Calculate FFMI, FMI, and SMMI from height, weight, body fat percentage or mass, and optional skeletal muscle mass, or interpret a reported InBody score.

The device score uses model-specific logic; this mode calculates FFMI, FMI, and optional SMMI from measured inputs.

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.

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.

If you want to review the background research, look up FFMI/FMI/SMMI using reputable sources such as PubMed/Google Scholar and major medical/academic sites. Search terms to start with include “fat-free mass index (FFMI)”, “fat mass index (FMI)”, and “skeletal muscle mass index (SMMI)”.

What is an InBody Score?

The InBody Score is a proprietary summary number shown on some InBody bioelectrical impedance analysis (BIA) reports. It is intended as a quick, at-a-glance indicator of overall body composition (primarily the balance of muscle and fat relative to reference/“ideal” values). Because the exact scoring method is not publicly disclosed and can vary by device/model, it cannot be reproduced exactly with a simple public formula.

How to Calculate FFMI, FMI, and SMMI?

The following steps outline how to calculate FFMI, FMI, and SMMI:

  1. Determine your body weight (W) and height (H).
  2. Determine your fat mass (FM). If you only have body fat percentage (%BF), compute FM = W × (%BF / 100).
  3. Compute fat-free mass (FFM) = W − FM.
  4. Compute FFMI = FFM / H².
  5. Compute FMI = FM / H².
  6. If you have skeletal muscle mass (SMM), compute SMMI = SMM / H² (optional).

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.