Smart scales explained: what are smart scales, are they accurate, and will they help you? 

Smart scales are any set of scales that communicate with an app on your phone or tablet to share data. Usually, this is via wi-fi or Bluetooth. Most smart scales are also body composition scales, offering much more than just a basic weight measurement. 

The variety of data that smart scales capture differentiates them from old-school bathroom scales, which only measure a person’s body weight.  

“Body composition analysis scales are like bathroom scales, but give you rather more data,” says Chris Zaremba, advanced personal trainer and founder of Fitness Over Fifty. “Standing on a basic set of bathroom scales, it’s easy to see you lost a kilo in weight, but it doesn’t show you where that weight has been lost from.”  

Smart scales measure weight, too, but they can also measure other metrics, which include: 

• Body mass index (BMI) – a measure that uses your height and weight to calculate if you are a healthy weight or not. 
• Body fat percentage – how much of your body mass is fat. 
• Muscle mass – the amount of muscle in your body. 
• Bone mass – what percentage of your weight is the bones in your body. 
• Water mass – shows how hydrated the body is. 
• Basal metabolic rate (BMR) – this tells you how many calories your body needs to function. 

Some models, such as the Withings Body Scan, can also measure more advanced metrics like pulse wave velocity – this measures arterial stiffness, which can be a predictor of cardiovascular risk; and electrodermal activity, which can measure activity in the sympathetic nervous system, which directs the body’s fight-or-flight response to stress. 

Others offer some additional measurements – although some of these are less useful metrics than others. The Xiaomi Mi Body Composition Scale 2 offers a balance test in addition to an array of body composition measurements. They can also provide estimates of protein levels and body age.

Smart scales can also tell you what percentage of your body is fat or muscle, with some models offering segmental body composition. These give you a clearer idea of the distribution of fat and muscle around your body, by separating your body into different areas. This is potentially more precise than a single, overall reading, which won’t necessarily tell you where the fat is in your body. 

Smart scales send all this data to an app which you can then refer to, whether that’s to monitor changes over time, track progress towards a goal, or in some cases, for giving data to your doctor. 

They work by using bioelectric impedance analysis (BIA). When you stand on the scales, a safe, low voltage current passes through the body. The difference in voltage between the source and the receiver electrodes shows how much resistance there is. 

Resistance is determined by your body composition, as tissue with a high level of water (such as muscle) will offer less resistance than tissues with less water and more insulating properties (such as fat).  

A series of complicated equations allow smart scales to estimate your body composition based on the level of resistance as the current passes through the body. 

There is debate around the accuracy of BIA as a measure of body composition. According to one study, the variety of methods used to calculate BIA means that there can be considerable differences in the measurements from one device to the next.  

Zaremba explains: “Body composition monitors are not perfect, but they certainly are better than an ordinary scale. The numbers don’t mean that much on their own. I’ve done comparisons of different models, and they can come up with quite different numbers. They are inaccurate when you compare one make or model with another one.

“But where they are helpful is when you use the same one on a regular basis, as you can see what the trend is. That’s the key thing – such a monitor is reliable and accurate when comparing itself with itself”. 

It’s also important to recognise that there are plenty of factors that can influence the readings – such as hydration levels, when you last ate or exercised, went to the loo or showered. Even room temperature can be an influence.

When using smart scales, Zaremba recommends: “Look at your body composition numbers as a seven-day moving average. It makes sense to do it at the same time every day if you can. You’re then eliminating any daily or weekly variables.”  

This might mean weighing yourself first thing in the morning before eating but after visiting the bathroom (sometimes described as your ‘dry weight’). When you do it is less important than being consistent. As long as you keep how and when you weigh in consistent, then the readings will be consistent too – even if they aren’t completely accurate. 

At the least, they’ll be able to give you a rough idea of how you are doing, show trends over time, and potentially flag up areas that you might want to focus on. 

Giving you an insight into your body composition is where smart scales can offer significantly more information than traditional scales.  

For a more comprehensive explanation, see our body composition article. But in summary, having an insight into this (even if the measurements aren’t entirely accurate) can help identify potential risk factors for your health. 

Some smart scales, such as the Withings Body Comp and Eufy P2 Pro can also give you an idea of where that fat is stored. Research has shown that having higher levels of abdominal fat – often in the form of visceral fat (fat around your internal organs) – is associated with increased risk of heart disease. So, knowing where your fat is stored may help to reduce some of these risks. They can also tell you if your muscle mass is increasing, or your fat mass is reducing. 

This means they are great for tracking the effectiveness of any changes you make to your diet or exercise routine.  

Finally, smart scales are easy to use and very convenient. All you need to do is stand on them for a few seconds and wait for your results – which you can refer to via the app. This is much quicker and easier than using skinfold callipers to measure body fat percentage – which often require a second pair of hands) or getting a DEXA scan.

The biggest issue with smart scales is their accuracy (or inaccuracy). Some studies suggest that smart scales are not always as precise as we would like, and will often overestimate or underestimate body fat percentages, sometimes quite significantly. 

This could present a misleading picture of your body composition, which may lead you to miss potential risk factors as a result. 

The technology is advancing at a rapid pace, and as these companies continue to collect data and refine their algorithms, we are likely to see smart scales improving in terms of accuracy and the variety of health metrics they can measure. 

So, should you buy smart scales? If you’re looking for the most definitive, accurate way of measuring your body composition, then there are other methods that are more effective, but less convenient or more expensive, such as Bod Pods, which measure total body volume, and DEXA scans.  

If you’re willing to accept their limitations, however, smart scales can be an incredibly convenient tool for providing a rough guide to your body composition. These scales can also alert you to potential risk factors for your health before you might otherwise become aware of them.  

This is important as we age. Zaremba says: “Understanding your body composition gets more urgent as you get older because it does take longer for your body to react to changes.” 

And if you’re someone who enjoys tracking your progress, smart scales can help keep you motivated and support you in achieving your goals. For most, they can be a helpful tool to aid in reaching their health and fitness goals. Just remember: if you are concerned about any measurements, see your doctor for professional advice and help.