Neither Garmin nor Apple Watch is universally more accurate. Each one leads in different measurements, and the gap between them shifts depending on what you’re tracking. Apple Watch consistently outperforms Garmin in optical heart rate monitoring, while Garmin edges ahead in GPS precision, VO2 max estimation, and recovery metrics like heart rate variability. For features like sleep staging and calorie burn, both platforms struggle with similar limitations.
Heart Rate: Apple Watch Has a Clear Edge
A prospective study published in Cardiovascular Diagnosis and Therapy compared several wearables against a medical-grade ECG. The Apple Watch Series 3 achieved a concordance correlation of 0.96 with the ECG, meaning its readings closely matched the gold standard. The Garmin Vivosmart HR scored 0.89, a noticeable step down. For context, the chest strap (Polar H7) scored 0.98, so the Apple Watch came remarkably close to chest-strap accuracy from the wrist.
The Garmin tended to underestimate heart rate by about 2 beats per minute on average, a small but consistent bias. At rest, both devices performed well (concordance of 0.85 or higher). The real separation showed up during intense exercise. At treadmill speeds of 8 and 9 mph, none of the wrist-worn devices maintained strong accuracy, but the Apple Watch degraded less than its competitors.
If heart rate accuracy during hard workouts is your priority, Apple Watch is the better wrist-based option. Pairing either watch with a chest strap still beats both.
GPS and Distance Tracking
GPS accuracy is where things get closer, and where Garmin’s hardware depth becomes relevant. In a detailed New York City test by DC Rainmaker, four watches recorded nearly identical distances over a 15.5 km route: the Apple Watch Ultra logged 15.50 km, the Apple Watch Series 8 came in at 15.56 km, the Garmin Epix at 15.58 km, and the Garmin Forerunner 955 at 15.43 km. All four were within about 150 meters of each other over that distance.
Both the Apple Watch Ultra and the tested Garmin models used multiband (dual-frequency) GPS, which reduces signal bounce off buildings and trees. The errors that did show up were predictable: tracks drifting into buildings or through tree cover. In open terrain, the differences between brands are negligible for most runners and cyclists. In dense urban environments or heavy forest, individual run variability matters more than brand choice.
Garmin offers multiband GPS across a wider range of models, including mid-priced options like the Forerunner 265. Apple limits it to the Ultra line. If you run in challenging GPS environments and don’t want to pay Ultra prices, Garmin gives you more options.
VO2 Max Estimation
VO2 max is the single number many endurance athletes care about most, and Garmin’s estimate tends to land closer to lab-tested values. One user who had their VO2 max measured in a lab at 50 mL/kg/min found their Garmin estimated 49, while the Apple Watch Ultra reported 41. That’s a meaningful gap.
Garmin has been refining its VO2 max algorithm (developed with Firstbeat Analytics) for over a decade. It factors in heart rate response, pace, temperature, and altitude. Apple introduced cardio fitness estimates more recently and uses a simpler model. Apple’s number isn’t useless for tracking trends over time, but if you want an estimate that’s close to what you’d get in a sports science lab, Garmin is more reliable.
Heart Rate Variability and Recovery Data
Heart rate variability (HRV) measures the tiny fluctuations between heartbeats and serves as a window into your nervous system’s recovery state. Both watches track it, but they measure it differently, and those differences matter.
Garmin records HRV continuously throughout the night using a metric called RMSSD, which captures beat-to-beat variation in real time. Because the optical sensor runs continuously during sleep, the data is relatively complete. Apple Watch uses a different statistical method (SDNN) and samples your heart rate intermittently, turning the sensor on every few seconds or minutes rather than running it nonstop. This makes Apple’s HRV reading more of an estimate than a direct measurement.
Garmin then rolls this nightly data into features like HRV Status, Body Battery, and Training Readiness, giving you a morning snapshot of how recovered you are. Apple displays HRV in the Health app but doesn’t build as much actionable guidance around it. For athletes who plan training intensity around recovery metrics, Garmin’s approach provides a more complete picture.
Blood Oxygen (SpO2) Readings
Both watches include a pulse oximeter, but neither is medical-grade, and their accuracy diverges in interesting ways. A review published in The Lancet Respiratory Medicine found the Apple Watch had an absolute mean difference of 0.8% compared to a clinical pulse oximeter in patients with chronic lung disease. That’s a small and clinically reasonable margin.
Garmin’s SpO2 sensor showed larger errors, particularly at altitude. The difference between Garmin readings and clinical pulse oximetry grew from 4.7% at sea level to 13.1% at 5,500 meters. At a mountain peak of 4,559 meters, Garmin readings differed from arterial blood gas analysis by 7%. Some studies found Garmin’s accuracy could improve to within 2.9% under better conditions, but it remained less reliable than Apple Watch’s sensor overall.
If you’re using SpO2 for altitude acclimatization or monitoring a respiratory condition, Apple Watch delivers more trustworthy readings.
ECG and Heart Rhythm Detection
Apple Watch is one of the few consumer devices with FDA-cleared ECG capability. In a clinical trial of roughly 590 subjects, the Apple Watch ECG app correctly identified atrial fibrillation 99.5% of the time and sinus rhythm 100% of the time in classifiable recordings. About 11.5% of recordings came back inconclusive due to poor signal quality. When those inconclusive results are factored in, the detection rate for AFib drops to 86.5%.
Garmin does not offer an ECG feature on any current watch. If detecting irregular heart rhythms is something you care about, Apple Watch is the only option between the two.
Calorie Burn: Neither Wins
A 2025 study from the University of Mississippi found that wearable devices (Apple Watch included) tracked heart rate with a mean error of about 4.4% and step counts within 8.2%, but calorie burn estimates had a mean error of nearly 28%. That level of inaccuracy held across walking, running, cycling, and mixed-intensity workouts, and across different body types.
Garmin’s calorie estimates face similar challenges. Both brands rely on heart rate, movement data, and user-entered body metrics to model energy expenditure, and both struggle because the relationship between heart rate and calorie burn varies enormously between individuals. Treat calorie numbers from either watch as rough guides rather than precise measurements.
Sleep Tracking
A study published in Sensors that evaluated five commercial sleep trackers against polysomnography (the gold-standard sleep study) was unable to determine the accuracy of sleep stage detection for any of the tested devices. The researchers couldn’t access the detailed data needed to evaluate how well these watches identified REM, deep, and light sleep stages.
Both Garmin and Apple break your night into sleep stages, but without strong validation data, it’s hard to say which is more accurate. Garmin tends to provide more granular sleep metrics, including a sleep score, Body Battery impact, and HRV trends overnight. Apple’s sleep data is cleaner and simpler but less detailed. Neither should be treated as a reliable diagnostic tool for sleep disorders.
Which Watch Fits Your Priorities
The accuracy question comes down to what you’re measuring. Apple Watch is the stronger choice for heart rate during exercise, blood oxygen readings, and heart rhythm monitoring via ECG. Garmin pulls ahead for VO2 max estimation, recovery and HRV tracking, and offers multiband GPS across more price points. GPS distance accuracy is essentially a tie at the high end of both lineups.
For general fitness tracking where you want reliable heart rate data and health monitoring features like ECG, Apple Watch is hard to beat. For endurance training where VO2 max trends, recovery guidance, and training load management drive your decisions, Garmin’s platform is built around those metrics in a way Apple’s isn’t. Calorie burn and sleep staging remain weak spots for both.