How do smart scales work? 

For instance, you can use your weight and height to calculate your body mass index (BMI). However, BMI doesn’t take into account how much muscle or water your body is made up of, so it can sometimes present a slightly misleading picture.  

People with a lot of muscle might be wrongly classed as obese. Conversely, people with little muscle but a high body fat percentage, could be assessed as being a ‘healthy’ weight. This is despite the fact that high levels of body fat (especially visceral fat – the fat that wraps itself around your internal organs) are associated with a host of health risks, such as diabetes, high blood pressure, and stroke.  

Some smart scales also measure body composition. These can give you a breakdown of your body fat percentage and muscle mass (among other metrics). Giving you a snapshot of what your weight consists of may be more insightful than BMI alone.

As well as these measurements, smart scales often connect to an app on your phone. This allows you to easily monitor your weight and body composition over time, and will help you to track changes, supporting you in reaching any health and fitness goals you might have. 

It all sounds very impressive, and smart scales can offer a lot when used as a supporting tool. We’ll explain exactly how they work, and what the limitations are as well.  

The first big difference between traditional analogue scales and smart scales is the way they can connect to other devices, typically your smartphone. Most smart scales use one of two methods to connect: Bluetooth or wi-fi. Some scales, such as the Withings Body Scan, Body Comp and Body Cardio, or the QardioBase X, offer both options. 

Wi-fi allows smart scales to connect to your phone even when the phone isn’t next to the scales. This means that data can be easily shared with the app, even when the phone isn’t next to the scales.  

Smart scales that use Bluetooth to connect to an app, such as the Fitbit Aria Air, require the app to be open and within close range in order to transfer the data from the scales into the app. 

Whichever method of sharing data is used by your smart scales, the main benefit of this wireless connectivity is that it makes tracking your data much easier. Gone are the days of having to manually write everything down, or trying to remember it all.

Most apps will show you your data for up to the previous 12 months. This gives you a comprehensive history of all your measurements, and makes it easy to see any changes over time. 

You can then spot whether any lifestyle changes are helping or hindering your goals, and make any adjustments. 

Not all smart scales measure body composition – for example the Fitbit Aria Air only measures your body weight. But body composition scales can estimate measurements such as body fat and muscle mass, using a method called bioelectrical impedance analysis (BIA).  

BIA works by sending a small electrical signal through your body. Your body composition scales will have electrodes built into them, which measure how much resistance the electrical signal meets on its journey from the scales travelling up one leg, down the other leg, and back to the scales.

Many manufacturers advise that any scales that use BIA are unsuitable for people with a pacemaker or other implanted device fitted. 

Body tissue with a high-water content, such as muscle, will have lower resistance than tissues with more insulating properties, such as body fat. The smart scales will then calculate the level of resistance (using proprietary algorithms) and combine this with other data such as your age, height and gender, to give an estimate of your body composition. Greater resistance will therefore generally correlate with higher body fat percentage. 

Most of these scales will offer a few basic body composition metrics, such as body fat percentage, muscle mass and body water. But some, such as the Xiaomi Mi Body Composition Scale 2 or the Withings Body Comp, will offer more metrics, such as visceral fat estimates.  

Some of the more advanced scales, like Withings Body Cardio scales, offer heart health metrics like pulse wave velocity, or nerve health (Withings Body Comp) or even a six-lead ECG (Withings Body Scan). These types of scales also rely on electrodes to measure these extra features. 

We’ve covered this in depth in our guide to smart scales. But briefly, there are question marks around the accuracy of the body composition measurements they provide (although weight measurements do tend to be reasonably accurate).  

This is especially true for smart scales that only use electrodes in the main body of the scales. Because the electrical current will search for the quickest route back to the electrodes, the signal will typically only measure resistance in the lower body and then estimate resistance for the upper body to give a reading for your overall body composition. 

Scales such as the Withings Body Scan and Tanita BC-545N use a handle to send the electrical current from hands to feet. In theory, this should lead to a higher level of accuracy than smart scales that don’t have the handles.  

There are also other variables that can influence the readings you get. Things such as when you last used the toilet, ate or drank something, exercised, and even the ambient temperature of the room, can all affect the measurements. 

So, if smart scales are not that accurate, what’s the point? Well, even though a specific measurement might not be completely precise, most smart scales will be consistent with themselves. This means that over time, any trends you notice are likely to be a true reflection of what’s happening in your body. If you’re noticing a decline in your body fat over, say, a three-month period, then it’s probably true that your body fat levels really are starting to decline.  

In this respect, smart scales are probably most beneficial to help motivate and support you in reaching your goals.  

As we have said in other reviews, smart scales won’t give you as precise a body composition score as the likes of a DEXA scan (sometimes written as DXA, which stands for Dual X-ray Absorptiometry, which supplies an in-depth analysis of your body composition, and is considered the ‘gold standard’ in this respect). But what they will do is give you a rough guide, highlighting changes in your body composition over time. They are also much quicker and more convenient to use. 

If you’re considering taking the plunge and getting some smart scales of your own, make sure you read our guide to the five things to look for when buying smart scales, to help you get the most suitable product for your needs and budget.