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Along with a connected watch, I also bought a connected bathroom scale.

Both are giving me plenty of indicators, very extended ones. And sometimes, I wonder where and how they can extract or deduce them.

A simple one: my bathroom scale dares to tell me my total body water level.
The software displays (I've only had the scale for one week) this for my weight:

enter image description here

and this for my total body water:

enter image description here

These two curves don't move in the same direction nor the opposite one.
From what could the total body water be determined and is it reliable?
The scale has so few values about me and I don't see how it could measure total body water.

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    $\begingroup$ Welcome to Biology StackExchange! Your scale very likely uses a small electric current to measure the electrical properties of your body. It turns out that water and fat have slightly different electrical properties, which can easily be measured. This is called Bioelectrical impedance analysis and is an established method for estimating body composition. The accuracy depends on the quality of your scale, but also on various other (physiological) factors. $\endgroup$
    – Domen
    Commented Feb 4, 2022 at 15:36
  • $\begingroup$ @Domen Thanks a lot! And just for my curiosity, when my scale displays 71.3 kg and 53.3% of total body water level, does it means that I weight "33.3 kg without water"? $\endgroup$ Commented Feb 4, 2022 at 18:12
  • $\begingroup$ Yes. Human body consists of approximately 53 % of water (by weight). Given your weight, you are composed of 38 kg of water and 33 kg of everything else. $\endgroup$
    – Domen
    Commented Feb 4, 2022 at 20:18
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    $\begingroup$ @Domen feel free to turn that comment into an answer. $\endgroup$
    – MattDMo
    Commented Feb 5, 2022 at 21:51
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    $\begingroup$ @Domen Yes, it is useful to everyone, and you deserve the answer reward for it. $\endgroup$ Commented Feb 6, 2022 at 9:48

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Your scale exploits the difference in electrical properties of water and fat to measure the composition of your body. This method is called bioelectric impedance analysis (BIA), and it is usually used for estimating the composition. There exists several other, more accurate methods, such as weighing a person under water (hydrostatic weighing), dual-energy X-ray absorptiometry (DXA), CT, and MRI – the last two usually regarded as the gold standard1.

The basic principle of BIA is to use a small electric current through your body and measure its electrical response. Roughly speaking, cell membranes can be regarded as electric capacitors, water as a good conductor, and fat as a bad conductor. In the simplest model, human body is therefore represented as a resistor and a capacitor in a series ($RC$ circuit), and electrical impedance (combination of electrical resistance and electrical capacitance) is measured at a single frequency (e.g. $50\;\mathrm{kHz}$).

The measurement together with your physical dimensions (height and weight), your age, and your gender – all of which you had to disclose to your bathroom scale – are then put into a BIA formula, which the manufacturer has obtained by calibrating its device with one of the more accurate methods. An example of such formula2 is $$FFM = (0.34 \times10^4) \frac{h^2}{Z} + 15.34 h + 0.273 m - 0.127t + 4.56 S - 12.44,$$ where $FFM$ is fat-free mass, $h$ body height (in meters), $Z$ electrical impedance (in Ohms), $m$ body mass (in kilograms), $t$ age (in years), and $S=0$ (female), $S=1$ (male).

The accuracy of BIA depends on the quality of the device, but there are also other important factors, such as the environment, ethnicity, phase of menstrual cycle, and underlying medical conditions3.


  1. https://jim.bmj.com/content/66/5/1.10
  2. https://www.ncbi.nlm.nih.gov/labs/pmc/articles/PMC4608267/
  3. https://www.ncbi.nlm.nih.gov/labs/pmc/articles/PMC2543039/
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