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Body Composition & FMI Calculator

Calculate your fat mass, fat-free mass, BMI, FMI, FFMI, and height-normalized FFMI in one place using your weight, height, and body fat percentage.

1. Select Your Units

2. Body Measurements

Weight and height are used to calculate BMI, FMI, and FFMI. All index calculations use metric units internally.

3. Body Fat Percentage

Enter your estimated body fat percentage from a body composition assessment, such as a DXA scan, bioelectrical impedance (BIA), or skinfold measurement. The calculator does not measure body fat. Accuracy depends on the body fat estimate you provide.

4. What Will Your Results Show?

Fat mass and fat-free mass describe two components of your body weight. BMI relates total body weight to height squared, FMI relates fat mass to height squared, and FFMI relates fat-free mass to height squared. Normalized FFMI applies an additional adjustment to a reference height of 1.80 meters (approximately 5 ft 11 in).

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How Does the Body Composition & FMI Calculator Work?

The Body Composition & FMI Calculator uses your body weight, height, and body fat percentage to calculate six body composition measurements: fat mass, fat-free mass, BMI, FMI, FFMI, and normalized FFMI. These metrics describe different aspects of body composition and show how total body weight, fat mass, and fat-free mass relate to height.

Metric What it means Formula How it is used
Body Fat Percentage The percentage of total body weight made up of fat mass. Fat mass ÷ body weight × 100 A basic body composition measure. The calculator uses your entered percentage as an input.
Fat Mass (FM) The total mass of body fat, expressed in pounds or kilograms. Body weight × body fat % ÷ 100 Estimates how much of your total body weight consists of fat mass.
Fat-Free Mass (FFM) Everything in your body weight that is not fat mass. Body weight − fat mass Includes muscle, bone, organs, water, and other nonfat components.
Body Mass Index (BMI) Total body weight relative to height squared. Weight (kg) ÷ height² (m²) A general weight-for-height index that does not distinguish fat from fat-free mass.
Fat Mass Index (FMI) Fat mass adjusted for height squared. Fat mass (kg) ÷ height² (m²) Helps compare fat mass relative to body height.
Fat-Free Mass Index (FFMI) Fat-free mass adjusted for height squared. Fat-free mass (kg) ÷ height² (m²) Helps compare fat-free mass across people of different heights.
Normalized FFMI FFMI adjusted to a reference height of 1.80 meters. FFMI + 6.1 × (1.80 − height in meters) A height-adjusted comparison metric, not a diagnostic test.
BMI = FMI + FFMI BMI can be separated into fat mass and fat-free mass indexes. BMI = FMI + FFMI Shows the mathematical relationship between the three indexes.
Measurement Accuracy Results depend on the accuracy of the body fat percentage entered. Measured or estimated body fat % DXA, BIA, and skinfold methods can produce different estimates.

1. Enter Your Measurements

Enter your body weight and height in pounds and feet/inches, or switch to kilograms and centimeters. All index calculations use metric units internally.

2. Add Your Body Fat Percentage

Use a body fat percentage from a reasonably consistent assessment method. The calculator uses it to estimate fat mass and fat-free mass.

3. Compare Your Results

BMI uses total weight, FMI uses fat mass, and FFMI uses fat-free mass. Together, these measurements offer more context than BMI alone.

1. Fat Mass Formula
FM (kg) = body weight (kg) × body fat % ÷ 100

2. Fat-Free Mass Formula
FFM (kg) = body weight (kg) − fat mass (kg)

3. Body Mass Index Formula
BMI = body weight (kg) ÷ height² (m²)

4. Fat Mass Index Formula
FMI = fat mass (kg) ÷ height² (m²)

5. Fat-Free Mass Index Formula
FFMI = fat-free mass (kg) ÷ height² (m²)

6. Normalized FFMI Formula
Normalized FFMI = FFMI + 6.1 × (1.80 − height in meters)
This formula adjusts FFMI to a reference height of 1.80 meters (approximately 5 feet 11 inches). It is a commonly used height-normalization convention, not a biological cutoff or diagnostic rule.

7. Mathematical Relationship
BMI = FMI + FFMI
Because total body weight equals fat mass plus fat-free mass, the sum of FMI and FFMI equals BMI when all three use the same height.

Practical Example
Body weight: 176.37 lb (80 kg)
Height: 5 ft 10.87 in (180 cm)
Body fat percentage: 20%
Height squared: 1.80 × 1.80 = 3.24 m²

Fat mass: 80 × 0.20 = 16 kg (35.27 lb)
Fat-free mass: 80 − 16 = 64 kg (141.10 lb)
BMI: 80 ÷ 3.24 = 24.69
FMI: 16 ÷ 3.24 = 4.94
FFMI: 64 ÷ 3.24 = 19.75
Normalized FFMI: 19.75 + 6.1 × (1.80 − 1.80) = 19.75

Verification:
FMI + FFMI = 4.94 + 19.75 ≈ 24.69 (BMI)
Small differences may occur because the displayed values are rounded.

US-to-Metric Unit Conversions
Weight (kg) = weight (lb) ÷ 2.2046226218
Height (cm) = (feet × 12 + inches) × 2.54

Scientific Background
Kouri EM et al. (1995): Fat-Free Mass Index in Users and Nonusers of Anabolic-Androgenic Steroids. Clinical Journal of Sport Medicine, 5(4), 223–228.
The 1.80-meter FFMI height-adjustment convention is associated with this published approach.

Fat mass and fat-free mass are calculated estimates, not direct measurements. Fat-free mass is not identical to muscle mass, and FFMI is not the same as skeletal muscle mass index. FMI and FFMI interpretation may vary with age, sex, body composition, measurement method, and individual circumstances. These measurements are not sufficient to make a medical diagnosis. This content is for informational purposes only and does not constitute medical advice.

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Fitness Calculators
Body Composition & FMI Calculator

The Body Composition & FMI Calculator uses your weight, height, and body fat percentage to estimate fat mass, fat-free mass, BMI, FMI, FFMI, and normalized FFMI. BMI relates total body weight to height, while FMI and FFMI separately account for fat mass and fat-free mass. Normalized FFMI applies an additional height adjustment using a 1.80-meter reference. Results depend on the accuracy of your entered body fat percentage and are intended for informational purposes.

Medical Disclaimer: The calculator results and information provided in this article are for educational and informational purposes only and do not constitute medical, health, or professional advice. Always consult with a qualified healthcare provider before making any decisions related to your health, diet, or fitness goals. Individual results may vary, and this tool should not be used to diagnose or treat any medical condition.

How Do You Calculate Body Composition From Body Fat Percentage?

You can estimate body composition by using your total body weight and body fat percentage to calculate fat mass and fat-free mass. For example, a person weighing 176.37 pounds (80 kg) with 20% body fat has approximately 35.27 pounds (16 kg) of fat mass and 141.10 pounds (64 kg) of fat-free mass. These values depend on the accuracy of the body fat percentage entered and should not be confused with direct measurements of muscle or other tissues.

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What Is FMI and How Do You Calculate Fat Mass Index?

Fat Mass Index (FMI) measures fat mass relative to height squared, using kilograms and meters. To calculate FMI, divide fat mass in kilograms by height in meters squared. FMI provides additional context beyond BMI by focusing specifically on fat mass, although it cannot independently determine a person's health status.

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What Is the Difference Between BMI, FMI, and FFMI?

Body Mass Index (BMI) relates total body weight to height without distinguishing between fat and fat-free tissue. Fat Mass Index (FMI) uses only fat mass, while Fat-Free Mass Index (FFMI) uses fat-free mass in the same height-adjusted calculation. Mathematically, BMI equals FMI plus FFMI, so the three measurements can help explain how body composition contributes to total body weight.

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How Do You Calculate FFMI and Normalized FFMI?

Fat-Free Mass Index (FFMI) is calculated by dividing fat-free mass in kilograms by height in meters squared. Normalized FFMI adds a height adjustment using the formula FFMI + 6.1 × (1.80 − height in meters), based on a 1.80-meter reference height. This adjusted index is used for comparison purposes and should not be interpreted as a direct measurement of muscle mass or a medical diagnosis.

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What Is Fat-Free Mass and How Is It Different From Muscle Mass?

Fat-free mass includes all components of body weight other than fat mass, including muscles, bones, organs, water, and other tissues. Muscle mass is only one component of fat-free mass, which means the two measurements are not interchangeable. Changes in hydration can affect estimated fat-free mass even when actual muscle tissue has not changed significantly.

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How Can You Track Fat Mass and Fat-Free Mass During Weight Loss?

Tracking body weight and body fat percentage over time can help estimate changes in fat mass and fat-free mass during weight loss. For more consistent comparisons, use similar measurement methods and conditions while accounting for possible changes in hydration and measurement error. Long-term trends are generally more informative than relying on a single body composition measurement.

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