Body fat scales

Body fat scales send a very low electrical current through your body via your feet. 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 resistance, the more fat you have.

The scales use that data, together with information you enter, such as gender, age, height and level of fitness to calculate your body fat percentage.

Some body fat scales aren’t suitable for people with pacemakers as the electrical current could interfere with the pacemaker. If you have a pacemaker, check with your doctor before using a body fat scale. The readings may be unreliable for children, athletes and bodybuilders, people with metal plates or screws in their body, and pregnant women.

• The display should be big and clear so it’s easy to read when standing on the scales.
  
• Displayed data should be easy to understand and stay on screen long enough to read easily. Ideally, you should be able to redisplay the data without weighing yourself again.
  
• Controls should be clearly labelled and easy to use. Programming the scales should be straightforward.
• Instructions should be easy to understand, with diagrams and advice on how to interpret your results.
• User profiles let you save sets of individual information. There should be enough for everyone who will regularly use the scales.
• Being able to use them on carpet is useful if you don’t have hard floors. Scales don’t generally measure weight effectively on carpet.
• Other information such as BMI (Body Mass Index), muscle mass, bone mass and the percentage of water in your body is displayed in many models, but we didn’t test these features. BMI is often used by health professionals to assess whether a person is a healthy weight, but this measurement has limitations. It doesn’t consider muscle mass, which is heavier than fat, and can be unreliable for elite athletes, children and some ethnic groups.
• Wireless connectivity on some models lets you link to your smartphone or tablet via an app. This means you can store your details online and keep track of your progress over time. If the brand has other products, such as blood pressure monitors or fitness bands, you can synchronise the results.

To check the scales’ body fat accuracy, we took readings from 15 men and women. We then compared the results with those obtained from a DEXA (Dual-Energy X-ray Absorptiometre) scan, a machine used in hospitals for determining body fat composition.

To assess weight accuracy, we used calibrated 20kg and 100kg weights. Weight sensitivity measures how sensitive the scales are in incremental changes of 100g.

Most scales in our test tended to under-read body fat percentage, so your actual body fat percentage will be higher than what’s calculated. For example, if a scale got a good body fat accuracy score – this is an average difference of about 3 to 4% – compared with the DEXA scan. This means if your actual body fat percentage is 20%, the scale may read it as only 16%. The models with an OK body fat accuracy had a difference of about 4 to 5%.

We also tested if the scales were easy to use. We assessed how easy the display was to read; how easy the controls were to programme and push with your toe; feet positioning; and how stable the scales were to stand on.

For our latest update we’ve also given the scales a score for body fat deviation based on the reading that was furthest away from the DEXA scan. For example, one model gave a body fat reading that was 17.5% different from the DEXA scan. The lowest deviation recorded was 7.3%. Previously tested models have been rescored to reflect this which is why some of the overall scores may have changed.