FMI
Fat Mass Index (FMI)
FMI relates fat mass to height squared, so it shows the amount of fat independently of build.
Short answer
FMI shows the amount of fat relative to height, so unlike percent body fat it does not swing simply because muscle changed. Paired with FFMI it explains what your BMI is made of.
What the value represents
FMI uses the same conversion as BMI, but only fat mass enters the numerator. It answers the question of how much fat a person carries relative to their height.
Compared with percent body fat it has one advantage: it does not move simply because muscle mass moved. Percent body fat is a share of weight, while FMI relates fat to an unchanging height.
That is exactly why FMI and percent body fat can develop differently from the same measurement, and why they complement each other.
- Fat mass related to height, in kg/m².
- The amount of fat on a stable basis that does not shift with training.
- It does not represent fat distribution or where in the body the fat sits.
How the value is produced
- A calculated value: fat mass (kg) ÷ height² (m²).
- Fat mass is an estimate that appears as the difference between measured weight and estimated fat free mass.
- On the result sheet it is accompanied by Z-scores and T-scores against a manufacturer data set.
How to read the current value
- Read it alongside FFMI. The two indices sum to BMI and show how weight divides between fat and non-fat tissue.
- The Z-score and T-score express position relative to the manufacturer reference set, not relative to a clinical norm.
- FMI does not tell you where the fat is stored. Distribution is what visceral fat and waist circumference are for.
- The manual does not publish recommended FMI ranges, so there is no manufacturer band to compare the number against.
How to read change over time
- A falling FMI at a stable height means a genuine loss of fat mass, not just a shift in a ratio.
- When percent body fat falls but FMI does not move, fat free mass most likely increased while fat stayed where it was.
- Short-term swings here also come mainly from hydration, because it affects the estimate of fat free mass and therefore of fat.
General principles: how to read change between measurements →
What to read it with
- Fat Free Mass Index (FFMI) – the other half of the BMI breakdown.
- Percent Body Fat – a share that also responds to changes in muscle mass.
- Body Fat Mass in kilograms – the absolute value before normalisation to height.
- Visceral Fat Area – the distribution information the index does not carry.
Common misunderstandings
- FMI is not the same as percent body fat. They differ in the denominator: height squared instead of weight.
- The index says nothing about whether the fat is subcutaneous or visceral.
- The manufacturer reference scores are not health thresholds.
What can affect the result short term
- Hydration and measurement conditions, which affect the estimate of fat mass.
- Food and drink shortly before the measurement.
- The accuracy of the entered height.
What to do next
- Use FMI when you want to follow fat without the distortion caused by changing muscle mass.
- Combine it with percent body fat — the two developing differently is itself useful information.
- For fat distribution, reach for visceral fat and waist circumference.
What the value does not determine
- It says nothing about fat distribution in the body.
- It shares the uncertainty of the bioimpedance estimate of fat mass.
- The reference scores come from a manufacturer data set, not from clinical norms.
In more depth, with sourcesIn more depth: why a fat mass index complements percent body fatExpand the detailed explanation
Percent body fat has total weight in the denominator, and total weight changes with both fat and lean tissue. The fat mass index has height in the denominator, which is stable in an adult. That separates the development of the fat component from the development of everything else.
In practice this means someone who gained muscle while fat stayed the same will see percent body fat fall but FMI hold steady. Both values falling together means fat mass genuinely decreased. The two indicators behaving differently is therefore diagnostically useful rather than contradictory.
Both values nevertheless rest on the same estimate of fat mass and share its uncertainty. The ESPEN clinical guidelines warn that body composition estimates are sensitive to changes in fluid distribution, and that sensitivity carries into FMI exactly as it does into percent body fat.
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.
When is FMI more useful than percent body fat?
When your muscle mass is changing at the same time. Percent body fat moves in that situation even without a change in fat, whereas FMI relates fat to a stable height and shows the fat component on its own.
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.