Fat Mass Index (FMI)
The fat mass index divides your total body fat by your height squared, giving a clearer picture of body fatness than BMI alone.
Plain English
Patient-Friendly Explanation
The fat mass index (FMI) is a number that measures how much of your body weight is fat, adjusted for your height. It works like BMI, but uses your actual fat mass instead of total weight — so it can tell the difference between a heavy athlete and someone carrying excess fat. Alongside muscle measurements, it gives a truer picture of body composition during weight loss.
Medical
Clinical Definition
Fat mass index (FMI) is calculated as fat mass (kg) divided by height in metres squared, partitioning the BMI formula into its fat and fat-free components (FMI + FFMI = BMI). It requires body-composition measurement — typically bioelectrical impedance analysis (BIA) or dual-energy X-ray absorptiometry (DXA) — but provides a more accurate index of adiposity than BMI, which cannot distinguish fat from lean mass. Age- and sex-specific FMI reference percentiles allow clinicians to track whether weight loss is reducing actual fat rather than muscle, and to diagnose sarcopenic obesity, defined by a high FMI with a low fat-free mass index.
Key Insight
Why It Matters
The scale cannot tell you whether you are losing fat or muscle. Body-composition tracking with FMI helps our doctors adjust your plan so you lose fat while protecting the muscle that keeps your metabolism healthy.
Sources
Reviewed against standard medical references.
Medically reviewed by
Family Physician & Registered Acupuncturist, Accord Medical Clinic. MBBS (NUS), Graduate Diploma in Family Medicine (NUS), Graduate Diploma in Acupuncture (Singapore College of TCM).
Last reviewed: August 2026
This page is general health education, not medical advice. Whether any treatment is appropriate for you is a decision made in consultation after assessment.
