Wearable guides
Apple Watch PPG Sensor: Technical Deep-Dive
Apple Watch uses reflective optical sensing with green LEDs and photodiodes for heart rate, and can also use infrared light. Models that support Blood Oxygen add red and infrared light for wellness SpO2 estimates. Apple does not publish exact LED wavelengths or detector counts for every generation.
Apple describes the optical heart sensor as green LEDs paired with light-sensitive photodiodes, flashed hundreds of times per second. Apple states the sensor supports 30–210 beats per minute and compensates for low signal by increasing LED brightness and sampling rate. The sensor can also use infrared light; which LED colour is used depends on the watch model, activity level and whether the wearer is asleep (Apple Support: heart rate).
Blood Oxygen measurements use red and infrared light and require the wearer to keep still for a 15-second reading. Apple says the app is for general fitness and wellness and is not intended for medical use (Apple Support: Blood Oxygen). The separate Irregular Rhythm Notification feature, which analyses PPG pulse timing while the wearer is still, was classified by the FDA through De Novo request DEN180042 in 2018. It is a notification feature, not a diagnosis.
Independent studies give a mixed picture. Against ECG during treadmill, cycling and elliptical exercise, Apple Watch heart rate reached a Lin's concordance of 0.92, versus 0.996 for a Polar H7 chest strap (Gillinov et al., 2017). In the Apple Heart Study, the positive predictive value of an irregular pulse notification for simultaneous AF on an ECG patch was 0.84 (Perez et al., 2019). For sleep, Apple Watch Series 6 matched polysomnography on 53% of epochs for sleep stage plus wake (Miller et al., 2022).
Sourced figures
Every number in this guide comes from the source linked beside it. Each study result applies to that study's devices, participants and protocol only.
Apple Watch heart rate vs ECG during exercise: Lin's concordance 0.92 (Polar H7 chest strap 0.996).
Gillinov et al. (2017), Medicine & Science in Sports & Exercise · Apple Watch research record
Apple Heart Study: positive predictive value of an irregular pulse notification for simultaneous AF on ECG patch 0.84 (95% CI 0.76–0.92).
Perez et al. (2019), Apple Heart Study, New England Journal of Medicine · Apple Watch research record
Apple Watch Series 6 sleep stage + wake epoch agreement with polysomnography: 53% (κ 0.20).
Miller, Sargent & Roach (2022), Sensors · Apple Watch Series 6 research record
Apple Watch Series 6 SpO2 vs a standard pulse oximeter: r = 0.89, bias −0.23, limits of agreement −3.49 to 3.04.
Spaccarotella et al. (2022), Journal of Clinical Medicine · Apple Watch Series 6 research record
Apple Watch Series 7 SpO2 root mean squared error at rest in people with COPD: 3.5% (4.1% after sit-to-stand).
Støve et al. (2023), Respiratory Care · Apple Watch Series 7 research record
What published studies show
Results quoted from 32 published validation studies, each against a clinical reference. A result applies to the model and protocol tested, not to every model the brand sells. From the wearable accuracy database (v1.0.0).
Heart rate at rest2 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Apple Watch Series 9 / Ultra 2 | 5.91% (MAE 3.73 bpm)Mean absolute percentage error | Polar H10 + Kubios HRV | n = 39, Serial morning measurements over 14 days, 316 readings | O'Grady et al. (2024)Independent |
| Apple Watch Series 6R-R intervals and BPM at rest | 1.15%Mean absolute percentage error | ECG | n = 78, Lab: rest, talking, movie, before/ | Bonneval et al. (2025)Independent |
- Apple Watch Series 9 / Ultra 25.91% (MAE 3.73 bpm)
Mean absolute percentage error · vs Polar H10 + Kubios HRV · n = 39, Serial morning measurements over 14 days, 316 readings
O'Grady et al. (2024) Independent
- Apple Watch Series 61.15%
Mean absolute percentage error, R-R intervals and BPM at rest · vs ECG · n = 78, Lab: rest, talking, movie, before/after walking
Bonneval et al. (2025) Independent
Heart rate during exercise9 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Apple Watch Series 6 | High agreement in squat, deadlift, cable row (rho > 0.832)Study finding (no single number) | ECG chest strap (Polar H7/ | n = 29, Resistance exercise (5 exercises) | Støve et al. (2022)Funding not reported |
| Apple Watch SE | Significantly lower MAE than Polar Pacer Pro and Galaxy Watch 6Study finding (no single number) | ECG chest strap (Polar H7/ | n = 37, Low/ | Jeong et al. (2026)Independent |
| Apple Watch Series 8 | Skin pigmentation did not predict error (P = 0.62); 50% of missing data from dark-skin participantsStudy finding (no single number) | ECG chest strap (Polar H7/ | n = 28, Rest, cycling, recovery | Mulholland et al. (2026)Independent |
| Apple Watch Series 9 | Less accurate for darker skin pigmentationStudy finding (no single number) | ECG chest strap (Polar H7/ | n = 30, 10-min treadmill | Chase et al. (2024)Independent |
| Apple Watch Series 4 | 0.9%–4.3% (range by intensity)Coefficient of variation | ECG chest strap (Polar H7/ | n = 25, Sitting, walking, running, sprints | Düking et al. (2020)Independent |
| Apple Watch Series 6 | < 5% in all 5 activitiesCoefficient of variation | ECG chest strap (Polar H7/ | n = 60, Sitting, walking, running, resistance, cycling | Hajj-Boutros et al. (2023)Funding not reported |
- Apple Watch Series 6High agreement in squat, deadlift, cable row (rho > 0.832)
Study finding (no single number) · vs ECG chest strap (Polar H7/H10) · n = 29, Resistance exercise (5 exercises)
Støve et al. (2022) Funding not reported
- Apple Watch SESignificantly lower MAE than Polar Pacer Pro and Galaxy Watch 6
Study finding (no single number) · vs ECG chest strap (Polar H7/H10) · n = 37, Low/moderate/high-intensity treadmill
Jeong et al. (2026) Independent
- Apple Watch Series 8Skin pigmentation did not predict error (P = 0.62); 50% of missing data from dark-skin participants
Study finding (no single number) · vs ECG chest strap (Polar H7/H10) · n = 28, Rest, cycling, recovery
Mulholland et al. (2026) Independent
- Apple Watch Series 9Less accurate for darker skin pigmentation
Study finding (no single number) · vs ECG chest strap (Polar H7/H10) · n = 30, 10-min treadmill
Chase et al. (2024) Independent
- Apple Watch Series 40.9%–4.3% (range by intensity)
Coefficient of variation · vs ECG chest strap (Polar H7/H10) · n = 25, Sitting, walking, running, sprints
Düking et al. (2020) Independent
- Apple Watch Series 6< 5% in all 5 activities
Coefficient of variation · vs ECG chest strap (Polar H7/H10) · n = 60, Sitting, walking, running, resistance, cycling
Hajj-Boutros et al. (2023) Funding not reported
3 more results in the database. Heart rate accuracy across devices →
Overnight heart rate1 study
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Apple Watch Series 6 | ICC 0.96Intraclass correlation | ECG | n = 53, Overnight, sleep laboratory | Miller et al. (2022)Other maker funded |
| Apple Watch Series 6 | 1.5 bpm (absolute bias)Mean absolute error | ECG | n = 53, Overnight, sleep laboratory | Miller et al. (2022)Other maker funded |
- Apple Watch Series 6ICC 0.96
Intraclass correlation · vs ECG · n = 53, Overnight, sleep laboratory
Miller et al. (2022) Other maker funded
- Apple Watch Series 61.5 bpm (absolute bias)
Mean absolute error · vs ECG · n = 53, Overnight, sleep laboratory
Miller et al. (2022) Other maker funded
HRV (RMSSD)2 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Apple Watch Series 9 / Ultra 2 | 28.88% (MAE 20.46 ms)Mean absolute percentage error | Polar H10 + Kubios HRV | n = 39, Serial morning measurements over 14 days, 316 readings | O'Grady et al. (2024)Independent |
| Apple Watch Series 6 | ICC 0.67Intraclass correlation | ECG | n = 53, Overnight, sleep laboratory | Miller et al. (2022)Other maker funded |
| Apple Watch Series 6 | 22.5 ms (absolute bias)Mean absolute error (absolute bias) | ECG | n = 53, Overnight, sleep laboratory | Miller et al. (2022)Other maker funded |
- Apple Watch Series 9 / Ultra 228.88% (MAE 20.46 ms)
Mean absolute percentage error · vs Polar H10 + Kubios HRV · n = 39, Serial morning measurements over 14 days, 316 readings
O'Grady et al. (2024) Independent
- Apple Watch Series 6ICC 0.67
Intraclass correlation · vs ECG · n = 53, Overnight, sleep laboratory
Miller et al. (2022) Other maker funded
- Apple Watch Series 622.5 ms (absolute bias)
Mean absolute error (absolute bias) · vs ECG · n = 53, Overnight, sleep laboratory
Miller et al. (2022) Other maker funded
Sleep stages (4-stage)4 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Apple Watch Series 8 | κ = 0.53Cohen's kappa | Polysomnography (PSG) | n = 62, Sleep laboratory, 1 night | Schyvens et al. (2025)Funding not reported |
| Apple Watch Series 8 | 50.5–86.1% (range across stages)Sensitivity | Polysomnography (PSG) | n = 35, Inpatient, 1 night | Robbins et al. (2024)Other maker funded |
| Apple Watch Series 8deep sleep duration | Deep sleep −43 minMean bias (signed) | Polysomnography (PSG) | n = 35, Inpatient, 1 night | Robbins et al. (2024)Other maker funded |
| Apple Watch Series 6 | 53%Epoch-by-epoch agreement | Polysomnography (PSG) | n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together | Miller et al. (2022)Other maker funded |
| Apple Watch Series 6 | κ = 0.20Cohen's kappa | Polysomnography (PSG) | n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together | Miller et al. (2022)Other maker funded |
| Apple Watch Series 8 | κ = 0.3Cohen's kappa | Polysomnography (PSG) | n = 26, Two Korean sleep centres, in-lab PSG | Lee et al. (2023)Independent |
- Apple Watch Series 8κ = 0.53
Cohen's kappa · vs Polysomnography (PSG) · n = 62, Sleep laboratory, 1 night
Schyvens et al. (2025) Funding not reported
- Apple Watch Series 850.5–86.1% (range across stages)
Sensitivity · vs Polysomnography (PSG) · n = 35, Inpatient, 1 night
Robbins et al. (2024) Other maker funded
- Apple Watch Series 8Deep sleep −43 min
Mean bias (signed), deep sleep duration · vs Polysomnography (PSG) · n = 35, Inpatient, 1 night
Robbins et al. (2024) Other maker funded
Epoch-by-epoch agreement · vs Polysomnography (PSG) · n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together
Miller et al. (2022) Other maker funded
- Apple Watch Series 6κ = 0.20
Cohen's kappa · vs Polysomnography (PSG) · n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together
Miller et al. (2022) Other maker funded
- Apple Watch Series 8κ = 0.3
Cohen's kappa · vs Polysomnography (PSG) · n = 26, Two Korean sleep centres, in-lab PSG
Lee et al. (2023) Independent
1 more result in the database. Sleep accuracy across devices →
Sleep vs wake (2-stage)3 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Apple Watch Series 8 | > 90% (all devices)Sensitivity | Polysomnography (PSG) | n = 62, Sleep laboratory, 1 night | Schyvens et al. (2025)Funding not reported |
| Apple Watch Series 8 | ≥ 95%Sensitivity | Polysomnography (PSG) | n = 35, Inpatient, 1 night | Robbins et al. (2024)Other maker funded |
| Apple Watch Series 6 | 88%Epoch-by-epoch agreement | Polysomnography (PSG) | n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together | Miller et al. (2022)Other maker funded |
| Apple Watch Series 6 | κ = 0.30Cohen's kappa | Polysomnography (PSG) | n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together | Miller et al. (2022)Other maker funded |
- Apple Watch Series 8> 90% (all devices)
Sensitivity · vs Polysomnography (PSG) · n = 62, Sleep laboratory, 1 night
Schyvens et al. (2025) Funding not reported
- Apple Watch Series 8≥ 95%
Sensitivity · vs Polysomnography (PSG) · n = 35, Inpatient, 1 night
Robbins et al. (2024) Other maker funded
Epoch-by-epoch agreement · vs Polysomnography (PSG) · n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together
Miller et al. (2022) Other maker funded
- Apple Watch Series 6κ = 0.30
Cohen's kappa · vs Polysomnography (PSG) · n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together
Miller et al. (2022) Other maker funded
Blood oxygen (SpO2)8 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Apple Watch Series 7 | 3.5% (rest)Root-mean-square error | Medical-grade pulse oximeter | n = 36, Rest, COPD outpatients | Støve et al. (2023)Funding not reported |
| Apple Watch Series 6 | r = 0.89Pearson correlation | Medical-grade pulse oximeter | n = 265, Spot readings | Spaccarotella et al. (2022)Independent |
| Apple Watch Series 6 | −0.23 (LoA −3.49 to 3.04)Mean bias (signed) | Medical-grade pulse oximeter | n = 265, Spot readings | Spaccarotella et al. (2022)Independent |
| Apple Watch Series 7 | 2.2% (RMSE 2.9%)Mean absolute error | Medical-grade pulse oximeter | n = 49, Spot readings, with and without chronic pulmonary disease | Jiang et al. (2023)Independent |
| Apple Watch Series 7 | 2.9%Root-mean-square error | Medical-grade pulse oximeter | n = 49, Spot readings, with and without chronic pulmonary disease | Jiang et al. (2023)Independent |
| Apple Watch Series 8 | Met ISO RMS ≤ 4% at normoxia and SpO2 < 90%; lowest bias and RMSD of the threeStudy finding (no single number) | Medical-grade pulse oximeter | n = 18, Induced hypoxemia (FiO2 14%, 12%, 10%), ISO 80601-2-61 | Walzel et al. (2023)Independent |
- Apple Watch Series 73.5% (rest)
Root-mean-square error · vs Medical-grade pulse oximeter · n = 36, Rest, COPD outpatients
Støve et al. (2023) Funding not reported
- Apple Watch Series 6r = 0.89
Pearson correlation · vs Medical-grade pulse oximeter · n = 265, Spot readings
Spaccarotella et al. (2022) Independent
- Apple Watch Series 6−0.23 (LoA −3.49 to 3.04)
Mean bias (signed) · vs Medical-grade pulse oximeter · n = 265, Spot readings
Spaccarotella et al. (2022) Independent
- Apple Watch Series 72.2% (RMSE 2.9%)
Mean absolute error · vs Medical-grade pulse oximeter · n = 49, Spot readings, with and without chronic pulmonary disease
Jiang et al. (2023) Independent
Root-mean-square error · vs Medical-grade pulse oximeter · n = 49, Spot readings, with and without chronic pulmonary disease
Jiang et al. (2023) Independent
- Apple Watch Series 8Met ISO RMS ≤ 4% at normoxia and SpO2 < 90%; lowest bias and RMSD of the three
Study finding (no single number) · vs Medical-grade pulse oximeter · n = 18, Induced hypoxemia (FiO2 14%, 12%, 10%), ISO 80601-2-61
Walzel et al. (2023) Independent
4 more results in the database. SpO2 accuracy across devices →
Atrial fibrillation detection4 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Apple Watch Series 6smartwatch ECG app, inconclusive tracings counted | 85% (spec 75%)Sensitivity | 12-lead ECG | n = 201, Tertiary cardiology service, single-lead ECG algorithms | Mannhart et al. (2023)Funding not reported |
| Apple Watch Series 6smartwatch ECG app, inconclusive tracings counted | 75%Specificity | 12-lead ECG | n = 201, Tertiary cardiology service, single-lead ECG algorithms | Mannhart et al. (2023)Funding not reported |
| Apple Watch + KardiaBand (SmartRhythm 2.0)AF episodes ≥ 1 h | 97.5% episode sensitivity (PPV 39.9%)Sensitivity | Insertable cardiac monitor | n = 24, Ambulatory, 31,348.9 h of recordings | Wasserlauf et al. (2019)Funding not reported |
| Apple Watch (irregular rhythm notification)PPG irregular pulse notification | PPV 0.84 (95% CI 0.76–0.92)Positive predictive value | ECG patch | n = 419297, Siteless pragmatic study; ECG patch after notification | Perez et al. (2019)Maker funded |
| Kardia Band on Apple Watch (single-lead ECG)automated ECG read; 57 of 169 recordings non-interpretable | Sens 93% / spec 84% (interpretable recordings)Sensitivity | 12-lead ECG | n = 100, Before and after cardioversion | Bumgarner et al. (2018)Funding not reported |
- Apple Watch Series 685% (spec 75%)
Sensitivity, smartwatch ECG app, inconclusive tracings counted · vs 12-lead ECG · n = 201, Tertiary cardiology service, single-lead ECG algorithms
Mannhart et al. (2023) Funding not reported
Specificity, smartwatch ECG app, inconclusive tracings counted · vs 12-lead ECG · n = 201, Tertiary cardiology service, single-lead ECG algorithms
Mannhart et al. (2023) Funding not reported
- Apple Watch + KardiaBand (SmartRhythm 2.0)97.5% episode sensitivity (PPV 39.9%)
Sensitivity, AF episodes ≥ 1 h · vs Insertable cardiac monitor · n = 24, Ambulatory, 31,348.9 h of recordings
Wasserlauf et al. (2019) Funding not reported
- Apple Watch (irregular rhythm notification)PPV 0.84 (95% CI 0.76–0.92)
Positive predictive value, PPG irregular pulse notification · vs ECG patch · n = 419297, Siteless pragmatic study; ECG patch after notification
Perez et al. (2019) Maker funded
- Kardia Band on Apple Watch (single-lead ECG)Sens 93% / spec 84% (interpretable recordings)
Sensitivity, automated ECG read; 57 of 169 recordings non-interpretable · vs 12-lead ECG · n = 100, Before and after cardioversion
Bumgarner et al. (2018) Funding not reported
VO2max estimate3 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Apple Watch Series 10 | 13.2% (MAE 6.79 mL/kg/min)Mean absolute percentage error | Indirect calorimetry | n = 40, Maximal treadmill test (modified Åstrand) | Lambe et al. (2026)Independent |
| Apple Watch | 13.31% (MAE 6.92 mL/kg/min)Mean absolute percentage error | Indirect calorimetry | n = 30, Maximal treadmill test (modified Åstrand) | Lambe et al. (2025)Independent |
| Apple Watch Series 7 | 15.79% (ICC 0.47)Mean absolute percentage error | Indirect calorimetry | n = 19, Graded cycle ergometer test | Caserman et al. (2024)Independent |
- Apple Watch Series 1013.2% (MAE 6.79 mL/kg/min)
Mean absolute percentage error · vs Indirect calorimetry · n = 40, Maximal treadmill test (modified Åstrand)
Lambe et al. (2026) Independent
- Apple Watch13.31% (MAE 6.92 mL/kg/min)
Mean absolute percentage error · vs Indirect calorimetry · n = 30, Maximal treadmill test (modified Åstrand)
Lambe et al. (2025) Independent
- Apple Watch Series 715.79% (ICC 0.47)
Mean absolute percentage error · vs Indirect calorimetry · n = 19, Graded cycle ergometer test
Caserman et al. (2024) Independent
Energy expenditure (calories)6 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Apple Watch Series 9 | Inconsistent across all groupsStudy finding (no single number) | Indirect calorimetry | n = 30, 10-min treadmill | Chase et al. (2024)Independent |
| Apple Watch Series 4 | 13.5%–27.1% (range by intensity)Coefficient of variation | Indirect calorimetry | n = 25, Sitting, walking, running, sprints | Düking et al. (2020)Independent |
| Apple Watch Series 6 | 14.68–24.85% (poor in all activities)Coefficient of variation | Indirect calorimetry | n = 60, Sitting, walking, running, resistance, cycling | Hajj-Boutros et al. (2023)Funding not reported |
| Apple Watch Series 8physical activity energy expenditure, 10 min | +21.6 kcalMean bias (signed) | Indirect calorimetry | n = 52, 10-min recumbent cycling intervals | Kostrna et al. (2026)Independent |
| Apple Watch Series 6walking | 19.8% walk / 24.4% runMean absolute percentage error | Indirect calorimetry | n = 20, Outdoor 2-km walk (6 km/ | Le et al. (2022)Independent |
| Apple Watch | Lowest EE error of 7 devices, but no device below 20%Study finding (no single number) | Indirect calorimetry | n = 60, Sitting, walking, running, cycling | Shcherbina et al. (2017)Independent |
- Apple Watch Series 9Inconsistent across all groups
Study finding (no single number) · vs Indirect calorimetry · n = 30, 10-min treadmill
Chase et al. (2024) Independent
- Apple Watch Series 413.5%–27.1% (range by intensity)
Coefficient of variation · vs Indirect calorimetry · n = 25, Sitting, walking, running, sprints
Düking et al. (2020) Independent
- Apple Watch Series 614.68–24.85% (poor in all activities)
Coefficient of variation · vs Indirect calorimetry · n = 60, Sitting, walking, running, resistance, cycling
Hajj-Boutros et al. (2023) Funding not reported
- Apple Watch Series 8+21.6 kcal
Mean bias (signed), physical activity energy expenditure, 10 min · vs Indirect calorimetry · n = 52, 10-min recumbent cycling intervals
Kostrna et al. (2026) Independent
- Apple Watch Series 619.8% walk / 24.4% run
Mean absolute percentage error, walking · vs Indirect calorimetry · n = 20, Outdoor 2-km walk (6 km/h) and run (10 km/h)
Le et al. (2022) Independent
- Apple WatchLowest EE error of 7 devices, but no device below 20%
Study finding (no single number) · vs Indirect calorimetry · n = 60, Sitting, walking, running, cycling
Shcherbina et al. (2017) Independent
Apple Watch PPG Sensor: Technical Deep-Dive Comparisons
Whoop vs Apple Watch: What the Sleep-Lab Studies Found
Two sleep-lab studies tested WHOOP and Apple Watch on the same people on the same night, against polysomnography (PSG) for sleep and ECG for heart rate. Miller and colleagues (2022) tested WHOOP 3.0 and Apple Watch Series 6 in 53 adults; Schyvens and colleagues (2025) tested WHOOP 4.0 and Apple Watch Series 8 in 62 adults. Overnight heart rate was close to ECG for both. WHOOP 3.0 tracked overnight HRV more closely than Apple Watch S6. For sleep stages the two studies point in different directions, so neither device clearly wins there.
View Comparison →Garmin vs Apple Watch: Heart Rate Accuracy from Published Studies
In the studies that tested both brands, Apple Watch agreed more closely with ECG for exercise heart rate and overnight heart rate. Series 10 and Fenix 8 were not the models tested.
View Comparison →Apple Watch Ultra 2 vs Garmin Fenix 8: Specs Guide
Choose the Garmin Fenix 8 if battery life, built-in mapping, and training tools come first. Choose the Apple Watch Ultra 2 if you use an iPhone and value tighter Apple integration, cellular connectivity, and a brighter specified display. Both offer optical heart-rate sensing, multi-band positioning, ECG support in eligible regions, and a 40 m recreational dive rating. This comparison uses the 49 mm Ultra 2 and 47 mm Fenix 8 AMOLED as the clearest like-for-like models.
View Comparison →Fitbit vs Apple Watch: Heart Rate and Sleep Accuracy from Published Studies
Independent studies favor Apple Watch for exercise heart rate (concordance with ECG 0.92 vs 0.67 for Fitbit Blaze in Gillinov 2017). For sleep staging, Fitbit Sense (κ 0.42) and Charge 5 (κ 0.41) came close to Apple Watch Series 8 (κ 0.53) in Schyvens 2025. Both brands ran large AFib studies, but with different definitions, so their headline numbers are not directly comparable.
View Comparison →Samsung Galaxy Watch vs Apple Watch: PPG Sensor and Health Features
Samsung Galaxy Watch7 and Watch Ultra pair only with Android phones; Apple Watch pairs only with iPhone, so the phone usually decides. Both have an FDA-cleared or authorized ECG feature. Samsung adds cuffless blood pressure estimates that need cuff calibration every 28 days. In independent testing, Apple Watch had the lowest heart-rate error and Samsung's older Gear S2 the highest of seven devices (Shcherbina 2017); no study has compared current models.
View Comparison →Oura Ring vs Apple Watch: Finger PPG vs Wrist PPG Accuracy
Oura is a ring built mainly for sleep and recovery; Apple Watch is a full smartwatch with ECG. Three studies tested both against polysomnography. In Miller 2022 (53 adults), Oura Gen 2 had higher sleep-stage agreement than Apple Watch Series 6 (61% vs 53%) while Apple Watch tracked overnight heart rate more closely (ICC 0.96 vs 0.85); overnight HRV agreement was similar (0.67 vs 0.63). In the independent Lee 2023 study, four-stage kappa was 0.35 for Oura Gen3 and 0.30 for Apple Watch Series 8. An Oura-funded study (Robbins 2024, 35 adults) reported stage sensitivity of 76.0–79.5% for Oura Gen3 and 50.5–86.1% for Apple Watch Series 8.
View Comparison →Google Pixel Watch vs Apple Watch: PPG Sensor and Health Ecosystem
Pixel Watch 3 pairs with Android phones and Apple Watch with iPhone, so the phone usually decides. Google lists a multi-path optical heart-rate sensor, red and infrared SpO2 sensors, ECG-compatible electrical sensors, a cEDA skin-conductance sensor and skin temperature on Pixel Watch 3. Apple lists an electrical heart sensor, third-generation optical heart sensor, blood oxygen and temperature sensors on Series 10. Both quote about a day of battery. No study has compared the two.
View Comparison →Withings ScanWatch vs Apple Watch: Sleep Accuracy in a Published Study
The published head-to-head result is sleep staging, and it used the original ScanWatch, not ScanWatch 2. Battery life below is Withings’ specification, not a study.
View Comparison →Research records
Device records collect datasets, papers, and the sourced figures on this page. For accuracy across the brand, read How accurate is Apple Watch heart rate?.
Also named in this guide: Polar H7
Frequently Asked Questions
What PPG wavelengths does Apple Watch use?
Apple documents green light for optical heart-rate measurements, infrared light for some background readings, and red plus infrared light for Blood Oxygen measurements on supported models. It does not publish exact nominal wavelengths for every generation.
How accurate is Apple Watch heart rate?
Apple does not publish a numeric heart-rate accuracy specification. In one ECG-referenced exercise study, Apple Watch reached a Lin's concordance of 0.92 versus 0.996 for a Polar H7 chest strap (Gillinov et al., 2017). Accuracy varies with fit, motion and activity type.
Is Apple Watch SpO2 FDA cleared?
No. Apple says the Blood Oxygen app is for general fitness and wellness and not intended for medical use. The Irregular Rhythm Notification feature is a separate PPG feature that the FDA classified through De Novo request DEN180042 in 2018.
Sources
- Apple. Monitor your heart rate with Apple Watch
- Apple. How to use the Blood Oxygen app on Apple Watch
- U.S. Food and Drug Administration. FDA record
- Gillinov et al. (2017). Variable Accuracy of Wearable Heart Rate Monitors during Aerobic Exercise. Medicine & Science in Sports & Exercise. doi:10.1249/MSS.0000000000001284
- Perez et al. (2019), Apple Heart Study. Large-Scale Assessment of a Smartwatch to Identify Atrial Fibrillation. New England Journal of Medicine. doi:10.1056/NEJMoa1901183
- Miller, Sargent & Roach (2022). A Validation of Six Wearable Devices for Estimating Sleep, Heart Rate and Heart Rate Variability in Healthy Adults. Sensors. doi:10.3390/s22166317
- Spaccarotella et al. (2022). Assessment of Non-Invasive Measurements of Oxygen Saturation and Heart Rate with an Apple Smartwatch: Comparison with a Standard Pulse Oximeter. Journal of Clinical Medicine. doi:10.3390/jcm11061467
- Støve et al. (2023). Assessment of Noninvasive Oxygen Saturation in Patients With COPD During Pulmonary Rehabilitation: Smartwatch versus Pulse Oximeter. Respiratory Care. doi:10.4187/respcare.10760