Today, keeping track of your health is more within reach than ever with the widespread distribution and use of wearable fitness trackers. But how accurate is your health device? Are you really getting those 10k steps a day, or did you really burn that many calories in your morning 5K run? Guava Health explores the accuracy of activity trackers produced by the five leading fitness tracker companies—Fitbit, Garmin, Apple, Oura, and Withings—to understand how reliable the data from wearable devices is and how effectively they can be used to monitor health behaviors.
Please note that the compilation of data will rely on information published in peer-reviewed journals. This is due to the fact that manufacturers are not legally obligated to disclose accuracy testing unless their products are classified as medical devices.
If you are still experiencing decision fatigue when it comes to choosing your device, check out this article to help you narrow down your selection.
While step counting is a specialty of Fitbit devices, a review of 67 studies revealed some variations in accuracy. On treadmills, Fitbits tended to underestimate steps, while in real-life scenarios, they often overestimated them. Overall, however, these discrepancies were generally considered acceptable. Despite its success in step counting, Fitbit's energy expenditure measurements were less accurate, particularly for higher-intensity activities, and it often underestimated distance during faster walking. Additionally, while Fitbit's sleep tracking aligns closely with some research-grade tools, a more recent 2023 study found that the Fitbit Inspire 2 overestimates vital sleep metrics when pitted against the gold standard. Compared to polysomnography (PSG) tests, researchers saw these overestimations for total sleep time, deep sleep, and REM. When it comes to cardiovascular health, the Fitbit Charge does pretty well. It only slightly overestimates VO2 max and slightly underestimates heart rate (HR), but becomes more accurate with increased activity. VO2 max is an important indicator of cardiovascular fitness. It measures the amount of oxygen your body can take up during exercise, so it tends to be higher in individuals who do a lot of aerobic activity, like running or swimming.
Accurately tracks: Steps, VO max, and sleep (sort of)
Inaccurately tracks: Distance and energy expenditure
In a comprehensive review of 32 studies from 2020, Garmin activity trackers demonstrated high accuracy in tracking steps, but limited studies were available for assessing speed, elevation, and sleep tracking. When it comes to distance accuracy, Garmin watches seem to overestimate distances at slow treadmill speeds and underestimate at high speeds. Furthermore, the accuracy of energy expenditure and HR tracking was found to be comparatively lower than that of other health trackers. A study from 2021 revealed that the Garmin Forerunner accurately predicted VO2 max, although there was notable variation among the group of 44 individuals.
Accurately tracks: Steps and VO2 max
Inaccurately tracks: Energy expenditure, distance, heart rate, and sleep
A review from 2020 shows the Apple Watch to be highly accurate as a fitness tracker when measuring daily step counts, particularly at a moderate intensity level. However, this preciseness fluctuated with the device overestimating at slower walking speeds and underestimating at faster ones. The review also found that energy expenditure was overestimated by the Apple Watch. While formal research from this 2019 study hasn't been shared discussing heart rate in healthy patients, researchers found that for patients with cardiovascular disease, the Apple Watch HR during exercise was accurate. This work was promising for potential use in HR-guided training programs for cardiac rehabilitation. Additionally, according to a 2021 report by Apple, the watchOS 7 and later models allow users to note if they're on heart rate-limiting medications, which modifies the algorithm used to calculate VO2 max, ensuring more accurate results.
Accurately tracks: Steps, heart rate, and VO2 max
Inaccurately tracks: Energy expenditure and speed
The Oura Ring stands out for its precision in sleep tracking, nighttime heart metrics, and body temperature, offering insights into readiness and overall wellness. This 2024 study published in the Journal of Clinical Sleep Medicine serves as a testament to Oura's sleep measurements, as it performed impressively well next to medical-grade sleep study tracking results. A study from 2020 indicates it's also great at reading nocturnal heart health metrics, but little research has been done to assess its daytime accuracy. Research does indicate, however, that when it comes to step counting and energy expenditure, it may be suboptimal. In 2023, this study concluded that while it can detect sweeping changes in energy expenditure, it became less sensitive as activity intensity increased. The ring struggles with step counting due to its position on the finger, often mistaking hand movements for steps. Additionally, without GPS, it cannot accurately track distance or speed unless paired with a phone. Oura rings also track VO2 max, but a blog post by Oura explains that it's a "broad estimate" and it won't be as accurate as lab measurements.
Accurately tracks: Sleep, nighttime heart rate variability and heart rate, body temperature
Inaccurately tracks: Steps, distance, speed
A recent study demonstrated a strong correlation between estimated steps from the Withings watches and those obtained using devices widely considered gold standards for step and activity estimation. Additionally, the Withings tended to assess relative energy expenditure levels with exceptional accuracy. Likewise, recent research suggests that while the Withings showed acceptable accuracy in measuring HR for general consumer use, it may not be suitable for situations requiring more precise HR measurements, such as healthcare or clinical trials. Notably, the Withings displayed lower accuracy in measuring HR during mobile activities like walking, gardening, and household tasks, compared to more sedentary activities such as desk work, eating or drinking, and sitting. According to Withings, the ScanWatch does measure VO2 max, however, there's been no research published on its accuracy, so it's safe to assume it's not medical grade.
Accurately tracks: Energy expenditure, heart rate (sedentary), and steps
Inaccurately tracks: Heart rate (mobile)
Back to the age-old question: How accurate are fitness trackers?
In short, they're not always exact, but they do point us in the right direction. The compilation of research further pushes for the narrative that people should treat these fitness trackers as tools to give them relative information about our overall health. Rather than focusing too deeply on the individual metrics, they should zoom out and focus on the general trends.
Health applications can help people visualize and identify patterns in the information their device is tracking around the clock, ultimately supporting them in making more informed health decisions.
While this may not have been the answer you wanted to hear, have no fear! This doesn't mean you should stop aiming to hit your daily 10,000 steps or disregard the metrics entirely. This information just suggests that it is advisable to avoid basing lifestyle and your health decisions solely on these numerical values.
This story was produced by Guava Health and reviewed and distributed by Stacker Media.
