Fat Mass Index (FMI) Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/10/2026
The fat mass index (FMI) is calculated with fat mass (kg) ÷ height² (m²). For 80 kg, 1.80 m, and 20% body fat, the FMI is 4.94 kg/m².
Explanation
Body mass index (BMI) doesn't distinguish fat mass from muscle mass: two people with the same BMI can have very different body compositions. The fat mass index (FMI) and the fat-free mass index (FFMI) break BMI down into its two parts: FMI relates fat mass alone to height squared, exactly as BMI relates total weight to height squared. The sum of FMI and FFMI approximately reconstitutes BMI (bone mass aside), which shows whether a high BMI reflects excess fat mass or, conversely, significant muscle mass. FMI is calculated from three figures: weight, height, and body fat percentage, the latter measured by bioelectrical impedance, skinfolds (see our Jackson-Pollock skinfold calculator), or DEXA scan. As a guide, FMI values seen in adults range from about 2-3 kg/m² for a very lean athlete to over 12-15 kg/m² in marked obesity, with a zone generally considered favorable between 3 and 6 kg/m² for men and 5 and 9 kg/m² for women — benchmarks that vary with age, ethnicity, and the body fat measurement method, and that don't replace a healthcare professional's advice.
Example: 80 kg, 1.80 m, 20% body fat
Inputs
Weight: 80 kg. Height: 1.80 m. Body fat percentage: 20%.
Calculation
Fat mass = 80 × 20% = 16 kg. FMI = 16 ÷ (1.80 × 1.80) = 16 ÷ 3.24 ≈ 4.94 kg/m².
Result
This person has a fat mass index of 4.94 kg/m².
Frequently asked questions
What is the advantage of FMI over body fat percentage alone?
Body fat percentage doesn't account for height: two people at 20% body fat but of very different sizes carry very different absolute amounts of fat. FMI, by relating fat mass to height squared, allows comparing people of different builds on a common basis, exactly as BMI does for total weight.
Do FMI and FFMI add up exactly to BMI?
Almost: FMI + FFMI reconstitutes BMI to within a few tenths, the gap corresponding mainly to how bone mass and body water are distributed between the two compartments depending on the measurement method. This breakdown remains a useful approximation for interpreting a BMI, not an exact mathematical identity.
Does FMI accuracy depend on the body fat measurement?
Entirely: FMI inherits the uncertainty of the body fat measurement, which varies by ± 3-4 points between a consumer impedance scale and a reference DEXA measurement. For tracking over time, the key is to always use the same method and the same measurement conditions (fasted, at the same time of day), since the relative change is more reliable than the absolute value.