If you are looking at the app and seeing a place to enter body fat %, that's mainly for using the Aria 2 scale which measures and loads the body fat % automatically. It is not required and can just be left blank if entering weight manually.
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Best AnswerIt probably uses the same method as the Fitbit scales. No methods are totally accurate. I would imagine yours is ballpark the same as Fitbit's.
Here's an article about weighing in Fitbit scales, some of which would apply to any.
How can I get the most accurate reading on my Fitbit scale?
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All scales work on the same principal: biolectrical impedance analysis (BIA). That method is very sensitive to hydration, hence the recommendations in the help article cited by @JohnnyRow : step on your scale consistently at the same time of the day, preferably first thing in the morning.
I can’t comment about the Yunmai scale, but based on my daily use of the Fitbit Aria for almost six years, changes in body fat % correlate pretty well with changes in body weight. If you manage to get your weigh-ins (weight + BF%) to your Fitbit account, I suggest you link your account to TrendWeight (see link in my signature). Short term fluctuations do not mean much, longer term changes are what matter.
Other ways to assess body fat: 1) tape meter (take measurements of your waist circumference), 2) the mirror.
Dominique | Finland
Ionic, Aria, Flyer, TrendWeight | Windows 7, OS X 10.13.5 | Motorola Moto G6 (Android 9), iPad Air (iOS 12.4.4)
Take a look at the Fitbit help site for further assistance and information.