Wearable guides
Fitbit PurePulse 2.0: PPG Sensor Deep-Dive
Fitbit introduced PurePulse 2.0 with the Sense and Versa 3 in 2020: a multi-path optical heart-rate sensor with more LED and photodiode paths and an updated algorithm. Fitbit devices use it for heart rate, HRV, SpO2 and sleep features; Fitbit does not publish a numeric heart-rate accuracy specification.
Fitbit announced PurePulse 2.0 alongside the Sense and Versa 3 in August 2020 as a multi-path optical heart-rate sensor with an updated algorithm. The Sense also added an EDA sensor for stress management, an ECG app and a skin temperature sensor (Fitbit Sense launch).
Independent heart-rate results vary by model. Against ECG during treadmill, cycling and elliptical exercise, a Fitbit Blaze reached a Lin's concordance of 0.67, the lowest of the wrist devices tested, versus 0.996 for a Polar H7 chest strap (Gillinov et al., 2017). In a study of 22 adults that included a Fitbit Charge HR, correlations with ECG heart rate ranged from 0.67 to 0.95 across the four wrist devices, and all of them underestimated heart rate (Wallen et al., 2016).
For rhythm screening, the Fitbit Heart Study reported a positive predictive value of 98.2% for an irregular rhythm detection being confirmed as concurrent AF on an ECG patch (Lubitz et al., 2022). Daily Readiness now combines heart rate variability, recent sleep and resting heart rate; the original activity component was replaced by resting heart rate (Google Health Help: readiness). Estimated Oxygen Variation shows how much blood oxygen fluctuated during sleep rather than an absolute SpO2 value, and is shown separately from the nightly average SpO2 (Fitbit Help: estimated oxygen variation).
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.
Fitbit Blaze heart rate vs ECG during exercise: Lin's concordance 0.67 (Polar H7 chest strap 0.996).
Gillinov et al. (2017), Medicine & Science in Sports & Exercise
Fitbit Heart Study: positive predictive value of irregular rhythm detection for concurrent AF on ECG patch 98.2% (95% CI 95.5–99.5%).
Fitbit Alta HR performed as well as or better than actigraphy for sleep/wake classification against polysomnography.
Four wrist devices including Fitbit Charge HR: heart-rate correlation with ECG 0.67–0.95; all underestimated heart rate.
What published studies show
Results quoted from 16 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 |
|---|---|---|---|---|
| Fitbit Charge 4 | 7.8 bpm (seated rest)Mean absolute error | Holter ECG | n = 23, Rest, sedentary, low- and high-intensity phases | Nissen et al. (2022)Independent |
| Fitbit Charge 4 | ICC 0.80 (COPD) / 0.79 (controls)Intraclass correlation | ECG chest strap (Polar H7/ | n = 55, 5 days free-living | Hermans et al. (2024)Funding not reported |
- Fitbit Charge 47.8 bpm (seated rest)
Mean absolute error · vs Holter ECG · n = 23, Rest, sedentary, low- and high-intensity phases
Nissen et al. (2022) Independent
- Fitbit Charge 4ICC 0.80 (COPD) / 0.79 (controls)
Intraclass correlation · vs ECG chest strap (Polar H7/H10) · n = 55, 5 days free-living
Hermans et al. (2024) Funding not reported
Heart rate during exercise5 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Fitbit Charge 6mixed rest/stress/walking | 4.5 bpmMean absolute error | ECG chest strap (Zephyr BioHarness / | n = 45, Rest, cognitive stress, steady and intermittent walking in neutral/ | Gielen et al. (2026)Funding not reported |
| Fitbit Inspire 3mixed rest/stress/walking | 9–14 bpm (group range)Mean absolute error | ECG chest strap (Zephyr BioHarness / | n = 45, Rest, cognitive stress, steady and intermittent walking in neutral/ | Gielen et al. (2026)Funding not reported |
| Fitbit Charge 4squats | 15.7 bpm (squats)Mean absolute error | Holter ECG | n = 23, Rest, sedentary, low- and high-intensity phases | Nissen et al. (2022)Independent |
| Fitbit Versa | 4.1%–19.1% (range by intensity)Coefficient of variation | ECG chest strap (Polar H7/ | n = 25, Sitting, walking, running, sprints | Düking et al. (2020)Independent |
| Fitbit Sense | 4.14–10.76%Coefficient of variation | ECG chest strap (Polar H7/ | n = 60, Sitting, walking, running, resistance, cycling | Hajj-Boutros et al. (2023)Funding not reported |
| Fitbit Blazeall exercise conditions | rc = 0.67Lin's concordance correlation | ECG | n = 50, Gillinov: treadmill, bike, elliptical | Gillinov et al. (2017)Funding not reported |
- Fitbit Charge 64.5 bpm
Mean absolute error, mixed rest/stress/walking · vs ECG chest strap (Zephyr BioHarness / Faros) · n = 45, Rest, cognitive stress, steady and intermittent walking in neutral/hot/cold chambers
Gielen et al. (2026) Funding not reported
- Fitbit Inspire 39–14 bpm (group range)
Mean absolute error, mixed rest/stress/walking · vs ECG chest strap (Zephyr BioHarness / Faros) · n = 45, Rest, cognitive stress, steady and intermittent walking in neutral/hot/cold chambers
Gielen et al. (2026) Funding not reported
- Fitbit Charge 415.7 bpm (squats)
Mean absolute error, squats · vs Holter ECG · n = 23, Rest, sedentary, low- and high-intensity phases
Nissen et al. (2022) Independent
- Fitbit Versa4.1%–19.1% (range by intensity)
Coefficient of variation · vs ECG chest strap (Polar H7/H10) · n = 25, Sitting, walking, running, sprints
Düking et al. (2020) Independent
- Fitbit Sense4.14–10.76%
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
- Fitbit Blazerc = 0.67
Lin's concordance correlation, all exercise conditions · vs ECG · n = 50, Gillinov: treadmill, bike, elliptical
Gillinov et al. (2017) Funding not reported
Sleep stages (4-stage)3 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Fitbit Sense | κ = 0.42Cohen's kappa | Polysomnography (PSG) | n = 37, Sleep laboratory, 1 night | Schyvens et al. (2025)Funding not reported |
| Fitbit Charge 5 | κ = 0.41Cohen's kappa | Polysomnography (PSG) | n = 39, Sleep laboratory, 1 night | Schyvens et al. (2025)Funding not reported |
| Fitbit Sense 2 | 61.7–78.0% (range across stages)Sensitivity | Polysomnography (PSG) | n = 35, Inpatient, 1 night | Robbins et al. (2024)Other maker funded |
| Fitbit Sense 2deep sleep duration | Deep sleep −15 minMean bias (signed) | Polysomnography (PSG) | n = 35, Inpatient, 1 night | Robbins et al. (2024)Other maker funded |
| Fitbit Sense 2 | κ = 0.42Cohen's kappa | Polysomnography (PSG) | n = 26, Two Korean sleep centres, in-lab PSG | Lee et al. (2023)Independent |
| Fitbit Sense 2 | 64.6%Epoch-by-epoch agreement | Polysomnography (PSG) | n = 26, Two Korean sleep centres, in-lab PSG | Lee et al. (2023)Independent |
- Fitbit Senseκ = 0.42
Cohen's kappa · vs Polysomnography (PSG) · n = 37, Sleep laboratory, 1 night
Schyvens et al. (2025) Funding not reported
- Fitbit Charge 5κ = 0.41
Cohen's kappa · vs Polysomnography (PSG) · n = 39, Sleep laboratory, 1 night
Schyvens et al. (2025) Funding not reported
- Fitbit Sense 261.7–78.0% (range across stages)
Sensitivity · vs Polysomnography (PSG) · n = 35, Inpatient, 1 night
Robbins et al. (2024) Other maker funded
- Fitbit Sense 2Deep sleep −15 min
Mean bias (signed), deep sleep duration · vs Polysomnography (PSG) · n = 35, Inpatient, 1 night
Robbins et al. (2024) Other maker funded
- Fitbit Sense 2κ = 0.42
Cohen's kappa · vs Polysomnography (PSG) · n = 26, Two Korean sleep centres, in-lab PSG
Lee et al. (2023) Independent
- Fitbit Sense 264.6%
Epoch-by-epoch agreement · vs Polysomnography (PSG) · n = 26, Two Korean sleep centres, in-lab PSG
Lee et al. (2023) Independent
Sleep vs wake (2-stage)3 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Fitbit Charge 5 | > 90% (all devices)Sensitivity | Polysomnography (PSG) | n = 62, Sleep laboratory, 1 night | Schyvens et al. (2025)Funding not reported |
| Fitbit Sense | > 90% (all devices)Sensitivity | Polysomnography (PSG) | n = 62, Sleep laboratory, 1 night | Schyvens et al. (2025)Funding not reported |
| Fitbit Sense 2 | ≥ 95%Sensitivity | Polysomnography (PSG) | n = 35, Inpatient, 1 night | Robbins et al. (2024)Other maker funded |
| Fitbit Alta HR | As good as or better than actigraphyStudy finding (no single number) | Polysomnography (PSG) | n = 34, Sleep laboratory, 3 nights incl. disrupted night | Chinoy et al. (2021)Funding not reported |
- Fitbit Charge 5> 90% (all devices)
Sensitivity · vs Polysomnography (PSG) · n = 62, Sleep laboratory, 1 night
Schyvens et al. (2025) Funding not reported
- Fitbit Sense> 90% (all devices)
Sensitivity · vs Polysomnography (PSG) · n = 62, Sleep laboratory, 1 night
Schyvens et al. (2025) Funding not reported
- Fitbit Sense 2≥ 95%
Sensitivity · vs Polysomnography (PSG) · n = 35, Inpatient, 1 night
Robbins et al. (2024) Other maker funded
- Fitbit Alta HRAs good as or better than actigraphy
Study finding (no single number) · vs Polysomnography (PSG) · n = 34, Sleep laboratory, 3 nights incl. disrupted night
Chinoy et al. (2021) Funding not reported
Total sleep time1 study
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Fitbit Sense 2 | −74.5 min (older adults)Mean bias (signed) | Polysomnography (PSG) | n = 19, 1 night | Searles et al. (2026)Funding not reported |
- Fitbit Sense 2−74.5 min (older adults)
Mean bias (signed) · vs Polysomnography (PSG) · n = 19, 1 night
Searles et al. (2026) Funding not reported
Blood oxygen (SpO2)1 study
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Fitbit Charge 4 | ICC 0.32 (COPD, overnight)Intraclass correlation | Medical-grade pulse oximeter | n = 30, 4 nights | Hermans et al. (2024)Funding not reported |
- Fitbit Charge 4ICC 0.32 (COPD, overnight)
Intraclass correlation · vs Medical-grade pulse oximeter · n = 30, 4 nights
Hermans et al. (2024) Funding not reported
Atrial fibrillation detection3 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Fitbit Sensesmartwatch ECG app, inconclusive tracings counted | 66% (spec 79%)Sensitivity | 12-lead ECG | n = 201, Tertiary cardiology service, single-lead ECG algorithms | Mannhart et al. (2023)Funding not reported |
| Fitbit Sensesmartwatch ECG app, inconclusive tracings counted | 79%Specificity | 12-lead ECG | n = 201, Tertiary cardiology service, single-lead ECG algorithms | Mannhart et al. (2023)Funding not reported |
| Fitbit (PPG irregular rhythm algorithm)PPG irregular rhythm detection | PPV 98.2% (95% CI 95.5–99.5%)Positive predictive value | ECG patch | n = 455699, Remote trial; 1-week ECG patch after detection | Lubitz et al. (2022)Maker funded |
| Fitbit Ionic + FibriCheck algorithmthird-party PPG algorithm (FibriCheck), not Fitbit's own | Sens 97% / spec 100%Sensitivity | 12-lead ECG | n = 73, Around cardioversion, 1-min PPG | Selder et al. (2023)Independent |
- Fitbit Sense66% (spec 79%)
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
- Fitbit Sense79%
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
- Fitbit (PPG irregular rhythm algorithm)PPV 98.2% (95% CI 95.5–99.5%)
Positive predictive value, PPG irregular rhythm detection · vs ECG patch · n = 455699, Remote trial; 1-week ECG patch after detection
Lubitz et al. (2022) Maker funded
- Fitbit Ionic + FibriCheck algorithmSens 97% / spec 100%
Sensitivity, third-party PPG algorithm (FibriCheck), not Fitbit's own · vs 12-lead ECG · n = 73, Around cardioversion, 1-min PPG
Selder et al. (2023) Independent
Energy expenditure (calories)4 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Fitbit Versa | 8.0%–32.3% (range by intensity)Coefficient of variation | Indirect calorimetry | n = 25, Sitting, walking, running, sprints | Düking et al. (2020)Independent |
| Fitbit Sense | 13.44–29.66% (poor in all activities)Coefficient of variation | Indirect calorimetry | n = 60, Sitting, walking, running, resistance, cycling | Hajj-Boutros et al. (2023)Funding not reported |
| Fitbit Sense 2physical activity energy expenditure, 10 min | +3.14 kcalMean bias (signed) | Indirect calorimetry | n = 44, 10-min recumbent cycling intervals | Kostrna et al. (2026)Independent |
| Fitbit Surge | No device achieved EE error below 20%Study finding (no single number) | Indirect calorimetry | n = 60, Sitting, walking, running, cycling | Shcherbina et al. (2017)Independent |
- Fitbit Versa8.0%–32.3% (range by intensity)
Coefficient of variation · vs Indirect calorimetry · n = 25, Sitting, walking, running, sprints
Düking et al. (2020) Independent
- Fitbit Sense13.44–29.66% (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
- Fitbit Sense 2+3.14 kcal
Mean bias (signed), physical activity energy expenditure, 10 min · vs Indirect calorimetry · n = 44, 10-min recumbent cycling intervals
Kostrna et al. (2026) Independent
- Fitbit SurgeNo device achieved EE error below 20%
Study finding (no single number) · vs Indirect calorimetry · n = 60, Sitting, walking, running, cycling
Shcherbina et al. (2017) Independent
Respiratory rate1 study
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Fitbit Charge 4 | ICC 0.84 (COPD) / 0.77 (controls)Intraclass correlation | ECG chest strap (Polar H7/ | n = 55, 5 days free-living | Hermans et al. (2024)Funding not reported |
- Fitbit Charge 4ICC 0.84 (COPD) / 0.77 (controls)
Intraclass correlation · vs ECG chest strap (Polar H7/H10) · n = 55, 5 days free-living
Hermans et al. (2024) Funding not reported
Fitbit PurePulse 2.0: PPG Sensor Deep-Dive Comparisons
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 →Fitbit vs Garmin: Heart Rate Accuracy from Published Studies
Older-model studies give mixed results. During exercise, the Garmin Forerunner 235 agreed more closely with ECG than the Fitbit Blaze (concordance 0.81 vs 0.67; Gillinov 2017). For sleep, Fitbit Alta HR performed as well as or better than actigraphy while Garmin Fenix 5S and Vivosmart 3 did worse (Chinoy 2021), and Fitbit Sense and Charge 5 (κ 0.42/0.41) beat Garmin Vivosmart 4 (κ 0.21) in Schyvens 2025.
View Comparison →Research records
Device records collect datasets, papers, and the sourced figures on this page. For accuracy across the brand, read Fitbit heart rate accuracy.
Also named in this guide: Polar H7
Frequently Asked Questions
How does Fitbit sleep staging work with PPG?
Fitbit estimates sleep stages from heart-rate patterns, HRV and movement. Fitbit does not publish a stage-classification accuracy figure. In a polysomnography study of seven consumer devices, Fitbit Alta HR performed as well as or better than actigraphy for sleep/wake classification (Chinoy et al., 2021).
Is Fitbit Sense EDA sensor useful for stress monitoring?
EDA (electrodermal activity) measures small changes in skin conductance linked to sympathetic arousal. Fitbit introduced it on the Sense for stress management. It needs good skin contact and is a wellness feature, not a diagnostic test.
Does Google acquisition affect Fitbit PPG technology?
Fitbit is now part of Google, and Fitbit health features also run on Google Pixel Watch. Google describes the Pixel Watch 3 heart-rate sensors as multi-path, and its updated Daily Readiness uses HRV, sleep and resting heart rate (Google: Pixel Watch 3 Daily Readiness).
Sources
- Google / Fitbit. Fitbit debuts Sense, its most advanced health smartwatch, Versa 3 and Inspire 2
- Gillinov et al. (2017). Variable Accuracy of Wearable Heart Rate Monitors during Aerobic Exercise. Medicine & Science in Sports & Exercise. doi:10.1249/MSS.0000000000001284
- Wallen et al. (2016). Accuracy of Heart Rate Watches: Implications for Weight Management. PLOS ONE. doi:10.1371/journal.pone.0154420
- Lubitz et al. (2022), Fitbit Heart Study. Detection of Atrial Fibrillation in a Large Population Using Wearable Devices: The Fitbit Heart Study. Circulation. doi:10.1161/CIRCULATIONAHA.122.060291
- Chinoy et al. (2021). Performance of seven consumer sleep-tracking devices compared with polysomnography. Sleep. doi:10.1093/sleep/zsaa291
- Google. Understanding your readiness score
- Google. How do I track my estimated oxygen variation in the Fitbit app?
- Google. Google Pixel Watch 3: Updated Daily Readiness and new fitness metrics