How to use them
As with conventional scales, it’s best to measure yourself regularly and record the results rather than rely on one-off or occasional measurements.
• Make sure your feet are bare and clean for good contact with the scales’ sensor pads.
• Put the scales on a hard, level floor.
• Don’t take a reading immediately after waking, after a meal, or for 24 hours after excessive exercise or alcohol intake. Your body’s water content could be uneven or atypical, which will make the reading unreliable.
• Measure yourself at the same time of day under the same conditions.
All models on test tend to under-read body fat percentage, so don’t take the displayed values as absolute truth. The best ones under-read by about 3-5%, so, if they say your body fat percentage is 25%, it’s likely to actually be 28% or even 30%. Nevertheless, they’re still good for tracking your body’s change over time.
How they work
Body fat scales use bioelectric impedance analysis (BIA) to analyse your body composition.
There are more accurate ways of measuring body fat, such as underwater weighing and X-ray absorptiometry, but these are much less convenient as they require specialised equipment and expertise.
In BIA, a very low electrical current is sent through your body via your feet. The current is harmless and too low to be felt. However, all the tested models' instructions warn they aren't suitable for people with pacemakers as the scales' electrical current could interfere with the pacemaker. Many also warn the readings can be unreliable for children, athletes and bodybuilders, people with metal plates or screws in their bodies and pregnant women.
Tissue containing a lot of water, such as muscle, lets the current through easily, but fat contains comparatively little water, so it resists the current – the higher the impedance, the more fat there is in your body. The scales use that data, together with personal data you enter such as your height, age, sex and fitness level, to calculate your body-fat percentage.