Apple Watch vs Fitbit: Accurate, With Different Blind Spots

Apple Watch HRV has been tested directly: against a 3-lead ECG in 78 adults, the Series 6 measured beat-to-beat intervals almost exactly at rest (1.15 percent error) but its HRV reading far less closely (31.31 percent) (Bonneval et al., Sensors, 2025), and against a Polar H10 chest strap the Series 9 and Ultra 2 underestimated HRV by 8.31 ms on average (O'Grady et al., Sensors, 2024). No comparable review or study turned up for Fitbit's HRV specifically; its own published evidence base is concentrated on sleep-stage and heart-rate accuracy instead.

Separate validations of each brand found the same overall accuracy band but a different weak point. A 2017 treadmill study of two Apple Watches against a chest-strap reference found trivial bias at every intensity, with correlation strongest during walking (r=0.97) and weaker during running (r=0.81 to 0.86). A 2022 evaluation of a Fitbit Charge 4 against a Holter ECG, run alongside a Samsung Galaxy Watch, found the Fitbit's error stayed under 10 percent overall, with its best accuracy during seated rest and its weakest during high-intensity intervals, roughly the opposite pattern from a separate 2019 clinical-population study of Apple Watch, where accuracy improved rather than degraded as intensity rose.

Neither study tested the two brands' current models against each other directly, so this compares each brand's own separate evidence rather than a declared head-to-head winner.

Seven polysomnography studies across five Fitbit generations, one of them funded by Fitbit, found consistently high sensitivity for detecting sleep (93 to 96 percent) alongside much weaker specificity for detecting wake (13 to 62 percent) and systematic stage-classification bias. Apple Watch has now been tested alongside Fitbit in the same sleep lab three times. In 35 healthy adults, the Fitbit Sense 2 overestimated light sleep by 18 minutes and underestimated deep sleep by 15, while the Apple Watch Series 8 underestimated deep sleep by 43 minutes and overestimated light sleep by 45, in a study funded by Oura (Sensors, 2024 [funding listed from Oura]). In a six-device study, the Series 8's stage agreement, a Cohen's kappa of 0.53, edged the Fitbit Sense at 0.42 and the Fitbit Charge 5 at 0.41 (Sleep Advances, 2025). In a home sleep study, the Apple Watch Series 7 ranked ahead of the Fitbit Charge 5 overall, though both showed large stage biases (Sleep Medicine, 2026).

Both ECG apps were tested on the same patients against the same reference. In 201 patients at a tertiary cardiology center, 62 of them in atrial fibrillation, the Apple Watch 6 detected atrial fibrillation with 85 percent sensitivity and 75 percent specificity against a physician-read 12-lead ECG, and the Fitbit Sense with 66 percent and 79 percent. The Fitbit could not classify 21 percent of tracings and the Apple Watch 18 percent, which is what held both sensitivities down. The study was funded by Abbott, which made none of the devices tested.

Apple's has also been tested in a harder population, and the result is a genuine caution rather than a clean endorsement: a 2020 study of 50 post-cardiac-surgery patients who frequently cycle between AFib and normal sinus rhythm, a harder population than general screening, found the Apple Watch Series 4's rhythm notification caught AFib with only 41 percent sensitivity against continuous telemetry. That is a different population and a different question from the widely cited Apple Heart Study accuracy figures, not a contradiction of them, but it means Apple's ECG accuracy is not a single fixed number either.

A peer-reviewed validation of the Fitbit Charge 2 found its score gave a consistent, unbiased read on relative fitness but overestimated actual lab-measured VO2 max, and that a simple equation using only age, resting heart rate, and body composition matched it for accuracy without requiring a run at all. A 2024 validation of the Apple Watch Series 7 against a graded cycling test with a metabolic gas analyzer found the opposite direction of average error, a significant underestimate of lab-measured VO2 max, with 15.79 percent mean absolute percentage error and an intraclass correlation of only 0.47, which the study's authors call poor reliability. A 2026 test of the newer Series 10 found the same direction: in 40 healthy adults it underestimated lab-measured VO2 max by 6.25 ml/kg/min on average, with 13.2 percent mean absolute percentage error, and placed only 5 of the 40 in their correct fitness percentile. Both estimates are usable as a trend, and neither should be read as a precise number.

A 2023 study comparing several consumer smartwatches against a clinical-grade pulse oximeter in 49 adults found the Apple Watch Series 7 closest to the reference standard of the devices tested (mean absolute error 2.2 percent), with the highest rate of successful readings. No Fitbit was in that study. Fitbit’s reading has been tested on its own, in patients: in 30 people with COPD wearing a Fitbit Charge 4 alongside a medical wrist oximeter for four nights, agreement on oxygen saturation was poor (intraclass correlation 0.32) and the Fitbit read high, and in 33 hospitalized medical patients the Fitbit Sense 2 returned an oxygen reading for only 41 percent of the period, reading 1.7 points high on average with modest agreement (0.47). No study cited here tested a Fitbit and an Apple Watch for blood oxygen on the same people.