FFMI
Fat Free Mass Index (FFMI)
FFMI relates fat free mass to height squared, so it shows the amount of non-fat tissue independently of build.
Short answer
FFMI is BMI calculated from the non-fat part of the body alone. Paired with the Fat Mass Index it tells you whether your weight rests on lean tissue or on fat.
What the value represents
FFMI uses the same conversion as BMI, but puts fat free mass in the numerator instead of total weight. It therefore describes how much non-fat tissue a person carries relative to their height.
It is the companion to BMI that answers whether extra weight is lean tissue or fat.
Fat free mass includes water, protein and minerals, so it is not only muscle. FFMI is therefore not the same thing as a muscle index.
- Fat free mass related to height, in kg/m².
- The total amount of non-fat tissue — muscle, water, bone and organs together.
- It does not represent muscle mass alone, and it does not represent strength.
How the value is produced
- A calculated value: fat free mass (kg) ÷ height² (m²).
- Fat free mass is itself an estimate from bioimpedance, derived from total body water.
- On the result sheet the manual presents it alongside Z-scores and T-scores against a manufacturer data set.
How to read the current value
- Read it together with the Fat Mass Index. Only the pair splits BMI into its non-fat and fat components.
- A higher FFMI at the same BMI means a larger share of your weight is non-fat tissue.
- The Z-score and T-score on the result sheet show your position relative to the manufacturer reference set, not relative to a health norm.
- The manual does not publish recommended FFMI ranges, so there is no manufacturer threshold to read the value against.
How to read change over time
- FFMI moves slowly and responds sensitively to hydration, because water is part of fat free mass.
- A rise in FFMI after several months of strength training is expected; a jump between two consecutive measurements is more likely a product of measurement conditions.
- It is worth following FFMI and FMI together — they show whether the non-fat component, the fat component, or both are changing.
General principles: how to read change between measurements →
What to read it with
- Fat Mass Index (FMI) – the other half of the BMI breakdown.
- Fat free mass in kilograms – the absolute value before normalisation.
- Skeletal Muscle Index – a narrower indicator focused on muscle alone.
- BMI – FFMI and FMI together explain what your BMI is made of.
Common misunderstandings
- FFMI is not a muscle index. It also contains water, bone and organs.
- A higher FFMI does not automatically mean more muscle — it can also reflect a higher amount of body water.
- There is no single universal threshold for a good FFMI that would hold across age and sex, and the manufacturer does not publish one.
What can affect the result short term
- Hydration, which directly shifts the estimate of fat free mass.
- Training, sauna and fluid intake before the measurement.
- The accuracy of the entered height.
What to do next
- Use FFMI together with FMI when you want to know what your BMI is composed of.
- For tracking muscle specifically, reach for the Skeletal Muscle Index instead.
- Compare only measurements taken under comparable conditions, since water makes up a large part of fat free mass.
What the value does not determine
- It does not cover muscle alone — it also includes water, bone and organs.
- It is sensitive to hydration status.
- The reference scores come from a manufacturer data set, not from clinical norms.
In more depth, with sourcesIn more depth: splitting BMI into FFMI and FMIExpand the detailed explanation
FFMI and FMI add up to BMI, because fat free mass and fat mass together make up total weight and both are divided by the same height squared. That decomposition is the main reason the two indices are reported together — separately they lose most of their interpretive value.
In bioimpedance, fat free mass is derived from an estimate of total body water that assumes a roughly constant proportion of water in lean tissue. The ESPEN clinical guidelines point out that this assumption breaks down where fluid distribution is altered, shifting the estimate of fat free mass and, with it, FFMI.
The Z-scores and T-scores printed on the result sheet express position relative to the manufacturer reference set. They are not clinical norms and must not be confused with the Z-scores and T-scores used, for example, in bone density assessment.
Sources
- InBody 970 Result Sheet Interpretation (ENG_A_211104) — InBody Co., Ltd.
- Bioelectrical impedance analysis – part II: utilization in clinical practice — Clinical Nutrition (ESPEN guidelines)
On this page
Related values
Related articles
Where to find it
The Muscle-Fat Analysis bar chart and the Obesity Analysis section that follows it.
- Reviewed by
- MojeInBody content team
- Date
An InBody measurement is not a diagnostic tool. Results do not determine health status and do not replace a medical examination.
FAQ
Frequently asked questions
Answers to what people ask most often about this value.
What is the difference between FFMI and a muscle index?
FFMI counts the whole of fat free mass, including water, bone and organs. The Skeletal Muscle Index works with muscle only, which makes it the more specific measure for tracking muscle.
Do FFMI and FMI add up to my BMI?
Yes. Fat free mass and fat mass together make up total weight, and both indices use the same denominator, so their sum equals BMI.
Explanations make the most sense alongside your own trend.
The public guide is open to everyone. The client portal adds your own value, history and the context of a specific measurement.