The short answer
For weight, almost all smart scales are reliable: in a study carried out at Bichat hospital, the median error did not exceed 300 g. For body fat, none of them is reliable in absolute terms: the same scales were off by 2 to 4 kg compared with the reference examination. The right scale is therefore the one that measures weight accurately, syncs without effort and gives you a reliable trend. The Withings Body Smart, at 99.95 EUR, ticks these boxes with the best compatibility; the Beurer BF 720, at 39.99 EUR, is enough if you mainly want to track your weight.
The eight scales compared
| Scale | Official price | Electrodes | Graduation | Users | Connection | Power |
|---|---|---|---|---|---|---|
| Renpho Elis 1 | 27.99 EUR | 4, feet | not recorded | not recorded | Bluetooth | 3 AAA batteries |
| Beurer BF 720 | 39.99 EUR | 4, feet | 100 g | 8 | Bluetooth | 3 AAA batteries |
| Withings Body Smart | 99.95 EUR | 4, feet | 50 g | 8 | Wi-Fi and Bluetooth | 4 AAA batteries, 15 months |
| Garmin Index S2 | 149.99 EUR | feet, number not stated | not stated | 16 | Wi-Fi | 4 AAA batteries, up to 9 months |
| Renpho MorphoScan Nova | 189.99 EUR on promotion, 229.99 EUR otherwise | 8, feet and hands | 50 g | 8 on the scale | Bluetooth | USB-C battery, up to 5 months |
| Withings Body Comp | 199.95 EUR | feet, number not stated | 50 g | 8 | Wi-Fi and Bluetooth | 4 AAA batteries, 15 months |
| Xiaomi Body Composition Scale S400 | not displayed by the brand | feet | 100 g | 36 | Bluetooth | 3 AAA batteries |
| Omron VIVA | not displayed by the brand | not stated | not stated | 4 and a guest | Bluetooth | 4 AAA batteries |
Impedance measurement: what it really measures
A current, not a direct measurement of fat
A body composition scale sends a very weak electric current from one foot to the other, or from the feet to the hands for models with a handle. Water and muscle conduct current well, fat much less so. The scale therefore does not « see » fat: it measures a resistance, then estimates body fat with an equation that takes your height, age and sex into account. Anything that changes your hydration, such as a meal, a workout or a glass of water, changes the result.
What the studies show
| Study | Participants | Result |
|---|---|---|
| Frija-Masson et al., Bichat hospital, 2021: Tefal, Terraillon, Withings Body Cardio versus DXA | 48 to 53 per scale | weight accurate to within 300 g; body fat underestimated by 2.2 to 4.4 kg in median |
| Nana et al., 2022: foot-to-foot Tanita scale versus DXA | 929 adults | body fat percentage not equivalent to DXA, even with a tolerance of 5 points |
| Siedler et al., 2023: 14 consumer devices versus a reference model | 73 | mean error of -3.5 to +11.7 points of body fat depending on the device |
The authors of the French study conclude that smart scales « are not accurate for body composition ». In other words: if your scale shows 28% body fat, your true value may be noticeably different. However, if you weigh yourself under the same conditions, the trend over several weeks remains useful: that is what you should follow.
More electrodes, more frequencies: an advantage to put into perspective
Models with eight electrodes, such as the Renpho MorphoScan Nova, send the current through the arms and legs, and estimate the composition of each limb. Multi-frequency models, such as the Body Smart or the Xiaomi S400, measure at several frequencies. On paper, this is more complete. But none of the studies cited shows that a consumer model reaches the accuracy of the reference examination: the gap between devices remains wide, and the number of indicators displayed, up to « more than 50 » at Renpho, says nothing about their accuracy.
Pacemaker wearers, pregnant women
All the brands recorded advise against impedance measurement for people with a pacemaker or another electronic implant: the current could interfere with the device. Withings offers a simple weighing mode that turns it off. For pregnancy, the guidance varies: Beurer and Eufy write not to use the scale, Omron to track weight only, and Withings recommends turning off impedance on the Body Comp, for lack of validation.
The detail, scale by scale
Withings Body Smart: the best balance
At 99.95 EUR, the Body Smart weighs to within 50 g and measures body fat, water, muscle and bone mass, visceral fat and standing heart rate. It connects over Wi-Fi, so it syncs without a phone nearby, recognises up to eight users and lasts 15 months on its batteries. It is compatible with Apple Health, Google Health Connect and Google Fit, and offers pregnancy, athlete and baby modes. One inconsistency to note: its product page states a frequency of 250 kHz, the brand’s comparison page 100 kHz.
Beurer BF 720: the essentials for under 40 EUR
The BF 720, at 39.99 EUR, weighs to within 100 g with four steel electrodes, and measures body fat, water, muscle and bone mass. It connects over Bluetooth only: you need to open the app to sync. For tracking your weight, it is more than enough.
Renpho Elis 1: the lowest price
At 27.99 EUR, the Elis 1 is the cheapest in the comparison. Four electrodes, three batteries, one app: it does the job of an entry-level smart scale. We did not record its graduation on the brand’s page.
Garmin Index S2: for Garmin watch users
At 149.99 EUR, the Index S2 connects over Wi-Fi and recognises up to 16 users. Its main appeal is its integration with the Garmin Connect ecosystem, with the training and sleep tracked by the watch. Its graduation and its compatibility with Apple Health or Google Fit are not stated on the pages consulted.
Renpho MorphoScan Nova: eight electrodes and a handle
The MorphoScan Nova, 189.99 EUR on promotion, adds a retractable handle with a screen: the current passes through the hands and feet, and the scale estimates the composition of each limb. It recharges over USB-C. It is the most complete on paper, although the studies do not allow us to say that it is more accurate than a four-electrode scale.
Withings Body Comp: extra measurements, double the price
At 199.95 EUR, the Body Comp takes the measurements of the Body Smart and adds a « vascular age » and a « nerve health » measurement. These are brand-specific indicators, which we could not compare with a medical reference. The extra cost is hard to justify for tracking your weight.
Xiaomi S400 and Omron VIVA: prices not displayed
The Xiaomi S400 claims 25 indicators, two measurement frequencies and up to 36 users, but it only works with the Xiaomi Home app. The Omron VIVA, presented as clinically validated, measures six indicators for four users. Neither brand displays a price on its official website.
Interpreting your numbers
BMI
The World Health Organization defines overweight as a BMI of 25 or more and obesity as a BMI of 30 or more. It points out that BMI is a « surrogate marker » of body fat, and that waist circumference helps refine the diagnosis.
Body fat, without a scale
Our body fat calculator applies the US Navy method, based on tape measure readings. It is also an estimate: compare it with your scale, and track both over time rather than relying on a single number.
The waist-to-hip ratio
It shows where fat is stored. Abdominal fat is more closely linked to cardiovascular risk than fat on the hips, and two tape measure readings are enough.
Does weighing yourself often help you lose weight?
This is the real question behind buying a smart scale. A meta-analysis published in 2015 in the International Journal of Behavioral Nutrition and Physical Activity answers it in three steps:
| Situation studied | Weight difference |
|---|---|
| Weighing yourself, changing nothing else (a single trial) | -0.5 kg, not significant |
| Adding weighing to a weight loss programme (4 trials) | -1.7 kg |
| Full programmes including weighing, versus little or no intervention (15 trials) | -3.4 kg |
In other words, the scale has no effect by itself: it helps when it is part of an approach, with diet, physical activity and a goal you follow. The real advantage of a smart model is that it keeps the log for you: weigh-ins are recorded automatically and the curve is drawn effortlessly.
And what about your mood? A 2016 meta-analysis covering 29 analyses found no link between weighing and anxiety, body image or eating disorders, but a small negative link with self-esteem. If the morning number weighs on you, switch to one weigh-in a week, or hide body fat in the app: it is the least reliable measurement anyway.
When and how to weigh yourself
- Always under the same conditions: in the morning, after going to the toilet, before eating and drinking, barefoot and with dry feet.
- On a hard, flat floor, never on a carpet.
- Not straight after exercise: sweat distorts the impedance measurement.
- Look at the weekly average, not the daily number: weight varies from one day to the next with water and the contents of the gut.
To turn these numbers into a goal, see our guide on the calorie deficit and our comparison of slimming programmes.
Our sources
- Frija-Masson J et al. Accuracy of smart scales on weight and body composition: observational study. JMIR mHealth uHealth, 2021 (PMID 33929337).
- Nana A et al. Agreement of anthropometric and body composition measures predicted from 3D camera, bioelectrical impedance and DXA. Obes Res Clin Pract, 2022 (PMID 35094958).
- Siedler MR et al. Assessing the reliability and cross-sectional and longitudinal validity of fifteen bioelectrical impedance analysis devices. Br J Nutr, 2023 (PMID 36404739).
- Madigan CD et al. Is self-weighing an effective tool for weight loss? Int J Behav Nutr Phys Act, 2015 (PMID 26293454).
- Benn Y et al. What is the psychological impact of self-weighing? A meta-analysis. Health Psychol Rev, 2016 (PMID 26742706).
- World Health Organization, « Obesity and overweight » fact sheet and technical report 894 (2000).
- Official websites and support pages of Withings, Renpho, Garmin, Beurer, Xiaomi, Omron and Eufy, recorded on 26/09/2026.