TBW/FFM
Total Body Water to Fat Free Mass (TBW/FFM)
TBW/FFM states what share of your fat free mass is water; the manual puts that at roughly 73 percent in a human body.
Short answer
TBW/FFM says what share of lean tissue is water; the manual gives roughly 73 percent. The value stays within a narrow band and serves mainly as a check: it helps you tell whether a change in fat free mass rests on real tissue or on water.
What the value represents
Fat free mass is everything except fat — muscle, organs, bone and the water within them. This metric says how much of it is water.
The manufacturer manual states that roughly 73 percent of fat free mass is water. It is one of the most stable values on the result sheet, because it stays close to that general physiological constant. That is why you should look not at the absolute number but at its small shifts and above all at their direction.
Unlike the water share of weight, this value is not distorted by the amount of fat. Fat is excluded from the denominator, so two people with very different fat percentages can be compared.
In practice it works as a check. When fat free mass changes, this value helps you judge whether the tissue itself changed or only the water within it.
- The share of total body water in fat free mass, expressed as a percentage.
- How watery non-fat tissue is — how much water falls to muscle, organs and the rest of the non-fat mass.
- It does not represent the total amount of water or how it is divided between the inside and outside of cells.
- It does not represent the quality or function of muscle tissue.
How the value is produced
- A calculated value: total body water (L) ÷ fat free mass (kg) × 100.
- Total body water is derived from impedance measured at several frequencies; the manufacturer presents it as a measured quantity.
- In the InBody model, fat free mass is calculated as the sum of total body water, protein and minerals. Water is therefore also the largest component of the denominator — which is why the ratio moves within a narrow band around the value the manual gives as general.
- The manual gives no recommended range or cut-off for this value. The figure of roughly 73 percent is a general physiological statement about the composition of fat free mass, not a reference bound for an individual.
How to read the current value
- The expected level according to the manual is around 73 percent. The manual gives no recommended range for this value, so there is no band on the result sheet you are supposed to land in.
- Because water is the main component of fat free mass, the value stays within a narrow span. A deviation of a few tenths of a percent is therefore relatively more informative than it would be in a metric with a wide range.
- A higher value means relatively more water per unit of non-fat tissue. It does not establish where that water is — only the ECW Ratio distinguishes water inside cells from water outside them.
- A lower value is not a sign of dehydration. Fat free mass also consists of protein and minerals, and their proportions differ between people.
How to read change over time
- Follow the value alongside fat free mass. When fat free mass rose and hydration rose with it, part of the gain is water rather than tissue.
- When fat free mass rose and hydration stayed the same, it is more likely that the tissue itself was added.
- Short-term swings of a few tenths of a percent are ordinary and reflect mainly measurement conditions and the phase of the cycle, not a real change in composition.
- If hydration rises over time while the ECW Ratio rises too, that is the same phenomenon described from two sides — more water in lean tissue, and a larger part of that water outside the cells. Even so it remains a description, not a conclusion about cause.
General principles: how to read change between measurements →
What to read it with
- Fat free mass (FFM) – the denominator, without which a change cannot be interpreted.
- Total body water in litres – the numerator.
- The ECW Ratio – adds the information about where that water sits: inside the cells or outside them.
- Water share of weight (TBW/WT) – the same idea related to the whole of weight including fat.
- Skeletal Muscle Mass and Soft Lean Mass – show whether the tissue component moved.
- Protein – the other main component of fat free mass, whose change shifts the ratio just as a change in water does.
Common misunderstandings
- The value is not a measure of whether you drink enough. It describes the composition of lean tissue, not current fluid intake.
- Higher hydration is not automatically a better result. It means only relatively more water in lean tissue, which can also accompany a shift of water out of the cells.
- The value is not the same as the water share of weight. They differ in the denominator: fat free mass instead of total weight.
- The 73 percent figure from the manual is a general characteristic of fat free mass, not a personal goal or a threshold.
- Because water is the main component of fat free mass, do not expect large swings. A change of five percentage points would point to a problem with measurement conditions rather than a real change in the body.
What can affect the result short term
- Fluid and salt intake before the measurement, and food shortly beforehand.
- Training, sauna and showering — they shift fluid between compartments.
- Time of day and body position; the manual recommends standing for at least 5 minutes beforehand.
- Menstruation, during which body water fluctuates; the manual advises against measuring in this period.
- Illness, fever and alcohol the day before.
- The quality of electrode contact — dry or unwiped skin worsens the impedance measurement that water is derived from.
What to do next
- Use the value as a check when following fat free mass. It is not a goal in itself.
- When fat free mass jumps, look here before attributing the gain to muscle.
- For the question of where the water is, move to the ECW Ratio; for how much there is in total, to total body water in litres.
- Compare only measurements with the same preparation. In a value with such a narrow span, non-comparable conditions show up markedly.
What the value does not determine
- It does not establish hydration status or dehydration.
- It does not say whether water is inside the cells or outside them.
- The manual gives neither a recommended range nor a threshold for it.
- It is an internal ratio of the model in which water is also the main component of the denominator, so it has a small range.
- It does not diagnose oedema, inflammation or any other condition.
On terminology
- On the result sheet and in the research parameters the value is labelled TBW/FFM. Despite the shorthand name, it is not hydration in the sense of fluid intake but the water share of fat free mass.
- The figure of roughly 73 percent is a general physiological statement from the manual, not a recommended range.
In more depth, with sourcesIn more depth: why hydration of fat free mass moves within such a narrow bandExpand the detailed explanation
The assumption of roughly constant hydration of lean tissue is one of the cornerstones of the whole of bioimpedance analysis. The method estimates a volume of water from impedance and derives fat free mass from it precisely through the relationship that water makes up around 73 percent of lean tissue. The InBody manual gives that figure and at the same time defines fat free mass as the sum of total body water, protein and minerals. TBW/FFM is therefore an internal ratio of the model and naturally stays close to that level — its strength lies not in the absolute value but in small shifts and their direction.
The ESPEN reviews of bioimpedance principles describe departure from the assumed hydration of lean tissue as the main systematic source of error in the method. If the actual water content of lean tissue departs from the assumption, the estimate of fat free mass shifts and with it everything derived from it — fat mass, percent body fat and the height-normalised indices. Following TBW/FFM therefore serves not only to describe tissue but also to tell the user when the rest of the result should be read more cautiously.
The second set of ESPEN guidelines, on the practical use of bioimpedance, confirms that caution: in states with altered fluid distribution, body composition estimates are less reliable. In such a situation the combination of TBW/FFM with the ECW Ratio is more informative, because the first shows how much water falls to lean tissue and the second whether it is divided in the usual way between the inside of cells and their surroundings.
The manual publishes neither a recommended range nor a cut-off for this value, and gives no equation for obtaining total body water from impedance. A public explanation must therefore not construct thresholds of its own or present 73 percent as an individual target. The figure describes the composition of fat free mass in general, not a required value for a particular person.
Sources
- InBody 970 Result Sheet Interpretation (ENG_A_211104) — InBody Co., Ltd.
- Bioelectrical impedance analysis – part I: review of principles and methods — Clinical Nutrition (ESPEN guidelines)
- 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 ECW Ratio Analysis and the separate Body Water result sheet.
- 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.
Does lower hydration mean I do not drink enough?
No. The value describes what share of lean tissue is water, not current fluid intake. Fat free mass also contains protein and minerals, and their proportions differ between people.
What value is correct?
The manual states that water makes up roughly 73 percent of fat free mass, but publishes neither a recommended range nor a threshold for this metric. Treat 73 percent as a general characteristic rather than a personal goal.
What use is the value if it barely moves?
That is precisely its use. When fat free mass changes markedly between measurements, this value helps you judge whether the change rests on real tissue or on water. A small shift in a narrow band means more here than a large swing in a metric with a wide range.
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.