When Apple launched its first smartwatch in 2015, there was a sense that the company didn’t exactly know what the device was for. But within a few years, it carved out an identity focusing mainly on health and fitness, with millions of owners using their Apple Watches to track metrics such as steps taken, minutes of exercise each day, and how often they stand. Nearly every other smartwatch soon followed Apple’s lead, and they all promise to help you get fit and stay healthy.
A new report, however, has cast doubt over the accuracy of one of the main algorithms used by fitness watches: measuring calories burned. A reporter for MailOnline [paywalled] tested devices by seven major manufacturers under scientific conditions, and found that only one came within 10 percent of the correct result. Fortunately for Apple Watch wearers, that one device was the Apple Watch Series 10.
The reporter visited a lab in London with the facilities to measure calorific usage to a high degree of accuracy. The lab measured his basal metabolic rate (BMR), ran a Dexa scan for body fat percentage, and took various other preliminary medical readings that would not be available to a fitness wearable. He then took a VO2 test while using a stationary bike and wearing a heart monitor strap and oxygen mask. All of this effort produced a calorific figure that’s about as accurate as possible, and could be compared with the readings taken by the fitness watches.
Most of the wearables (including the Apple Watch) underestimated the calories burned. Here’s the percentage of the correct score that they each gave, from least to most generous:
- Decathlon CW500 S: 37%
- WHOOP 5.0: 66%
- Fitbit Charge 6: 72%
- Samsung Galaxy Watch Ultra LTE (47mm): 78%
- Oura Ring 4: 86%
- Apple Watch Series 10: 92%
- Garmin Vivoactive 6: 112%
As you can see, Apple’s smartwatch came closest to the correct score, beating Garmin (the only overestimator) by four percentage points and Oura by six. Why the inaccuracies? Because, firstly, fitness wearables have to work with the data they have, which is limited. And, secondly, because some of the equations they work from “were developed more than a century ago and are not very accurate,” MailOnline says.
“Wearables are popular because people think it’s personalized data,” says Owen Hutchins, who runs the My Vital Metrics lab used for the test. “Unfortunately, the reality is that they’re using a whole bunch of algorithms to predict you—not to measure you.”
Apple recommends calibrating your watch to “improve the accuracy of your distance, pace, and calorie measurements” by walking or running at your normal pace for about 20 minutes with the Outdoor Walk or Outdoor Run selected in the Workout app. Apple says this will “improve the accuracy of your calorie calculations in the Workout app, and the calorie, distance, Move, and Exercise calculations in the Activity app.”