Fat Mass Index (FMI) Calculator
Calculate your fat mass index (FMI) from weight, height and body fat percentage and see the Kelly (2009) category, from fat deficit to obesity class III, plus your FFMI.
How it works
Fat mass (kg) = weight × body fat % ÷ 100 · FMI = fat mass ÷ height (m)² · FFMI = (weight − fat mass) ÷ height (m)² · normal FMI: men 3–6, women 5–9 kg/m² (Kelly 2009)
The fat mass index splits BMI into its fat and lean parts: FMI + FFMI = BMI. Because it measures fat relative to height, two people with the same BMI but different muscle can be told apart. Kelly, Wilson and Heymsfield (2009) used DXA data from NHANES to set FMI categories that match the BMI classes — for example an FMI above 6 (men) or 9 (women) corresponds to a BMI over 25, and 9 or 13 to obesity. Accuracy depends on how body fat was measured.
Worked example
A man weighing 70 kg at 175 cm with 20% body fat: fat mass = 70 × 0.20 = 14 kg; FMI = 14 ÷ 1.75² = 14 ÷ 3.0625 = 4.57 kg/m², in the normal range of 3–6. His FFMI is 56 ÷ 3.0625 = 18.29 kg/m².
Frequently asked questions
What is a healthy fat mass index?
Using the Kelly 2009 DXA reference, a normal FMI is 3–6 kg/m² for men and 5–9 kg/m² for women. Values above 9 (men) or 13 (women) correspond to obesity.
Why use FMI instead of body fat percentage?
Body fat percentage can fall simply because muscle increases, even if fat stays the same. FMI tracks fat mass itself relative to height, so it changes only when fat changes, which makes it a cleaner measure of adiposity.
How do FMI and FFMI relate to BMI?
They add up exactly: FMI + FFMI = BMI. A muscular person may have a high BMI because of a high FFMI while their FMI is normal.
Which body fat measurement should I use?
The reference values come from DXA, so DXA results fit best. Skinfolds, bioimpedance or the Navy tape method work for tracking change but can differ from DXA by several percentage points.
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This calculator is an educational aid for nutrition and health professionals. It does not replace a clinical assessment — measurement technique strongly affects the result, so interpret it with a qualified professional.