor your height at any fixed scale. So the vision model works the way an experienced coach sizes up a physique: it judges how muscular you look for your frame, from the size and fullness of the shoulders, chest, arms and legs, and how lean you look, from visible definition and where fat sits, then places that combination on the FFMI scale.

What comes back is an approximate score, such as around 22, with two or three sentences on what that range usually means in training terms. If the photo is too cropped, dark or covered to judge muscularity, the result is marked as tentative or you are asked for a clearer photo.

A clear, full-body photo taken straight on in fitted or minimal clothing, standing relaxed in even lighting, gives the most reliable FFMI estimate.

FFMI is sensitive to its inputs, so small errors grow. For someone weighing 80 kg at 1.80 m, every percentage point of body fat moves the score by about 0.25, which means a body fat read four points out shifts FFMI by a whole point. A visual judgment of muscularity adds error on top: a pump, flexing, a tan and overhead lighting make people look bigger and leaner, while loose clothing and flat light hide size. A wide-angle lens held close also exaggerates whatever body part is nearest the camera.

Use the result as a rough placement on the scale, not as proof of anything about yourself or anyone else. For a real FFMI, weigh yourself, measure your height against a wall, get a body fat figure from a DEXA scan or at least one consistent method, and work it out with the steps below. Even then, a repeat of the same method over time is more informative than any single score.

FFMI was formalized in a 1995 study by Kouri and colleagues, who were investigating anabolic steroid use among bodybuilders and needed a height-normalized way to compare muscularity across individuals — similar in spirit to how BMI normalizes weight for height, but built specifically around lean mass instead of total weight. It's remained in use ever since as an informal but widely-cited benchmark in strength sports.

These bands come from the original research population and general fitness community consensus, not a universal law — individual results vary by height, frame size, and measurement method.

Working Out Your FFMI Yourself

You need three numbers: weight in kilograms, height in meters and body fat as a percentage. If you use pounds and inches, multiply pounds by 0.4536 and inches by 0.0254 first.

Fat-free mass = weight × (1 − body fat as a decimal). FFMI = fat-free mass ÷ (height × height). Normalized FFMI = FFMI + 6.1 × (1.8 − height), which adjusts the score to a 1.8 m reference so tall and short people compare more fairly. The 6.1 factor comes from the original 1995 paper.

Worked through for someone 1.70 m tall weighing 70 kg at 20% body fat: fat-free mass is 70 × 0.8 = 56 kg. Height squared is 2.89, so FFMI is 56 ÷ 2.89 = 19.4. Normalized, that becomes 19.4 + 6.1 × 0.1 = 20.0.

Why the Body Fat Number Matters So Much

The body fat figure is the least reliable of the three inputs, and it moves the result more than people expect. Here is the same 80 kg, 1.80 m person run through the formula with four different body fat readings, to show how much the answer rides on that one input.

Nearly four FFMI points separate the top and bottom rows, for one body. That is the gap between two of the bands in the table above, which is why a score near a band boundary says very little on its own.

It also helps to remember what fat-free mass includes. Bone, organs and water all count, so a larger skeleton, a day of heavy carbohydrate intake or starting creatine can nudge the number without any change in muscle.