HRV accuracy: Oura vs Garmin vs Polar vs Fitbit vs Withings

Heart Rate Variability is the headline number on every modern wearable, but the value your watch shows is only as useful as the sensor and algorithm behind it. A figure from a Polar chest strap and a figure from a Fitbit on the same wrist on the same night can disagree by 20% or more. This guide cuts through the marketing and looks at what independent research has actually found.

The accuracy hierarchy in one paragraph

Chest straps are the gold standard for everyone. Among wrist devices, accuracy depends more on when the device measures (overnight beats spot checks) than on the brand. Optical sensors lose accuracy during motion, and every vendor smooths and filters the raw data before showing it to you. The trend within a single device matters far more than comparing absolute numbers across brands.

How to read this: rows aren't directly comparable without the reporting-style context. Garmin and Fitbit produce optical signals of similar accuracy, but Garmin averages seven nights while Fitbit shows a rawer single-night RMSSD. So Garmin trends look calmer; Fitbit trends look more reactive, even though both measure essentially the same thing. A 10% gap sounds small, but on a metric where 5 ms separates "recovery OK" from "overreached," it actually shifts decisions.

What one 2025 study measured on the same nights

Almost every accuracy figure published so far comes from a separate study with its own lab, protocol and volunteers, which is why the ranges above overlap so freely. Dial and colleagues (Physiological Reports, 2025) put five devices on the same people on the same nights against an ECG reference, so for once the ordering is directly comparable:

Polar: the consumer benchmark

Polar makes both the most accurate and the most variable HRV products in the consumer market, depending on which one you wear. The Polar H10 chest strap records full ECG-grade RR intervals and has been validated to within 1–2% of medical ECG in over a dozen studies (Gilgen-Ammann 2019, Schaffarczyk 2022). It is the device sport scientists actually use in research.

Polar's wrist-based watches (Vantage V3, Grit X2, Ignite) use optical sensors and report HRV from a 4-minute morning measurement (Nightly Recharge). These are accurate to within 5–10% of ECG when held still, which is fine overnight. Polar publishes its algorithm openly, which is rare in the industry.

Oura: the one that measures at the finger

Oura reads HRV optically at the finger through the whole night and reports overnight RMSSD. The validation record is unusually strong for a consumer device: Kinnunen 2020 (Physiological Measurement) found r² = 0.98 against medical-grade ECG for nightly average HRV with a mean bias of −1.2 ms, and Cao 2022 (Journal of Medical Internet Research) reproduced the correlation (r = 0.96 per night) while measuring a systematic under-read of roughly 15 ms. One caveat on the first of those: its authors work at Oura Health, which is why the same-night ranking above carries more weight for ordering the brands.

That combination matters more than it sounds. A tight correlation with a stable offset is exactly what a trend device should deliver: the absolute value is not the ECG value, but the shape of the curve is. So do not hold an Oura RMSSD next to a chest-strap RMSSD. Hold it next to your own Oura reading from last week.

Garmin: broad coverage, smoothed numbers

Garmin measures HRV optically at the wrist throughout the night via the Elevate sensor. The reported value is a 7-night moving average, which makes day-to-day comparison less useful but trend tracking very stable. Studies on Garmin Fenix and Forerunner watches show overnight RMSSD within 8–15% of chest-strap reference (Hernando 2018, Pasadyn 2019).

Garmin's "HRV Status" classification (Balanced, Low, Unbalanced) is calibrated against your personal 21-day baseline, so any number labeled "Balanced" reflects your trend, not population norms. That's actually a sensible design choice for a metric this personal. More on HRV interpretation →

Fitbit: recently improved, still optical

Fitbit reports HRV nightly on the Sense, Sense 2, Charge 6, and Pixel Watch (which uses Fitbit firmware). The number you see is RMSSD, calculated from optical sensor recordings during sleep. Independent comparisons against polysomnography (Henriksen 2018, Stahl 2016) show acceptable accuracy for the trend but consistent under-reporting versus chest straps by 10–20%.

The signal is most reliable in deep and stable sleep stages. If you toss often, Fitbit's HRV will be noisier. The "Heart Rate Variability Insights" tile in the app shows your weekly average against your typical range, that comparison is what matters, not the absolute ms value.

Withings: clinical-grade ECG on demand

Withings is the odd one out. The ScanWatch and ScanWatch 2 include FDA-cleared single-lead ECG, which the wearer triggers manually. During those 30-second readings, HRV is essentially clinical-grade. Continuous overnight HRV from the optical sensor, however, is closer to Fitbit's ballpark, accurate enough for trends, not for absolute comparison with research data.

For a user who values medical-grade single readings (e.g. atrial fibrillation screening), Withings is unique. For continuous HRV trend tracking, it sits in the same tier as Garmin and Fitbit.

Why your Garmin and your Polar disagree

Three reasons account for almost all cross-vendor differences:

• Time window. Polar may show a single-night RMSSD; Garmin shows a 7-night average. Even on identical recordings, those numbers will differ.
• Filter strength. Vendors discard "abnormal" RR intervals at different thresholds. Stricter filtering produces a higher RMSSD; looser filtering captures more real variability.
• Sensor placement. Wrist optical readings degrade with skin contact, hair, tattoos, and motion. Chest straps see the actual ECG.

Cross-device differences of 10–20% are normal and expected. Trying to compare your number against a friend's on a different brand is not a useful exercise.

What this means for your training

Use one device consistently. Track your own baseline for 14–28 days, then watch for departures from your norm, not from population norms or another brand's display. Recovery decisions should follow trend signals, not single readings. A 10% drop sustained over a week means more than any one-off spike.

If you switch wearables, expect a 2–4 week recalibration period before your new baseline stabilises. RMSSD specifically tends to read lower on optical wrist devices than on chest straps, so don't panic at the apparent drop.

Where MyBodyAI fits

MyBodyAI accepts HRV from Polar, Fitbit, Withings, Oura and Garmin sources and reads every night against your own thirty-day baseline and on a fixed curve that shifts to where your values usually sit; no age table sits behind it, because the published references (Nunan 2010) pool short daytime recordings and have no night-time strata. That means you can switch wearables, or use two devices in parallel, and still see a continuous trend in the 12 MyBodyAI health indices, instead of three disconnected dashboards.

If you want a quick read on your current value, the HRV Decoder tool takes any RMSSD number and contextualises it for your age group. For the underlying science, see the What is HRV primer.

Bottom line

• Most accurate consumer device: Polar H10 chest strap.
• Most accurate device you can sleep in: Oura Ring 4: around 6% mean error against ECG in the 2025 same-night comparison.
• Best wrist device for athletes: Polar Vantage / Garmin Fenix: published algorithms, validated overnight readings.
• Best for everyday users: Fitbit Sense or Pixel Watch: no manual measurement, decent trend accuracy.
• Best for clinical-grade single readings: Withings ScanWatch (FDA-cleared ECG).
• What matters most: consistency. Pick one, track it for a month, watch the trend.