Why You Can’t Trust Your Smart Scales
I love my smart scale, but only for one thing: the way it syncs with the app. That’s all. All of his other “smart” stats—like body fat, muscle mass, hydration, and more—are wild guesses with no reliable relationship to the truth. And this is true for all smart scales on the market.
I don’t blame anyone for wanting to know more than just their weight on their scale. Weight fluctuates for dozens of reasons: we can gain or lose muscle mass, we can gain or lose fat. We may be lighter when we are dehydrated, or heavier because we ate a lot of salt last night and are retaining water. Wouldn’t it be great to see the difference between all these things? Many smart Libras claim to be able to do this. But they simply don’t deliver on their promises.
How scales measure your body composition
Weighing yourself is quite simple: you step on a scale and it measures the force that gravity exerts on your body.
Body fat mass, muscle mass, bone mass, body water and other metrics (which we can summarize as “body composition”) are taken from something called bioimpedance analysis, or BIA.
The conductive foot pads on your scale (which can be nearly invisible layers in the scale’s material) conduct current through your body from one foot to the other. You will not feel this current and it is not dangerous, although if you have a pacemaker you may be advised to turn this feature off.
The scale can detect electrical resistance between one leg and the other, which means current flows up one leg, through the torso, and down the other leg. Essentially, the formula presents your legs and thighs as a kind of horseshoe, made up of a mixture of water and non-water. It uses your height (which you enter into the app when setting up your profile) to guess the length of that horseshoe. And based on the resistance it measures, it makes a guess about how much of your body is fatty tissue and how much is watery.
As you might expect, this approach has many limitations. The biggest one is just the number of assumptions the formula has to make about your body to go from “this is a 5-foot-6 person with foot resistance value X” to “your body is 32% fat.” much less than the estimate of the amount of bone or lean muscle tissue.
For example, it makes assumptions about how much fat, muscle, etc. is in the upper body compared to the lower body. It may also depend on how hydrated you are, how recently you’ve exercised, and whether you have artificial knees or hips.
A review article on BIA accuracy notes that because so many assumptions are required, even the best devices and algorithms will likely never be able to provide completely accurate and detailed information about a person’s body composition:
The fundamental problem is that BIA is a forecasting method that inherently requires simplifications and assumptions based on population averages, but is assumed to apply accurately to all subjects.
A 2016 Consumer Reports study found that the scales they tested differed from BodPod body fat measurements by 21% to 34%. This is not enough to accurately know your body fat percentage. If you really want a number, any number, then a simple calculation like the Navy Body Fat Estimator will probably be just as good.
Some people say that they like to see how their body fat (or muscle mass) changes over time and that it changes in the direction they expect. Your experience may vary in this regard. My Withings Smart Scale has never shown any meaningful changes to these numbers for me, even when I know my body composition has changed. (And yes, I tried athlete mode. Even though I have more muscle than the average person my size, athlete mode makes the numbers even less realistic to me.)
What about all the other numbers?
Smart scales are sold primarily on the number of features they have. A scale that can tell you 12 things looks better than one that can only tell you one or two things, right?
If you entered your height on the app and assume the scale is reasonably accurate in determining your weight, your BMI measurements should be correct because BMI is simply the ratio between your height and weight.
Some scales claim to measure your BMR (basal metabolic rate), but this is usually calculated based on your weight, height and age. You can get this using an online calculator .
Muscle, fat, bone, and body water values are usually just adjustments to the BIA’s body fat formula; for example, lean body mass is your total weight minus body fat. All of these measures have the same accuracy problems that we discussed above with respect to the BIA. I wouldn’t put much stock in any of them.
Some scales measure heart rate, which you can check yourself; just count your heart rate by watching your watch and see if it matches the number on your scale.
Look closely at the other numbers your scale reports and you’ll likely see that they’re calculated based on the data you put into your profile in the app, often in combination with your weight or other data from the scale.
So are these scales accurate enough to be useful? In my opinion, no. If you find that the readings are consistent enough to be tracked over time, go ahead and enjoy them. But I wouldn’t use smart scale numbers (other than weight) to make any decisions about what to eat or how to exercise.
Which smart scales should you buy?
We have a list of the best smart scales , and I’m going to highlight our features editor Lindsay Ellefson’s favorite (since my own smart scale has been discontinued). Her pick is the iHealth Nexus , which costs about $42 and often sells for close to $30. She loves how easily it syncs with other apps, which is the only reason to get a smart scale in my opinion.