Buzz
I appreciate my smart scale, but only for one feature: how it syncs with an app. Everything else, like body fat, muscle mass, hydration levels, and more, is pretty much an unreliable guess with no real accuracy. This is a flaw shared by all smart scales on the market.
It's understandable to want more information from your scale than just your weight. Weight changes for numerous reasons: muscle gain or loss, fat gain or loss, dehydration, or even water retention from eating too much salt. Wouldn't it be wonderful to accurately track all of these factors? That’s what smart scales promise, but unfortunately, they often fall short.
How scales estimate your body composition
Weighing yourself is easy: just step on the scale, and it measures the force gravity exerts on your body.
Body composition, which encompasses body fat, muscle mass, bone mass, body water, and other metrics, is derived from a method called bioimpedance analysis (BIA).
Your scale’s conductive foot pads (often barely visible within its structure) send a current through your body, traveling from one foot to the other. You won’t feel it, and it’s safe, although if you have a pacemaker, you might be advised to deactivate this feature.
The scale measures the electrical resistance between your feet, meaning the current travels up one leg, through your torso, and down the other leg. Essentially, the formula visualizes your legs and hips as a horseshoe shape made from a combination of water and non-water substances. Using your height (inputted during app setup), it estimates the horseshoe’s length and, based on the resistance, makes an educated guess about the proportion of fat versus water in your body.
As you might imagine, this method has several limitations. The most significant is the number of assumptions the formula has to make to go from 'this is a 5'6" individual with a foot-to-foot resistance of X' to 'your body is 32% fat,' and even more so when it tries to estimate the amounts of bone or lean muscle tissue.
For instance, it makes assumptions about the distribution of fat and muscle between your upper and lower body. Additionally, it can be influenced by factors like your hydration level, recent exercise, and the presence of artificial joints such as knees or hips.
A review article on the accuracy of BIA highlights that due to the many assumptions involved, even the most advanced devices and algorithms are unlikely to provide completely accurate or detailed body composition information for an individual.
A core issue is that BIA is a predictive method that inevitably involves simplifications and assumptions based on average population values, which are expected to be universally applicable to all individuals.
In a 2016 Consumer Reports study, it was found that the body fat measurements from the scales tested differed from the BodPod measurement by anywhere between 21% to 34%. This level of inaccuracy means that determining your true body fat percentage is uncertain. If you're simply looking for a number, a straightforward method like the Navy body fat estimator might be just as reliable.
Some individuals enjoy tracking their body fat or muscle mass changes over time, noting that the results generally align with their expectations. However, your experience may differ. For instance, my Withings smart scale has never shown any significant changes in these numbers, even when I know my body composition has altered. (And yes, I’ve tried enabling 'athlete' mode. Despite having more muscle than the average person my size, the results become even less accurate with it.)
What about the other numbers though?
Smart scales are often marketed based on the variety of metrics they can measure. A scale that offers 12 different stats might seem more impressive than one that only tracks a couple of metrics, wouldn't you agree?
If you enter your height into the corresponding app, and assuming the scale is reliable enough in measuring your weight, the BMI results should be accurate, as BMI is simply the ratio of your weight to your height.
Some scales advertise the ability to measure your BMR (basal metabolic rate), but this is generally calculated using your weight, height, and age. You can easily determine it with an online calculator.
Measurements of muscle, fat, bone, and body water are often just variations on the body fat estimation using BIA. For example, lean body mass is calculated by subtracting body fat from your total weight. These readings all face the same accuracy limitations as BIA itself. I wouldn’t place too much trust in any of them.
Certain scales can measure heart rate, which you can verify yourself; just count your pulse beats while looking at a clock, and see if it matches what the scale reports.
Take a close look at the other readings your scale provides, and you’ll likely find that they’re based on the information you’ve entered into your profile in the app, often combined with your weight or other data from the scale.
Are these scales accurate enough to be helpful? Personally, I don’t think so. If you find that the readings are consistent enough for you to track over time, feel free to use them for fun. But I wouldn’t rely on smart scale numbers (other than weight) to make decisions about diet or exercise routines.
Which smart scales are actually worth purchasing?
We’ve compiled a list of the best smart scales, and I’ll highlight the favorite of our Features Editor Lindsey Ellefson (since my own smart scale is no longer available). She recommends the iHealth Nexus, priced around $42, with frequent sales dropping it closer to $30. She loves how seamlessly it syncs with other apps—which is, in my opinion, the whole point of getting a smart scale.
