These types of scales are not really made for giving good quality snapshot numbers. The technology behind them is not that accurate. As has been mentioned above, they send a current through your body and measure resistance. Differences in resistance are due to water content in the body. Muscle has more water then fat. So a higher water percentage equates to more muscle mass. The calculations also use height (don't give yourself two extra inches like in your Tinder profile
), and - I think - age. So the scales measure two things: weight & electrical resistance. All of the other numbers the scales give you are just the results math formulas.
Some of the detailed numbers and analysis mentioned in this thread above are difficult to get in a laboratory much less from a $100 bathroom scale.
Any one single measurement on any of these scales is pretty much worthless if you are expecting accurate numbers. And comparing numbers between just-rolled-out-of-bed and right after hard workouts on the same day may be interesting but is not what these scales are designed to do. I did some research about getting a scale last year and everything I read suggested taking your weight at the same time every day and the best time to do that is when you wake up, after emptying your system, and before eating or drinking anything. Due to the six to eight hours of sleep and fasting that will give a more consistent measurement from day-to-day. Waiting until later in the day will likely skew results based on what you've eaten & drunk and what exercise or other activities you've done each day.
Does that make these scales a waste of money? Not at all. Over time you'll see trends in the numbers to help see if your fitness and health plans are working. Maybe you are trying to lose, maintain or gain weight, or trying to get more lean, or adding muscle mass at a faster rate than fat mass. Over the course of several weeks the numbers from these scales should show the trend. Personally, I use my scale every morning I'm at home and log the numbers in a spreadsheet. I also keep a seven day rolling average of those numbers and pay most attention to the trend of those 7 day averages.
Having dropped 48 pounds since Jan 1, 2020 (based on seven day rolling averages of course) the numbers and trends from my scale have been additional confirmation that my exercise and diet plans are working. That's in addition to how my clothes fit, blood pressure, my performance biking, running, lifting weights, and just looking in the mirror. I don't care if the fat or muscle ##s the scale gives me are spot on accurate this morning. But I can see that they've changed in the right direction compared to two months ago. And by looking at the rolling averages I can see whether I've been consistent or jumping around over time (happy to say I'm usually consistent although there was a definite blip following my birthday weekend earlier this month
)