Fitbit vs Garmin: What the Evidence Actually Shows
No price, no comfort, no "which should you buy." Just what each device measures, how well-validated that measurement is, and where the two genuinely diverge.
Fitbit and Garmin measure several of the same core signals through meaningfully different windows and philosophies, from resting heart rate timing to how each brand chooses to report, or deliberately not report, blood oxygen. Every section below compares what's actually measured and how strong the evidence behind it is, drawn from each brand's individually-verified metric pages. For depth beyond a single metric, the full Fitbit and Garmin profiles cover each device on its own.
Which Measures Resting Heart Rate on a Wider Window?
Fitbit
Averages heart rate during non-movement periods across the full day, including daytime, rather than restricting the measurement to overnight.
Garmin
Reports the lowest 30-minute average heart rate recorded in the last 24 hours, a narrower, minimum-seeking window rather than a full-day average.
Where the evidence actually differs
These are genuinely different calculations, not just different labels: Fitbit averages across many low-movement moments through the day, Garmin specifically hunts for the single lowest 30-minute window in a rolling 24 hours. The same person could see meaningfully different resting heart rate numbers on the two devices simultaneously.
Which VO2 Max Estimate Has Stronger Independent Validation?
Fitbit
Cardio Fitness Score tracks the running pace-to-heart-rate relationship. A peer-reviewed validation of the Fitbit Charge 2 found it gave a consistent, unbiased read on relative fitness but overestimated actual lab-measured VO2 max, and that a simple age/resting-heart-rate/body-composition equation matched it for accuracy without requiring a run.
Garmin
Keeps the scientific "VO2 Max" label directly, estimated via the licensed Firstbeat Analytics algorithm during outdoor runs and walks. Garmin's own controlled validation reports roughly a 5 percent margin of error against lab spirometry, though independent testing using wrist-based optical heart rate rather than a chest strap reports larger errors.
Where the evidence actually differs
Both estimates are algorithm-derived, not lab-measured, and both carry real accuracy caveats found independently rather than just claimed by the brand: Fitbit's tends to overestimate, Garmin's error margin depends heavily on whether a chest strap or wrist sensor supplied the underlying heart rate.
How Differently Do the Two Brands Report Blood Oxygen?
Fitbit
Deliberately doesn't report a raw SpO2 percentage. "Estimated Oxygen Variation" is a relative overnight graph showing fluctuation, with a red, yellow, or green pattern flagging possible breathing disturbances.
Garmin
Pulse Ox reports an actual percentage, available as either an on-demand spot check or continuous all-day and overnight tracking depending on device settings.
Where the evidence actually differs
This is a genuine design-philosophy split, not a minor labeling difference. Fitbit deliberately withholds an absolute number in favor of a relative pattern; Garmin gives a direct percentage with more flexible measurement timing. Neither approach has an independent accuracy study specific to that brand's implementation on this site's research.
Does Garmin Have Anything Like Fitbit's ECG Feature?
Fitbit
Sense 2 offers AFib monitoring through an on-demand ECG app, similar in concept to Apple's, paired with a passive Irregular Heart Rhythm Notification using the same optical sensor as everyday heart rate tracking.
Garmin
No widely-offered ECG feature across the current lineup.
Where the evidence actually differs
This is a clean, real gap, not a close call: Fitbit's Sense 2 offers a genuine on-demand ECG (though narrower in device availability since it isn't on every current Fitbit model), Garmin's lineup doesn't have an equivalent feature widely available at all.
What Fitbit Has That Garmin Doesn't
On-demand ECG (Sense 2)
Garmin's lineup has no widely-offered equivalent.
Multi-input Stress Management Score
Calculated once daily from over 10 inputs across three categories, heart rate/EDA responsiveness, exertion balance, and sleep patterns, a broader input set than Garmin's continuous Stress Score.
What Garmin Has That Fitbit Doesn't
Race Predictor
Projects finish times from 5K to marathon using VO2 Max, training load, and performance history. No equivalent feature was found on Fitbit's page.
Absolute SpO2 percentage
Fitbit reports relative variation only, by deliberate design choice, never an absolute blood oxygen number.
This page won't tell you which to buy. Price, app ecosystem, and design all matter to that decision and none of them are evidence questions, so none of them are covered here. What the evidence actually supports: Fitbit and Garmin measure resting heart rate over genuinely different windows, report blood oxygen through opposite design philosophies, and each has one real feature (ECG for Fitbit, Race Predictor for Garmin) the other doesn't offer. For the full picture on either device, see the complete Fitbit or Garmin profile.