A body composition scale shows far more than your weight: body fat, water, muscle mass, sometimes a dozen other indicators. Yet these figures are not all worth the same. Weight is measured; the rest is estimated from a weak electrical current and an equation. Here is what scientific guidelines and the manufacturers’ own manuals say about these estimates, how to use them to follow a trend, and which model to choose for your profile in the Picksly Top 10.

This article explains how a consumer device works. A scale makes no diagnosis and does not replace a doctor’s advice.

What a body composition scale really measures

Weight: the only true measurement

The platform measures a force: your weight is a direct measurement. BMI (body mass index) is only a calculation, weight divided by the square of the height you entered in the app. For adults, the World Health Organization speaks of overweight from a BMI of 25 and of obesity from 30; it describes BMI as a surrogate marker of adiposity, not as a measurement of fat itself (WHO fact sheet, 8 December 2025).

Body fat, water, muscle: estimates

For everything else, the scale sends a very weak current between electrodes: under the feet on most models, under the feet and in handles on so-called “8-electrode” models. Water and lean tissue conduct the current well, fat much less so. The scale measures the body’s resistance to this current, the impedance, then converts it into a percentage of fat, water or muscle with an equation that takes your height, age and sex into account. This is bioelectrical impedance analysis.

The body fat percentage displayed is therefore not a measurement of your fat: it is an estimate, whose quality depends on the equation chosen by the manufacturer and on the state of your body at the time of weighing. The number of indicators on the box (13, 14 or 17 depending on the listing) says nothing about their reliability: they are all calculated from the same measurement.

The limits to know

The guidelines of the European Society for Clinical Nutrition and Metabolism (ESPEN), published in Clinical Nutrition (Kyle et al., 2004), conclude that impedance analysis performs well in healthy people with stable hydration, provided an equation validated for their age, sex and ethnicity is used. They do not recommend it for routine assessment at the extremes of BMI or when hydration is abnormal. They consider tracking over time possible for a BMI between 16 and 34, to be interpreted with caution.

Manufacturers say the same in their manuals. The Tanita RD-953 manual (PDF manual) states that body fat estimates vary with the amount of water in the body, and can be affected by dehydration or overhydration due, for example, to alcohol, menstruation, illness or intense exercise. The same manual states that the body fat function is not intended for pregnant women, professional athletes or bodybuilders.

Who should not use the impedance function

  • Pacemaker or other electronic medical implant: the Tanita manual says not to use the body composition function, because the current sent through the body could interfere with the implant. If in doubt, ask your doctor before buying.
  • Pregnancy: the Tanita manual restricts the scale to weight only. Its help page (Tanita FAQ) adds that there is no known risk, but that the reading would be wrong, because the scale cannot tell the fetus and amniotic fluid apart from fat; it advises waiting until after delivery to track body composition.

Instructions vary between manufacturers: read the manual of your model.

Using it well: follow a trend, not a value

The Tanita manual is clear: readings taken under different conditions do not have the same absolute values, but they can track a change as long as you always measure the same way. Hence a few simple rules, taken from that manual:

  • Weigh yourself at the same time of day, ideally about two hours after getting up, eating or intense exercise.
  • Keep the same conditions: bare, clean feet, no clothing, standing straight, the arch of each foot at the centre of the platform.
  • Compare series, not two weigh-ins: look at the change over several weeks rather than the difference with yesterday.
  • Keep the same scale: two models may use different equations, so their figures cannot be compared.

If a figure worries you, talk to your doctor rather than relying on the scale.

Which body composition scale for your profile?

The Picksly Top 10 scales ranks models by buyer rating, weighted by the number of reviews; the shop covered is Amazon France. The prices below were recorded on 27 and 28 October 2026: they change, so check the price shown before buying. Here we match the page’s price bands to the three Picksly profiles; this is an editorial choice, not a calculated score.

Budget: under €30

A 4-electrode scale connected to your phone by Bluetooth is enough to follow a trend. In the under €30 band: the Healthkeep at €18.74 (4.4/5 from 463 reviews), the RENPHO at €19.99 (4.5/5 from 149,591 reviews) and the GRIFEMA GA2001 at €13.97 (4.4/5 from 4,558 reviews).

Smart: €30 to €150

Between €30 and €80, the iHealth Fit costs €43 (4.5/5 from 4,842 reviews). If you only want weight and BMI, for example because the impedance function is not advised for you, the Withings Body (€59.95, 4.4/5 from 6,971 reviews) is limited to these two measurements according to its listing and syncs over Wi-Fi: see the €30 to €80 band. Between €80 and €150, 8-electrode models also send the current through the hands: the arboleaf at €109.99 (4.7/5 from 1,362 reviews) and the RunSTAR at €109.99 (4.6/5 from 5,449 reviews), in the €80 to €150 band.

Premium: over €150

High-end models give readings by body part (arms, legs, trunk). The TANITA BC-545N costs €249.99 (4.3/5 from 1,737 reviews), the TANITA RD-953 €199.99 (4.3/5 from 568 reviews) and the Beurer BF 1000 €249.99 (4.2/5 from 3,275 reviews): see the over €150 band. More measurements do not make a medical test: they are still impedance estimates, with the same contraindications.

Going further

If you are still unsure about the app, Wi-Fi or family sharing, our guide to choosing a smart scale covers these criteria. To also track your activity and sleep, a smartwatch or a health-tracking ring complements the scale; their readings are estimates too.

To know whether the price shown on Amazon is a good one, install the Picksly extension: it compares the page you are viewing with the offers we track, according to your profile. And for the models best rated by buyers, see our Top 10 smart scales.

FAQ

Is a body composition scale accurate?

For weight, yes: it is a direct measurement. Body fat, water and muscle mass are estimates that vary with hydration; according to the ESPEN guidelines, they are reliable mainly in healthy people with stable hydration, and useful for following a change.

When should I weigh myself?

Always at the same time and under the same conditions. The Tanita RD-953 manual advises waiting about two hours after getting up, eating or intense exercise, with bare, clean feet.

Can I use a body composition scale with a pacemaker?

Manufacturers advise against it: the Tanita manual says not to use the body composition function with a pacemaker or another electronic implant. Ask your doctor.

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Sources consulted on 17 October 2026: ESPEN guidelines on bioelectrical impedance analysis (Kyle et al., Clinical Nutrition, 2004); Tanita RD-953 scale manual and Tanita Europe FAQ on pregnancy; WHO fact sheet on obesity and overweight (8 December 2025). This article is not medical advice.

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