The TANITA body fat monitor/weighing scale uses BIA (Bioelectrical Impedance Analysis) technology. This method of body fat analysis has high repeatability and is easy to use; however, normal daily changes in hydration levels affect the measurements. When your hydration level is lower, you are likely to see a higher fat percentage. When your hydration level is higher, you are likely to see a lower fat percentage.
Our total body weight can also fluctuate, even up to 2.5 kg in the course of a day. This is partly caused by what you eat and drink, but also by the body's ever-changing hydration level. Things like exercise, sleep, hormonal fluctuations, salt intake, alcohol intake, dieting, the outside temperature, air humidity, stress, being sick and our toilet habits all affect the body's water level. If you measure daily, or even several times a day, this can lead to confusion as the differences can seem large. Even if you repeat a measurement immediately, you may see a change. This is normal. Changes of 1 to 2% in repeated measurements are due to the nature of BIA technology, but the end result is still an accurate representation of your body composition.
The electrodes in the scale send the same safe, low-frequency current through the body each time to measure impedance. When the same settings are used, the validated equations used to calculate and estimate results are the same each time. If the results change from measurement to measurement, even if the total body weight is the same, it means that the impedance in the body has changed. This may be due to a change in hydration levels and/or a change in skin conductivity (e.g. warm feet or hands vs cold feet or hands).
When your measurement results change from day to day it doesn't mean that the scale is broken, it could mean that your body water level and or weight is fluctuating. To minimise the effect of the normal daily body water and/or weight fluctuations on the measurements, it's advised to take measurements at the same time of day and under the same circumstances as much as possible.
To get the most accurate results from your TANITA, we have created some tips for the best measurement routine: How to get the best results from my TANITA?
In general we recommend to take measurements a few hours after waking up, before exercise rather than after, before eating or drinking a large meal rather than after, after going to the bathroom, in the same room/environment and when your body is on room temperature.
Furthermore we recommend to make an average of the weekly and montly readings, so you can monitor long term progress and so that the impact of daily fluctuations is minimised.
For more information about how the technology works see: