ECW Ratio – body water distribution

• A calculated value. The formula is extracellular water (L) ÷ total body water (L).
• Neither input volume is calculated from weight — both are derived from impedance measured at several frequencies. The manufacturer presents them as measured quantities rather than estimates.
• Separating the two compartments is possible because of the physics of the cell membrane. Low frequencies do not pass through it well and therefore reflect mainly extracellular water. High frequencies do pass through and also capture water inside cells. The difference lets both compartments be measured separately.
• The manufacturer does not publish the regression equation that turns measured impedance into a volume of water in litres. The value therefore cannot be independently recalculated.
• The recommended range of 0.360 to 0.390 is a manufacturer reference, not a measured quantity. Age and sex do not enter the calculation itself — they are used only for reference ranges and scores.

• The recommended range in the manufacturer manual is 0.360 to 0.390.
• The Evaluation Result Sheet uses a finer split: below 0.390, a band of 0.390 to 0.400, and above 0.400. That split gives no lower bound, whereas the recommended range does. Treat both as a manufacturer reference rather than a health threshold.
• A value around 0.38 corresponds to the roughly 3 : 2 ratio of intracellular to extracellular water the manual describes in a healthy population.
• A value above the range says one thing only: a relatively larger part of your water sits outside the cells. It does not establish whether the reason is hydration status, measurement conditions, a lower amount of muscle, or a medical cause.
• The manual itself notes that a higher ratio also appears in people without any particular diagnosis — it names lean women, older adults and people with a high proportion of fat. The reason given is a relative increase in extracellular water for nutritional reasons or where muscle is lacking.
• A lower value is not automatically a better result. The recommended range has a lower bound too, and below it a single number still confirms nothing.

• The ratio is among the values that respond most to measurement conditions. Compare only measurements taken the same way: ideally morning, fasted, after using the toilet, without training or sauna beforehand, and after a few minutes standing.
• A difference of 0.001 to 0.003 between two measurements sits within ordinary variation. The information is in the direction across several measurements, not in a single jump.
• When the ratio jumps once, start with the measurement conditions and with what preceded them — training, a salty meal, alcohol, prolonged sitting, phase of the cycle.
• A steadily rising ratio accompanied by falling muscle mass is a different situation from a one-off swing. In that case the numerator and denominator often move at once, so look up ICW and ECW in litres separately.
• The manufacturer Comparison Result Sheet shows the change in ECW Ratio between the current and previous measurement for the whole body and for each of the five segments. The segmental view often reveals that the change is concentrated in one area.

General principles: how to read change between measurements →

• Total body water and intracellular and extracellular water in litres – only these show whether the numerator, the denominator, or both moved.
• The segmental ECW Ratio – separates a whole-body shift from a deviation in one segment.
• Segmental lean mass – the manual explicitly warns that a raised ratio can overstate lean mass through water.
• Skeletal Muscle Mass – the manual itself names a shortage of muscle as a reason for relatively higher extracellular water.
• Body Cell Mass (BCM) – the manual recommends it as a nutritional indicator in people with oedema, fluid retention or ascites, where BMI and fat free mass tend to be distorted by excess extracellular water.
• The ECM/BCM ratio – a second view of the same balance between the extracellular and cellular parts of the body.
• Phase angle – another indicator derived directly from the electrical properties of tissue, which often moves alongside water balance.

• A raised ratio is not a diagnosis of oedema. The manual mentions oedema in connection with disturbed water balance, but it describes a physiological state, not an output of the device. Oedema is a clinical finding that a single bioimpedance number neither confirms nor excludes.
• The ratio does not measure dehydration. It describes how water is divided between two compartments, not whether you have enough of it.
• A higher ratio does not automatically mean you are retaining water in the sense of quantity. It also rises when water inside the cells falls — typically alongside a loss of cell and muscle mass.
• The value is not a percentage. 0.380 means 38 percent of water outside the cells, not 0.38 percent.
• The ratio cannot be compared across devices from different manufacturers or across methods. It arises from the model and frequencies of a particular device.
• The recommended range is not a medical norm. The manufacturer uses the terms recommended and reference range precisely so that the value is not read as a diagnostic threshold.

• Food and drink shortly before the measurement — the manual recommends measuring fasted, because stomach contents count towards weight and towards body composition.
• Training before the measurement — it raises perfusion and shifts fluid.
• Showering and sauna — hot and cold alike change circulation and fluid distribution.
• Body position. The manual recommends standing for at least 5 minutes beforehand, because moving from lying to standing lets gravity draw blood into the lower limbs.
• Time of day — water shifts towards the lower half of the body during the day, so the manual recommends measuring in the morning and repeating measurements under the same conditions.
• A full bladder; the manual recommends using the toilet before the measurement.
• Menstruation, during which body water fluctuates. The manual advises against measuring in this period.
• Higher salt intake, alcohol the day before, illness and fever.
• Room temperature outside the recommended 20 to 25 °C, and dry or insufficiently wiped skin on the electrodes.

• It does not diagnose oedema, inflammation, kidney disease, heart failure or any other condition.
• It does not establish whether someone is dehydrated or adequately hydrated.
• It cannot say whether a deviation arose from a gain in water outside the cells or a loss of water inside them. ICW and ECW in litres are needed for that.
• It cannot separate a real change in water balance from an effect of measurement conditions.
• The equation the device uses to calculate water volumes from impedance is not public, so the value cannot be independently recalculated.
• Values are not interchangeable between devices from different manufacturers or with reference methods for measuring body water.