Fitbit: What the Research Says About Each Metric
Fitbit pioneered consumer step tracking and still takes the most different approach to blood oxygen of any brand compared here.
Fitbit's identity was built on step counting before any of today's composite scores existed, and its more recent features often reframe familiar metrics in its own way rather than following the naming conventions other brands settled on. Below is what independent research and Fitbit's own help documentation say about each metric it tracks. A few items here rest on device-comparison research that hasn't been verified to the same depth as the rest, and are flagged plainly where that's the case.
How Does Fitbit Measure HRV?
Fitbit calculates HRV using RMSSD, averaged across the full night rather than reported as a single snapshot or tracked continuously through the day. It's overnight-only: Fitbit doesn't offer a daytime HRV reading at all, the same overnight-only design as Oura's ring but the opposite of Whoop's continuous, all-day approach.
What Fitbit calls it
"Heart Rate Variability," shown on the Health Metrics dashboard rather than surfaced as a standalone branded score the way Oura's HRV Balance or Garmin's HRV Status are.
How Does Fitbit Measure Resting Heart Rate?
Fitbit averages heart rate during non-movement periods across the full day, including daytime, rather than restricting the measurement to overnight the way Oura does.
What Fitbit calls it
"Resting Heart Rate," the same label as every other brand, with a daytime-inclusive averaging window specific to Fitbit.
Does Fitbit Have an ECG Feature?
Yes, on the Sense 2, which offers AFib monitoring through an ECG app similar in concept to Apple's, though narrower in device availability since it isn't on every current Fitbit model. Sense 2 pairs it with a second, passive feature, Irregular Heart Rhythm Notifications, which uses the same optical (PPG) sensor as everyday heart rate tracking to flag possible AFib in the background rather than requiring an on-demand spot check.
What Fitbit calls it
Officially the "Fitbit ECG App," branded the "Google ECG App" in newer Google-authored materials. Both it and Irregular Heart Rhythm Notifications carry FDA clearance and a CE mark, but they work differently: the ECG app is a one-time spot check a user runs on demand, while notifications monitor passively over time.
How Accurate Is Fitbit's Cardio Fitness Score?
Cardio Fitness Score estimates VO2 max by tracking the relationship between running pace and heart rate during a run, comparing that efficiency against resting heart rate, then benchmarking the result against age, sex, and weight norms. A peer-reviewed validation of the Fitbit Charge 2 found the score gave a consistent, unbiased read on relative fitness but overestimated actual lab-measured VO2 max, and that a simple non-exercise prediction equation using only age, resting heart rate, and body composition matched it for accuracy without requiring a run at all.
What Fitbit calls it
"Cardio Fitness Score," Fitbit's equivalent of Garmin's directly-labeled "VO2 Max" and Apple's "Cardio Fitness": three brands, three different names for estimates of the same underlying measure.
How Does Fitbit Measure Blood Oxygen?
Fitbit deliberately doesn't report a raw SpO2 percentage. "Estimated Oxygen Variation" is a relative overnight graph showing how much blood oxygen fluctuates, with a red, yellow, or green pattern flagging possible breathing disturbances, a genuinely different design from every other brand compared here.
What Fitbit calls it
"Estimated Oxygen Variation" (EOV). Apple, Garmin, and Oura all report some form of an actual oxygen-saturation reading; Fitbit's relative-graph approach is the outlier among the four.
What Does Fitbit's Daily Readiness Score Measure?
Daily Readiness Score, a Premium-only feature, weighs HRV, resting heart rate, and sleep quantity and architecture roughly equally with recent activity.
What Fitbit calls it
"Daily Readiness Score." Unlike Whoop's Recovery Score or Oura's Readiness Score, both free with the device, Fitbit gates this feature behind a Premium subscription.
Worth knowing
A real peer-reviewed evaluation found composite recovery scores across brands, Fitbit's included, are not methodologically equivalent to each other despite similar-sounding names.
How Accurate Is Fitbit's Step and Calorie Tracking?
Fitbit was the original consumer step-counting wearable, and step count remains one of the more reliable metrics across this entire device category. Calorie tracking is the weaker point, shared with every other brand compared here.
Worth knowing
A systematic review of 65 studies across 72 devices and 29 brands found energy-expenditure error exceeded 30% for every brand tested, even though step count and heart rate performed comparably better in the same review. This is a category-wide limitation, not something specific to Fitbit.
What Fitbit Doesn't Do
One real absence relative to its two closest competitors on price and positioning.
No CGM integration
Unlike Oura's live Dexcom Stelo integration, Fitbit has no continuous glucose monitoring feature as of this writing.
If step and activity tracking is the priority and a lower price point matters, Fitbit's original strength still holds up. If a native ECG or a free (not Premium-gated) recovery score matters more, that shapes which specific Fitbit model, if any, actually fits.