I've used digital weight and body-fat scales for years now, but I really wonder about their efficacy. I've obviously thought it's important to have one around, since your level of lean mass v. body fat is so closely linked with health, particularly among males.
There's also a cachet among fit males and bodybuilders along the lines of whether they have body fat far under 10 percent or not, but I've leave that discussion for another time.
Ultimately, behind the scenes, these "electrical impedance" scales use algorithms to estimate your body fat percentage. Many of them claim an accuracy of about 2.5% on either side of the reading; for example, if it says 10 percent, then your body fat probably falls somewhere between 7.5% and 12.5%.
But here's the wrinkle: much of the reading is going to be determined by the preferences you set before you even step on the scale, such as whether you're male or female, your age, and whether you have an athletically cut body or not (I suppose, the classic X shape). When I set up my last scale, my body fat went from something like 16% to 11% based purely on the latter setting, whether I was built like an athlete or not.
The internal software is basically estimating your body compostion based more on settings than any contact with your body.
And that's not all: I've noticed that the ambient air temperature makes a huge difference. I always get a lower body-fat reading with a room temperature of 70 or greater. I always get the lowest readings during the summer, regardless of my weight at the time.
For some reason, when I weigh more, the scale counts that as predominantly more muscle (how flattering!). Especially when the room is warm. And when you're wet, the BF reading also goes down significantly. The list goes on.
Your body composition does not change significantly week by week. So that when you step on the scale and the day-by-day fluctuations are four percent or more, you know the scale itself is a bit topsy-turvy. My sense is that most people who rely on scales get a big surprise when they test their body fat % in the most accurate manner in a performance lab, using calipers and pools, etc.
The next time you are pondering whether to deploy the plastic for a fancy "electrical impedance" machine, you might reconsider whether the full-length mirror is a more effective and affordable substitute.