Calculate FFMI, FMI and skeletal muscle index from your InBody weight, height and body fat, or compare a reported InBody score with the 80-point baseline.
Body Composition Index Formulas (FFMI, FMI, SMMI)
The following formulas are used to calculate FFMI, FMI, and SMMI from body-composition measurements. Use mass in kilograms (kg) and height in meters (m) to get results in kg/m².
FFMI (Fat-Free Mass Index) = FFM / height².
FMI (Fat Mass Index) = FM / height².
SMMI (Skeletal Muscle Mass Index) = SMM / height².
Skeletal muscle percentage = SMM / body mass × 100. This percentage converts the device-reported skeletal muscle value entered; it is not an independent measurement and should not be substituted for lean mass or fat-free mass.
These indices are commonly used in research (and sometimes clinical settings), but interpretation and cutoffs depend on age, sex, ethnicity, measurement method, and the reference population.
Primary research defines the mass/height² indices: VanItallie et al.(1990) and Schutz et al.(2002). Heymsfield et al.(2007) examines skeletal-muscle scaling to height. This tool uses entered total skeletal muscle mass; it does not substitute appendicular lean mass or apply population cutoffs.
How to Estimate Skeletal Muscle Mass
Choose Estimate muscle mass if you have your weight, height and age. The calculator uses the weight-and-height model in Lee et al. (2000) to estimate total skeletal muscle mass in kilograms and pounds. It also divides the estimated mass in kilograms by height in meters squared to give an estimated skeletal muscle mass index (SMMI).
Estimated SMM (kg) = 0.244 × weight (kg) + 7.80 × height (m) + 6.6 × sex coefficient − 0.098 × age (years) + study group adjustment − 3.3. The sex coefficient is 0 for female and 1 for male. The study used −1.2 kg for Asian participants, +1.4 kg for African American participants and 0 kg for White or Hispanic participants. Leaving the group unselected assumes a 0 kg adjustment.
For example, at 75 kg, 1.75 m and age 35, with the male coefficient and a 0 kg adjustment, the estimate is 31.8 kg (70.2 lb), or 10.39 kg/m². This mode is limited to ages 20–81 and BMI from 20 to under 30. The study reported a 2.8 kg standard error for the model. Use the estimate for general comparison and a measurement when you need to assess your own muscle mass.
What is an InBody Score?
The InBody Score summarizes body composition on some InBody reports. According to the U.S. manufacturer FAQ, the device uses height and a reference BMI to set a standard weight, then compares lean and fat mass with the corresponding reference amounts. The differences can move the score above or below its 80-point baseline.
InBody Japan’s worked examples show how the adjustment depends on the body composition pattern. In one example, the result calls for 8.1 kg more fat-free mass and 3.3 kg more body fat, giving 80 − 8.1 − 3.3 = 68.6, rounded to 69 points. The U.S. and Japanese documents give different reference BMI values for women, and the Japanese guide says the device can withhold the score when fluid balance falls outside its criteria. Use the score printed on your result sheet in Compare a reported InBody score to see how far it is from 80.
How to Calculate FFMI, FMI, and SMMI?
The following steps outline how to calculate FFMI, FMI, and SMMI:
- Determine your body weight (W) and height (H).
- Determine your fat mass (FM). If you only have body fat percentage (%BF), compute FM = W × (%BF / 100).
- Compute fat-free mass (FFM) = W − FM.
- Compute FFMI = FFM / H².
- Compute FMI = FM / H².
- If you have total skeletal muscle mass (SMM), compute SMMI = SMM / H² (optional). SMM cannot exceed fat-free mass. Keep total skeletal muscle distinct from appendicular lean mass.
- If SMM is entered, compute skeletal muscle percentage = SMM / body mass × 100.
Example Problem:
Use the following variables as an example problem to test your knowledge:
Body Weight (W) = 80 kg
Body Fat Mass (FM) = 20 kg
Height (H) = 1.78 m
Skeletal Muscle Mass (SMM) = 30 kg (optional). Results: FFM = 60 kg, FFMI = 18.94 kg/m², FMI = 6.31 kg/m², SMMI = 9.47 kg/m², and skeletal muscle percentage = 37.5%.
