Total Body Water on an InBody Scan: What the Number Means
Total body water on an InBody scan is the machine’s estimate of all the water in your body, in litres. It is split into intracellular water, held inside your cells, and extracellular water, found in blood, lymph and the spaces between cells. Because muscle holds a lot of water and fat holds very little, total body water rises and falls largely with lean mass. It is the foundation of the whole scan: the InBody estimates water first, then works out muscle and fat from it. Day to day, the figure moves with what you have eaten and drunk, so compare it only between scans done under the same conditions.
This post explains each water figure on the sheet, what moves them, and what they can and cannot tell you.
Why water comes first on an InBody
An InBody does not measure muscle or fat directly. It sends a small electrical current through your body at several frequencies and measures the resistance. Water, with the electrolytes dissolved in it, conducts the current; fat does not. So the first thing the machine can estimate is how much water you are carrying.
From there it uses known relationships. Lean tissue is mostly water, so total body water allows the machine to estimate lean mass. Subtract lean mass from body weight and you have an estimate of fat mass. That chain explains why a change in hydration can make your muscle or fat readings move, even if nothing real has changed. For the full walk-through of the sheet, see how to read your InBody results.
The water figures on the sheet
| Figure | What it is | What tends to move it |
|---|---|---|
| Total body water (TBW) | All water in the body | Lean mass, recent food and drink, sweat |
| Intracellular water (ICW) | Water inside cells | Muscle mass, mostly; rises as muscle grows |
| Extracellular water (ECW) | Water outside cells: blood, lymph, tissue fluid | Salt intake, hard training, inflammation, swelling |
| ECW ratio (ECW/TBW) | Share of total water that sits outside cells | Balance between the two above |
Not every InBody model shows every figure, but most models used in gyms and studios report at least total body water and the ECW ratio.
Intracellular water: the training signal
Muscle cells hold a great deal of water, along with the stored carbohydrate (glycogen) that binds water to it. As you build muscle, intracellular water tends to rise with it. Over months of consistent strength training, a gradual increase in intracellular water is a reasonable sign that lean tissue is increasing, provided the scans were done under similar conditions.
The flip side: a big drop in carbohydrate intake can lower glycogen and the water held with it, so intracellular water and lean mass can appear to fall in the first week or two of a low-carbohydrate diet. That is mostly water, not lost muscle.
Extracellular water and the ECW ratio
Extracellular water is the fluid outside your cells. The ECW ratio expresses it as a share of total body water, and the InBody sheet shows a normal range for it next to your result.
A slightly higher ratio than usual on a given day can follow:
- A very hard training session in the previous day or two, particularly heavy lower-body work.
- A salty meal or a lot of alcohol the night before.
- Long periods of sitting or standing, such as a long-haul flight.
- Minor injuries or local inflammation.
A ratio well outside the normal range repeatedly, or noticeable swelling in the ankles, legs or elsewhere, is not something a fitness scan can explain. Swelling can have many causes, some of which need medical attention. Check with your GP first if you notice persistent swelling, or if you have a heart, kidney or liver condition. A studio scan is not a diagnostic test.
Why the number moves day to day
People are often surprised by how much total body water can shift between scans a few days apart. The usual reasons:
- Food and drink. A large meal, or a litre of water before the scan, changes both weight and the reading.
- Carbohydrate. A high-carbohydrate day stores more glycogen, and more water with it.
- Salt. Higher sodium intake encourages the body to hold more fluid outside cells.
- Sweat. A hot day or hard session beforehand lowers water temporarily.
- Alcohol. It can leave you relatively dehydrated the next morning.
- Menstrual cycle. Many women retain more fluid at certain points of the cycle.
The same shifts explain much of the scale-weight noise people see. Our post on water weight fluctuations covers that in more detail.
Getting a reading you can compare
Consistency is everything. A simple checklist before each scan:
- Same time of day, ideally morning.
- No food for two to three hours beforehand and only normal sips of water.
- No exercise beforehand that day.
- Use the toilet before stepping on.
- Avoid alcohol the night before.
- For women, note where you are in your cycle and compare scans at similar points if possible.
Our guide on how to prepare for an InBody scan explains the reasoning behind each step.
Is the body water figure a hydration test?
Not really. It is tempting to read a lower total body water as “dehydrated”, but the figure reflects body size and muscle as much as hydration. A larger, more muscular person will always have more water than a smaller one. Comparing your water to someone else’s tells you very little.
For day-to-day hydration, the NHS suggests pale yellow urine is a simple, useful guide, and its guidance on water and drinks covers how much fluid most adults need. That is more practical than reading a scan.
Who should not be scanned
InBody scanners pass a small current through the body, so they are not used by people with pacemakers or other implanted electronic medical devices. Manufacturers also generally advise against scanning during pregnancy.
How we use it
In the studio on Archway Road, we read the water figures alongside everything else on the sheet, and alongside how training is actually going. A rising intracellular water trend with improving strength is encouraging. A one-off jump in extracellular water after a hard leg session is usually just the session. You can read more about how scans fit into coaching on the Highgate personal training page.
Questions we get asked
What is a good total body water number?
There is no single good number. It depends on your size and muscle mass. The trend between scans matters more than the figure itself.
Why did my muscle drop when I barely changed anything?
Most likely hydration or glycogen changed. Check your pre-scan routine was the same both times.
Should I drink more water before a scan to improve it?
No. Drinking extra changes the reading rather than improving anything real, and makes comparison harder.
Is an InBody scan part of training sessions?
See our personal training page for what is included and current prices.
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