Enter your body weight, body fat mass (or body fat %), height, and optional skeletal muscle mass to calculate FFMI, FMI, and SMMI. These indices can be derived from results you might see on a body composition report (including some InBody BIA reports), but they are not the proprietary InBody Score.

Body Composition Indices (Research Formulas)

By Fat Mass
By % Body Fat

Enter your values and click Calculate. Outputs: FFMI, FMI, and SMMI.

Unit hint: most adults are roughly 45–150 kg (100–330 lb).
Unit hint: body fat mass cannot exceed body weight.
Unit hint: SMM should not exceed your fat-free mass (or total body weight).
Unit hint: most adults are roughly 1.4–2.1 m. If using feet (ft), enter total feet as a decimal (example: 5 ft 10 in = 5.833 ft).
Use “Reset” to clear saved results. Avoid saving on shared devices.

This calculator is for educational purposes only and is not medical advice or a diagnosis. Body-composition estimates (including BIA) can vary with hydration, food intake, exercise, pregnancy, edema, and some medical conditions. If you’re under 18, pregnant, have a history of eating disorders, or are concerned about your results, consider discussing them with a qualified clinician.

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.