Useful or a flop: How accurate are body fat scales?

Useful or a flop: How accurate are body fat scales?

Almost everyone complains about at least one to two kilograms being too much on their hips when they weigh themselves. But a normal scale only gives an unfiltered value. How exactly the kilos are composed remains unclear. Body fat scales should help here. However, there is a lack of accuracy.

While normal bathroom scales only measure weight, body fat scales work according to a different principle, known as bioimpedance analysis (BIA). It is based on the fact that fat, water and muscle are conductive to different degrees. The scale's electrodes send an imperceptible, weak current through the body via the feet. Because of their electrolytes, water and muscle are better conductors than fat cells. The sensors of the scale now measure the total resistance. Based on this and taking into account the entries for weight, age, gender and height, the body fat percentage is determined.

Restriction to legs and groin

However, since current always chooses the path of least resistance, the measurement is limited to the lower half of the body, i.e. the legs and groin region. So the current runs up one leg, flows through the abdomen and flows down the other leg. The fat percentage in the rest of the body, i.e. in the arms, upper body and stomach, is not taken into account.

People who correspond to the apple type – i.e. have slim legs and a round stomach – are therefore classified better than it corresponds to reality. This is fatal insofar as the visceral fat, which is located in the abdomen, is considered to be particularly dangerous. According to the NDR , it is responsible for dyslipidemia, diabetes and high blood pressure.

Body fat scales often only measure the fat levels in the legs.

Body fat scales often only measure the fat levels in the legs. Image: © Pixabay 2016

Many factors can affect the result

But the result can also be falsified by other factors. Electricity flows better when your feet are wet, your bladder is full or your skin has been freshly creamed. This can cause the fat content to be up to 30 percent lower than it actually is, according to Zeit Online . In addition, the values of a target figure were stored in the scale's algorithm. If your own proportions deviate noticeably from these, this can also have an impact on the values determined. It can happen that too little fat mass is taken into account in slim people, while overweight people may have too high a fat percentage displayed. As a result of all these factors, large measurement errors can occur with commercially available bathroom scales with a body fat analysis function.

Schon eingecremte Haut kann das Ergebnis einer Körperfettwaage verfälschen.

Skin that has already been moisturized can falsify the result of a body fat scale. Image: © Thinkstock/Jupiterimages 2016

Alternatives to the usual body fat scale

More accurate values can be achieved with models that determine their result using several electrodes that can be attached to hands and feet. However, these quickly move within a price range of several hundred euros. If you want to know exactly, however, you cannot avoid going to the sports doctor. Bioimpedance analysis is also often used here, but the expert can rule out disruptive factors and works with up to eight electrodes that are distributed over the entire body and collect meaningful data.

Alternatively, however, calipometry can also be used. The body fat percentage is determined by measuring the skin folds – for example on the stomach and arms. The added values are then evaluated by the doctor using a table. Finally, the DAX method also provides exact fat values. However, this method, which is primarily used to measure bone density, is an X-ray examination. For this reason, the examination involves a low level of radiation exposure, which is why it is not suitable for children, adolescents or pregnant women.


Unfortunately, the values of body fat scales are not absolutely reliable. Too few of the devices are equipped with sensors that take the whole body into account when measuring. According to Fit For Fun , if you still want to use the body fat scale as a guide, you should always use it under the same conditions – for example, always in the morning, always before or after exercise. These factors have an additional influence on the result.


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