What a bathroom scale actually measures

A scale that reports body fat passes a tiny current up one leg and down the other and measures how easily it travels. It is harmless for most people, but anyone with a pacemaker or another electronic implant, and anyone who is pregnant, should switch that feature off, which is what the makers say too. Fat resists the current more than muscle and water do. From that reading, plus the height, age and sex you typed in, an equation guesses your body fat. The guess moves with how much water you drank, whether you have exercised, and the time of day. It can be several percentage points out on any given morning. It is not useless, but it is a trend instrument, not a measurement.

Muscle mass, bone mass, water and the rest are derived from the same reading and the same details, so they inherit the same guesswork. Treat them as rough indications that move together.

Weigh yourself at the same time of day, in the same state, on the same floor. Day-to-day changes are mostly water; over weeks the direction of travel is real, so look at the weekly average rather than any one reading.

Check the floor it will live on. Check that you can read the display from standing height. Ask whether you need the app to see your history, because a scale that keeps nothing without a phone is two things that can break.