A smartwatch glowing on a runner’s wrist may look authoritative, but wearable data is best viewed as a useful guide, not a precise medical measurement.
Wearable fitness trackers promise to count your steps, decode your sleep, track your heart rate, estimate calories burned and even detect early signs of illness. But are fitness trackers accurate?
The answer depends a lot on what you are measuring, which device you are using, and how you interpret the results. In general, wearables are better at showing trends over time than delivering exact medical-grade measurements.
Most Wearables Aren’t Medical Devices
The most important point to understand is that most consumer wearables are not regulated like hospital-grade medical devices. Some features, such as electrocardiogram (ECG) functions or irregular heart rhythm alerts, have received regulatory clearance. Yet most data these devices provide are generated for wellness and self-monitoring rather than diagnosis.
Most trackers rely on what’s called photoplethysmography, or PPG, which uses light reflected through the skin to estimate heart rate and related signals. They also use accelerometers and motion sensors to infer activity, steps, sleep and calorie expenditure.
These measures are proxies for gold-standard clinical tools such as ECG for heart rhythm and polysomnography for sleep staging. This matters because a wearable can be directionally helpful without being precise enough to substitute for medical testing.
many wrist-worn devices perform quite well when compared with ECG, especially when the user is still and the signal is stable.
Accuracy tends to fall during exercise. Motion, sweat, changing blood flow and imperfect sensor contact can all interfere with optical readings. That is why heart rate data during intense intervals or strength training can be much less dependable.
Other metrics are more problematic. Sleep staging, calorie burn and blood oxygen readings often show wider gaps from clinical reference standards. That doesn’t mean the data are useless. It means they should be interpreted cautiously, especially when users start treating them as exact measurements rather than estimates.
Here’s Information on Data Accuracy in 9 Top Wearable Trackers
1. Oura Ring (Generation 4)
The Oura Ring has an advantage because it is worn on the finger: blood flow signals are easier to capture than at the wrist. That helps explain why Oura tends to perform well for resting heart rate, heart rate variability (HRV) and overnight recovery metrics.
Oura’s sleep tracking is among the strongest in the consumer market. In a 2024 study, the Oura demonstrated a sensitivity of 76-80% and precision of 77-80% for discriminating between sleep stages, with no significant difference from polysomnography for wake, light sleep, deep sleep or REM sleep estimation. Yet like all wearables, it is still not a replacement for a formal sleep study.
Step counts are generally solid, with step counts tracked by Oura within 6% of research-grade measurements. However, physical activity intensity can still be underestimated or misclassified, with Oura underestimating moderate-to-vigorous physical activity by approximately 12%.
Its readiness, recovery and cardiovascular age are calculated based on combinations of measures. These may be useful for an individual to track and may be indicators of a specific issue, or improvement in health. However, they are based on proprietary algorithms and not independently validated clinical tools.
2. Apple Watch (Series 11)
Apple’s watch is among the most extensively studied devices on the market. Its ECG feature is relevant for atrial fibrillation screening. Irregular rhythm notifications show a 84% positive predictive value for Afib on a subsequent ECG. Apple Watch also performs well for resting heart rate, step counts and activity tracking during steady-state exercise.
Sleep tracking is decent, with the Apple Watch Series 8 showing a precision of 73- 88% for sleep stage discrimination. However, like most wrist-worn devices, it can misclassify wake time and certain sleep stages, underestimating waking by 7 minutes and deep sleep by 43 minutes.
Apple Watch tracks calories burned by combining user data (age, weight, gender), heart rate and movement to estimate energy expenditure. While generally reliable for tracking trends, its active calorie estimates have an error margin of 15% to 40% depending on activity type.
Its blood oxygen measurements should also be viewed cautiously, since wearable SpO2 readings are generally less dependable than medical pulse oximetry, particularly under conditions such as motion.
- Garmin (Forerunner 265) Garmin is a top choice for serious runners, cyclists and endurance athletes. Its GPS performance is excellent, which makes distance, pace and route tracking especially reliable outdoors.
Some Garmin models offer ECG capabilities for Afib detection, though unlike many others Garmin has not received FDA clearance for this indication. In a validation study using ambulatory Holter monitoring as the reference standard, Garmin smartwatch PPG-based AFib detection demonstrated a positive predictive value of 92% for confirmed Afib.
Heart rate tracking is generally strong, particularly during steady exercise, though like other wrist-based devices it struggles during abrupt intensity changes. Garmin also performs well in detecting sleep versus wake, with devices showing sensitivity ≥93% for detecting sleep versus wake. However, more granular sleep stage classifications are less consistent.
Calorie estimates can also vary widely, a limitation seen across nearly all wearable brands.
4. WHOOP 4.0
WHOOP focuses less on steps and more on strain, recovery and readiness, making it especially appealing to athletes and biohackers who care more about training load and recovery than daily step goals. Its continuous heart rate and heart rate variability tracking are among the most detailed available in the consumer space. However, similar to other trackers, it works best during steady state conditions and is less accurate during transitions.
For sleep-wake detection, WHOOP achieves 95% sensitivity for sleep but only 51% specificity for wake — frequently misclassifying wake periods as sleep. WHOOP showed the largest disagreement for REM sleep, overestimating by 21 minutes compared to other devices.
Interestingly, observational data from over 180,000 WHOOP users found that wearable-derived metrics correlate with mental health outcomes: individuals with higher HRV, lower resting heart rate, longer sleep duration and more physical activity reported lower levels of depression, anxiety and stress.
More frequent device wear was also associated with improved biometrics and healthier patterns over 12 weeks.
5. Fitbit (Charge 6 / Sense 2)
Fitbit helped define the consumer fitness tracker category. It still offers some of the strongest all-around monitoring. Step counts tend to be reliable in typical day-to-day conditions, and heart rate accuracy is generally good for a wrist-worn device.
Among wrist-worn devices tested during dynamic conditions including rest and varied-intensity walking, Fitbit showed the highest accuracy overall.
The Fitbit Heart Study (n = 455,699) demonstrated that the PPG-based irregular heart rhythm detection algorithm achieved a positive predictive value of 98% for concurrent Afib during ECG patch monitoring — the highest among the three large consumer-facing device trials. Of participants who received an irregular heart rhythm notification and subsequently wore an ECG patch, 32% were confirmed to have AFib.
Sleep tracking for Fitbit is useful, but not exact, with a sensitivity of 95-96% and specificity of 58-69% for detecting sleep. Fitbit Charge 4 notably underestimated deep sleep by 41 minutes and overestimated light sleep by 38 minutes.
Calorie burn estimates can be substantially off like other trackers. Additionally, the company’s stress and readiness features are interesting, though they are best interpreted as wellness insights rather than a diagnostic tool.
6. Samsung Galaxy Watch 8
Samsung’s latest smartwatches offer competitive heart rate tracking sleep analysis and smartwatch functionality. Studies suggest that heart rate and some HRV measurements correlate reasonably well with reference tools during sleep and resting conditions.
A 24-hour validation study found significantly high positive correlation between Samsung devices and ECG for heart rate, time-domain HRV and frequency-domain parameters during sleep. However, during awake time, accuracy decreases substantially.
Samsung also offers ECG and atrial fibrillation detection in some markets and use cases. For AFib detection, the Samsung Galaxy Watch demonstrated sensitivity of 85% and specificity of 79%.
Body composition estimates are less accurate. Like other consumer devices using bioelectrical impedance, they can be interesting for broad trends but are not precise enough for monitoring small day-to-day changes.
7. Withings ScanWatch 2
Withings has positioned itself closer to the medical side of consumer wearables, offering features such as ECG, SpO2 and sleep-related screening. That medical-adjacent branding is appealing, especially for users who want health monitoring without the distraction of a full smartwatch.
Like other wearable trackers, Withings’ ScanWatch performs well for detecting steady-state heart rate, is accurate in transitions between rest and activity. A 2025 meta-analysis of 26 studies (n = 17,349) found the Withings ScanWatch showed 89% sensitivity and 95% specificity for AFib detection, slightly lower than Apple Watch and Samsung devices.
8. Amazfit Balance / GTR 4
Amazfit has gained traction by offering a broad set of features at lower prices than premium competitors. Heart rate tracking is generally adequate for everyday wellness monitoring, and GPS has improved over time.
Like other wrist-worn devices using PPG, Amazfit detects heart rate well in steady-state settings but accuracy can decrease up to 30% during activity.
The brand has not been studied as extensively as Apple, Fitbit, Garmin or Oura. However, a 2025 meta-analysis of 26 studies examining smartwatch accuracy for Afib detection found that Amazfit demonstrated 100% sensitivity and 97% specificity — the highest among all devices tested, outperforming Apple Watch, Samsung and Fitbit.
Yet this finding should be interpreted cautiously given that Amazfit was represented by fewer studies than the more established brands.
There have been no studies on Amazfit’s sleep tracker or blood oxygenation sensing.
9. Google Pixel Watch 3
The Pixel Watch benefits from Google’s integration with Fitbit’s health platform, which gives it a relatively mature foundation for heart rate and sleep tracking. Like other wrist-worn devices utilizing PPG, Pixel Watch can assess steady-state heart rate well but performs worse during physical activity.
In a prospective multicenter validation study comparing 11 consumer sleep trackers against polysomnography, the Google Pixel Watch showed superiority in detecting deep sleep stages compared to other device types.
The Google Pixel Watch includes FDA-cleared irregular heart rhythm notifications and is linked to the Fitbit app, which demonstrates a high (98%) positive predictive value in detection Afib.
Key Takeaways of The Accuracy of Wearable Trackers
Wearable trackers can be useful tools for building self-awareness and spotting patterns over time. But they should be used as a compass, not a ruler.
If your Oura Ring shows poor recovery for several days in a row, that may be worth paying attention to even if the exact HRV number is not perfectly accurate. If your Apple Watch or Fitbit flags a possible irregular rhythm, that is worth getting checked by a clinician, even though the device itself is not making a diagnosis.
The technology is improving quickly, and some features are becoming genuinely clinically meaningful. But for now, most wearables are best understood as helpful consumer tools, not diagnostic instruments. The best wearable is the one you will actually use consistently, because an imperfect tracker you wear every day is far more valuable than a perfect one sitting on your nightstand.