FFM
Fat Free Mass (FFM)
Fat Free Mass is everything except fat — the sum of body water, protein and minerals in kilograms.
Short answer
Fat free mass is everything except fat — water, protein and minerals together. It is not muscle mass: roughly 73 percent of it is water, which is why it responds to hydration faster than any real change in tissue ever could.
- Body water40.2 kg73.5 %
- Protein10.8 kg19.7 %
- Minerals3.7 kg6.8 %
What the value represents
Fat free mass is a straightforward sum of the remaining three components of the model: weight without fat = body water + protein + minerals. It covers muscle, organs, bone, blood and all the water in the body.
According to the manual the largest tissue within fat free mass is muscle — apart from fat inside the muscle, every muscle cell belongs to fat free mass. That does not make fat free mass equal to muscle, though. Water makes up roughly 73 percent of fat free mass according to the manual, so water is in fact the largest share.
The manual also gives a practical link: more muscle mass means a higher basal metabolic rate, and therefore less energy surplus stored as fat. InBody calculates basal metabolic rate from fat free mass alone, using the equation BMR = 21.6 × FFM (kg) + 370.
- The mass of every body component except fat, in kilograms.
- The sum of body water, protein and minerals. It includes muscle, organs, bone and blood.
- The single input to the basal metabolic rate calculation in the equation InBody uses.
- It does not represent the amount of muscle. Muscle is the largest tissue inside it, but fat free mass also contains water, bone and organs.
How the value is produced
- A calculated value under the relationship FFM = total body water + protein + minerals.
- The input to the whole chain is body water derived from impedance. Fat free mass is therefore as sensitive to hydration as water itself.
- Fat mass is the mirror image of fat free mass: it arises as the difference between measured weight and fat free mass.
- The water share of fat free mass is reported separately on the Research Result Sheet as TBW/FFM; the manual gives roughly 73 percent as reference orientation for it.
- The reference range in brackets is a comparison against a reference; the manual does not publish the formula for it.
How to read the current value
- Read fat free mass against the reference range in brackets, not against other people values. A taller person naturally carries more.
- On its own it does not say how much muscle you have. Skeletal muscle mass, or soft lean mass, serves that purpose.
- When you want to compare fat free mass between people of different build, use the Fat Free Mass Index (FFMI), which relates it to height squared.
- Check the water share of fat free mass too. If it sits markedly above the reference orientation of roughly 73 percent, fat free mass is likely overstated by a fluid shift.
How to read change over time
- Fat free mass responds fast, because the largest part of it is water. The difference between morning and afternoon can be noticeable without any real change in tissue.
- A rise in fat free mass together with a rise in muscle mass and intracellular water over months is the expected picture during strength training.
- An abrupt rise between two measurements is more likely a product of measurement conditions or a fluid shift than a real gain in tissue.
- A fall in fat free mass during weight reduction is a signal to check. Look at whether muscle mass and protein fell too, or only body water.
General principles: how to read change between measurements →
What to read it with
- Body Fat Mass – the other side of the same balance; their sum is weight.
- Total body water and the water share of fat free mass – show whether the value has been shifted by hydration.
- Skeletal Muscle Mass – the narrower indicator that covers muscle alone.
- Soft Lean Mass – fat free mass without bone minerals.
- Fat Free Mass Index (FFMI) – normalisation to height for comparison between people.
- Basal metabolic rate – calculated from fat free mass, so the two move together.
Common misunderstandings
- Fat free mass is not muscle mass. It includes water, bone, organs and blood.
- A rise in fat free mass from one day to the next does not mean muscle gain. It usually means more water.
- Fat free mass is not the healthy part of your weight. It is a definitional category, not an assessment.
- High fat free mass alongside a raised ECW/TBW ratio may be overstated. The manual explicitly warns about overstatement where fluid is retained.
What can affect the result short term
- Hydration status, because water makes up roughly 73 percent of fat free mass.
- Training, sauna and showering before the measurement.
- Food and drink before the measurement; the manual recommends measuring fasted.
- Time of day and body position — the manual recommends standing for at least 5 minutes beforehand.
- The menstrual cycle, during which body water fluctuates.
What to do next
- Use fat free mass as an overall summary and look for detail in muscle mass and body water.
- If comparison with others or with an age group interests you, reach for the Fat Free Mass Index rather than the absolute value.
- During weight reduction, watch whether fat free mass falls only slightly. A marked fall means the loss is not coming from fat alone.
- Measure under the same conditions. Given the water share, fat free mass is one of the most sensitive values on the sheet.
What the value does not determine
- It does not cover muscle alone — it contains water, bone, organs and blood.
- It is highly sensitive to hydration status.
- Absolute values are not interchangeable with DEXA results.
- It can be overstated where fluid is retained.
- It does not diagnose any medical condition.
On terminology
- Fat free mass (FFM) and soft lean mass (SLM) are not the same: SLM is FFM without bone minerals.
In more depth, with sourcesIn more depth: the hydration constant and what it means for accuracyExpand the detailed explanation
Fat free mass arises from the estimate of total body water via the assumption that water makes up roughly 73 percent of it. This hydration constant is the key assumption of the whole of bioimpedance analysis. The manual gives it as reference orientation and, on the Research Result Sheet, lets it be checked through the TBW/FFM indicator, which displays the water share of fat free mass explicitly.
If the actual hydration of lean tissue departs from the assumption, the estimate of fat free mass shifts — and because fat mass is the difference, it shifts in the opposite direction. The ESPEN clinical guidelines on bioimpedance therefore name altered fluid distribution as a situation in which body composition estimates are less reliable. The manufacturer manual describes the same thing from the practical side when it warns that segmental lean mass can be overstated where the segmental extracellular water ratio is raised.
The manufacturer internal validation study compared InBody 970 with DEXA in 150 people and reported a correlation of r = 0.992 for fat free mass (R² = 0.983, SEE 0.7865 kg). That is not absolute agreement, though: mean fat free mass came out at 49.1 ± 12.9 kg on DEXA and 50.9 ± 13.6 kg on InBody 970, systematically somewhat higher. The study is an internal manufacturer validation in a Korean population aged 19 to 73, so it supports the traceability of change rather than any claim of interchangeability with DEXA.
Sources
- InBody 970 Result Sheet Interpretation (ENG_A_211104) — InBody Co., Ltd.
- Validation of InBody 970 (BWA) against DEXA — InBody Co., Ltd.
- Bioelectrical impedance analysis – part II: utilization in clinical practice — Clinical Nutrition (ESPEN guidelines)
On this page
Related values
Commonly confused with
Related articles
Where to find it
The first table at the top of the result sheet, labelled Body Composition Analysis.
- 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.
Is fat free mass the same as muscle mass?
It is not. Fat free mass is the sum of body water, protein and minerals — including bone, organs and blood. Muscle is its largest tissue, but muscle itself is described by skeletal muscle mass.
Why does my fat free mass swing by more than a kilogram?
Because water makes up roughly 73 percent of fat free mass according to the manual. A change of a litre in hydration propagates into fat free mass almost in full. That is why measuring under comparable conditions matters.
How is fat free mass related to basal metabolic rate?
Directly. InBody calculates basal metabolic rate with the equation BMR = 21.6 × FFM (kg) + 370, that is from fat free mass alone. Basal metabolic rate therefore carries no independent information — it moves exactly with it.
Explanations make the most sense alongside your own trend.
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