I have two electronic scales, one quite expensive from John Lewis, which does fat, water, muscle and BMI. The fat, water and muscle are given in percentages, and add up to more than 100. It claims to work to an accuracy of one tenth of a pound, but it always changes in largish amounts, after sticking at the same weight for several days. (In the morning, on empty). Sometimes it sticks when I have been to the loo again, and not eaten or drunk anything. The cheap one, from Lidl, I bought to check the first by, and it has the same problem of not showing small changes. They never tell the same amount, and the Lidl one is usually a higher weight than the JL one - but not always.
I had, briefly, an old-fashioned spring operated one -it was not better.

Penny