Your body-fat percentage moves if you drink half a litre

You step on the scale barefoot and it hands you a number with a decimal: 22.4 % fat. It did not measure that.

These scales send a very weak alternating current, one you cannot feel, in through one foot and out through the other. Fat conducts badly because it holds little water; muscle and blood conduct well because they are full of water and salts. The device measures how hard it is for that current to get through — the impedance — feeds that value, along with your height, age and sex, into a statistical equation, works out the total water in your body, derives your lean mass from it and subtracts. Your fat percentage is the leftover of a subtraction.

That is why it swings so much. If you are dehydrated there is less water inside you, the current has a harder time and the scale credits you with more fat than you have. Half an hour on a bike, a salty dinner, two coffees, a full bladder or cold feet will move the figure by a good half point, and none of those things put weight on you. On top of that, in foot-to-foot models the current runs up one leg and down the other: about your trunk and your arms it knows almost nothing.

Use it as a trend thermometer. Always weigh yourself on the same day of the week, just after getting up, after emptying your bladder and before drinking or eating, and write the figure down without looking at it. Compare it six weeks from now: that is where the information is.