Total Body Water (TBW)

• Derived from measured impedance. The manufacturer presents body water as a measured quantity rather than an estimate — unlike minerals, for example.
• The device measures impedance at eight frequencies across five body segments. Low frequencies do not cross the cell membrane and therefore reflect mainly extracellular water, while high frequencies do cross it and also capture intracellular water. Combining frequencies allows both parts to be established separately.
• The manual does not publish the regression equation that converts impedance into a volume of water. It describes the principle, not the formula — a real limitation when judging accuracy.
• The reference range in brackets next to the value is a comparison against a reference derived from your details. The manual does not publish the formula for it either; the bar length in the chart relates to ideal weight, not to your current weight.

• Read the value against the reference range in brackets, not against a general figure from the internet. The range follows your build, so two people get different ones.
• A higher value is not automatically better. The manual notes that the ratio of water to weight sits between 50 and 70 percent precisely according to the amount of fat — so more water often simply means less fat rather than better hydration.
• The litres alone cannot tell you whether the water is distributed in a balanced way. That is what the ECW/TBW ratio is for, with a reference range of 0.360 to 0.390 in the manual.
• The manual also gives health context for body water: losing roughly 10 percent of body water has an immediate negative impact and larger losses can be life-threatening. The measurement itself, however, does not diagnose hydration status.

• Body water responds fast. The difference between morning and evening, before and after training, or before and after a meal can be noticeable without anything having changed in body composition.
• When body water rises together with muscle mass and the intracellular part rises faster than the extracellular one, that is the expected direction during strength training.
• When body water rises but mainly through the extracellular part, and the ECW/TBW ratio rises with it, that is a different situation. Always look at the ratio, not just at the sum.
• The manual notes that an adult loses roughly 2.5 litres of water a day through breathing, sweat, urine and stool. Daily fluctuation in body water is therefore ordinary physiology, not measurement error.

General principles: how to read change between measurements →

• Intracellular and extracellular water – show which part the volume changed in.
• The ECW/TBW ratio – separates balanced distribution from a shift into the extracellular space.
• Fat free mass and the water share of fat free mass – water makes up roughly 73 percent of fat free mass according to the manual, so they move together.
• Skeletal Muscle Mass – muscle is the main water reservoir in the body.
• Segmental body water – shows whether the change affects the whole body or only one segment.

• Body water is not an indicator of fluid intake. It does not say how many litres to drink a day or whether you drank enough today.
• High body water does not automatically mean good hydration. It can reflect a higher proportion of muscle, or conversely a shift of water into the extracellular space.
• Litres of water cannot be compared between people. A taller and more muscular person naturally carries more, which says nothing about hydration.
• A fall in body water after training does not mean you lost weight in any meaningful sense. On the result sheet, though, such a fall does show up in fat free mass as well.

• Fluid and food intake shortly before the measurement.
• Training and sauna, after which both fluid distribution and total volume change.
• Time of day. The manual notes that water shifts towards the lower half of the body during the day and recommends measuring in the morning.
• Body position before the measurement. The manual recommends standing for at least 5 minutes, because fluid distribution changes when moving from lying to sitting to standing.
• Room temperature. The manual recommends 20 to 25 °C, because circulation changes in both cold and heat.
• The menstrual cycle, during which body water fluctuates.
• A full bladder — the water in it is not counted by the measurement.

• Do not read body water on its own. The decisive information lives in the split into intracellular and extracellular water and in their ratio.
• If you are following a trend, measure under the same conditions — morning, fasted, without training or sauna beforehand. Water is the fastest-responding component on the whole sheet.
• When fat free mass or muscle mass changes unexpectedly, check body water first. The explanation is often right there.
• If you notice sustained fluid retention or swelling, that belongs with a doctor. The measurement itself cannot establish the cause.

• It does not establish hydration status as a medical diagnosis.
• It does not say how much fluid you should drink.
• It does not diagnose oedema, fluid retention or kidney disease.
• The equation converting impedance into a volume of water is not published by the manufacturer.
• Values are not interchangeable with results from other methods or other devices.