FFMI Calculator — Fat-Free Mass Index

FFMI (Fat-Free Mass Index) is an index similar to BMI but describing only fat-free mass (muscle, bone, organs) relative to height. It is calculated by dividing fat-free mass in kilograms by height in meters squared. FFMI reflects actual muscularity better than BMI because it ignores fat tissue.

BMI divides total body weight by height squared and does not distinguish muscle from fat — a muscular person may register as "overweight" by BMI. FFMI considers only fat-free mass, making it a more accurate measure of muscularity. It does, however, require knowing your body fat percentage.

First, fat-free mass = weight × (1 − body fat%/100). Then FFMI = fat-free mass (kg) ÷ height² (m²). For an 80 kg, 180 cm person at 15% body fat: fat-free mass = 68 kg, so FFMI = 68 / (1.8 × 1.8) ≈ 21.0.

Normalized FFMI adjusts the result for height, because raw FFMI favors shorter people. The formula is FFMI + 6.1 × (1.8 − height in meters). This lets you compare muscularity across different heights on a common scale referenced to 180 cm.

Research suggests natural FFMI rarely exceeds 25. A value around 25 is considered the upper bound of genetic potential for most men training naturally. Results clearly above 25–26 are sometimes interpreted as a possible sign of anabolic-androgenic substance use.

Roughly for men: 16–17 is average muscularity, 18–19 above average, 20–21 good/athletic, 22–23 advanced, 24–25 very advanced (natural limit). For women values are about 3 points lower. These are indicative ranges.

Not automatically. FFMI is only a statistical index — a high result may stem from favorable genetics, long training experience or underestimated body fat. A normalized FFMI persistently above 25–26 in men is outside the typical natural range and is treated as a clue, not proof.

The most accurate methods are DEXA, hydrostatic weighing and air-displacement plethysmography (BodPod). Home methods include skinfold calipers, bioimpedance analysis (BIA) and circumference-based estimates. Home methods have an error of several percentage points, which directly affects FFMI accuracy.

At very high body fat, FFMI becomes less reliable because some fat-free mass rises along with total body weight. The index works best for people with moderate to low body fat, for whom it was originally developed based on studies of bodybuilders.

No. Results are indicative and depend directly on the accuracy of the body fat percentage you enter. The calculator does not replace professional body composition measurement (e.g. DEXA) or consultation with a trainer or doctor.