What does the ECW/TBW ratio mean?
It is one number divided by another. TBW is all the water in your body. ECW is the part sitting outside your cells — in blood plasma and the fluid between tissues. The ratio says what proportion of your water is on the outside.
The arithmetic
Your scan prints total body water, and usually splits it into intracellular fluid (ICF) and extracellular fluid (ECF):
- TBW = ICF + ECF
- ECW/TBW = ECF ÷ TBW
So a scan reading ICF 25.6 L and ECF 12.8 L gives TBW 38.4 L and a ratio of 0.333. If your report shows the two components but not the ratio, you can work it out from them.
What counts as normal
Roughly 0.36 to 0.39 is the range usually quoted for healthy adults. Lower tends to go with more muscle — muscle cells hold a lot of water inside them, which pushes the intracellular share up and the ratio down. Athletes often sit at the bottom of the range or just under it.
Values drifting above about 0.40 are where the figure starts attracting clinical attention.
What moves it
- Hard training. A heavy block causes muscle damage and inflammation, and fluid moves outward. This is normal and reverses with recovery.
- Inflammation or illness. The same mechanism for a less welcome reason.
- Losing muscle. Less intracellular water raises the ratio without anything being added outside.
- Sodium, sleep and time of day. Enough to move a single reading, which is the main argument for not reading too much into one.
Why one reading tells you almost nothing
Bioimpedance estimates water by passing a small current through you, so it is sensitive to how hydrated you were, when you last ate, whether you had trained that morning and how warm the room was. The measurement error on a single scan is comfortably larger than the change you are hoping to detect.
The ratio is useful as a trend, across scans taken under similar conditions — same time of day, similar hydration, not straight after a session. A steady climb over several months is a signal. Tuesday being higher than last Tuesday is not.
And if it is persistently high, that is a conversation with a doctor. A fitness app can show you the line; it cannot tell you why it is going up.
Seeing the trend without a spreadsheet
Fitlectic reads Evolt 360 and InBody PDFs on your phone and keeps the whole history, so ICF, ECF and the ratio plot across scans rather than sitting on separate sheets of paper. Where a report prints the components but not the ratio, it derives it — and never overwrites a figure the machine actually printed.
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