Body composition calculator
Body composition calculator
Turn weight, height, and body-fat percentage into fat mass, lean mass, BMI, FFMI, normalized FFMI, and FMI.
Track this number — the trend says more than today's reading.
Free — no card, 30 seconds. Your result comes with you.
What does a body composition calculator return?
BMI only sees total weight. FFMI indexes lean mass by height, while FMI indexes fat mass by height. Seeing both keeps a cut or bulk from collapsing into a single scale-weight number.
Fat Mass Index (FMI): the fat-mass part of BMI
FMI = fat mass in kilograms ÷ height in meters squared. It expresses fat mass relative to height. FFMI uses fat-free mass instead: muscle, bone, organs and water together. With unadjusted FFMI, FMI + FFMI = BMI.
For example, at 80 kg, 1.80 m and 20% body fat, fat mass is 16 kg. FMI is 16 ÷ 1.80² = 4.94 kg/m². Fat-free mass is 64 kg, giving FFMI 19.75 and BMI 24.69. The calculator converts pounds and inches to kilograms and meters before applying the same equation.
FMI is only as reliable as the body-fat estimate you enter. A changing smart-scale reading can reflect measurement conditions as well as body composition. This page does not assign universal “healthy FMI” bands or turn one reading into a diagnosis.
The height-normalized fat-mass and fat-free-mass indices were described by VanItallie and colleagues (1990). Compare with the Fat-Free Mass Index calculator or review your body-fat estimate.