Documentation checked: 21 October 2026. Both companies change their apps, plans and models often.
How WHOOP and Oura measure HRV
Both report RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD , a standard measure of how much the gap between one heartbeat and the next changes (Task Force, 1996), and both take it from sleep. They use different parts of the night.
WHOOP
WHOOP shows HRV in milliseconds with each Recovery, and its developer documentation names the field for RMSSD (WHOOP developer documentation). In the words the Dial study quotes, the value is a “dynamic average during sleep” weighted towards your last slow-wave (deep) sleep stage each night. WHOOP does not publish the exact weights. Its accuracy page says the optical ( PPG PPG An optical method that shines light into the skin and detects the pulse of blood with each heartbeat. Most watches, rings and phone-camera apps use it to estimate HRV. More about PPG ) sensor measures HRV, and that the WHOOP 5.0 and MG also use ECG ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG technology for some sleep measurements, HRV among them (WHOOP Support, accuracy).
Oura
Oura measures HRV only while you sleep. It takes 5-minute samples through the night and shows their mean as your average HRV, next to the highest single sample as your maximum HRV (Oura Member Care). Oura’s help page does not name the formula; the Dial study authors report that all the devices they tested, Oura included, show RMSSD.
Why your WHOOP and Oura numbers differ
HRV changes from one sleep stage to the next. In a sleep-lab study of 15 healthy young men, the median RMSSD was 67.3 ms in slow-wave sleep and 79.7 ms in REM sleep (Herzig and colleagues, 2018), so a value weighted towards deep sleep can sit below a whole-night average. Each company also cleans the data its own way, and no study we found shows a fixed direction for everyone. Expect a steady gap between the two apps, in either direction.
The model below draws one simulated night and shades the part each device reads. Why Oura and WHOOP show different HRV runs the same model with Garmin, Polar and Apple Watch alongside.
Same night, different numbers
A teaching model, not a measurement
Sleep stages lights out to waking
HRV every 5 minutes (RMSSD) 20 to 60 ms
- WHOOP
- 54.6 ms RMSSD
A sleep average weighted towards the last period of slow-wave (deep) sleep. The model uses that period alone.
Same night, same heart: WHOOP 54.6 ms and Oura 47.5 ms, because each device averages different hours.
The buttons work when JavaScript is on. The window for each device is listed in the description.
Two charts over eight hours of simulated sleep. The top chart is a hypnogram stepping between awake, REM, light and deep sleep, with deep sleep mostly in the first half and REM mostly in the second. The bottom chart is a line of HRV values, one per five minutes, that rise gently through the night and dip during wakeful minutes. One button per device shades the part of the night that device averages. WHOOP: 54.6 ms RMSSD. Oura: 47.5 ms RMSSD. Same night, same heart: WHOOP 54.6 ms and Oura 47.5 ms, because each device averages different hours.
Which is more accurate for HRV?
Oura’s rings agreed slightly more closely with a chest-strap ECG than WHOOP 4.0 in a 2025 home study. An older sleep-lab study, set up differently for the two devices, favoured WHOOP.
The 2025 home study
Dial and colleagues had 13 healthy adults (mean age 33) wear several devices at home with a Polar H10 chest-strap ECG as the reference, for 536 nights in total. For overnight RMSSD:
| Device | Concordance (CCC) | Average error (MAPE) | Average error in ms |
|---|---|---|---|
| Oura Ring Gen 4 | 0.99 | 5.96% | 3.93 |
| Oura Ring Gen 3 | 0.97 | 7.15% | 3.91 |
| WHOOP 4.0 | 0.94 | 8.17% | 4.17 |
Overnight RMSSD agreement with ECG (Dial and colleagues, 2025). A concordance of 1 means perfect agreement.
The authors rated the Oura rings highest, WHOOP’s accuracy as moderate and its agreement as acceptable. The difference in percentage error, and in error in ms, between the two Oura rings and WHOOP was not statistically significant. In a second analysis that compared each night with the person’s own usual values, WHOOP’s error was significantly higher than both rings’. The authors suggest that devices which did well may be suitable for following a trend.
The researchers could not copy WHOOP’s deep-sleep weighting, so they compared WHOOP’s value with the ECG’s all-night average, a window WHOOP does not use.
The same study tested two watches. Beside them, the WHOOP and both Oura rings sit at the closer end:
Overnight RMSSD against a Polar H10 chest strap
Dial and colleagues, 2025
13 adults, 536 nights at home; differences between Oura and WHOOP in percentage error were not statistically significant.
Source: Dial and colleagues, Physiological Reports, 2025. Every study we have checked, with its numbers, is in the device validation tracker.
A dot chart in two panels, one row per device tested by Dial and colleagues (2025) against a Polar H10 chest strap. The first panel shows concordance (CCC), where 1 is perfect agreement; the second shows the mean absolute percentage error (MAPE), where lower is closer. Oura Ring Gen 4: concordance 0.99, average error 5.96%. Oura Ring Gen 3: concordance 0.97, average error 7.15%. WHOOP 4.0: concordance 0.94, average error 8.17%. Garmin Fenix 6: concordance 0.87, average error 10.52%. Polar Grit X Pro: concordance 0.82, average error 16.32%. 13 adults, 536 nights at home; differences between Oura and WHOOP in percentage error were not statistically significant.
An earlier sleep-lab study
Miller and colleagues tested older models in 53 healthy young adults over one night in a sleep laboratory. WHOOP 3.0’s HRV agreed almost perfectly with the ECG (intraclass correlation 0.99, 4.5 ms low on average), while the Oura Gen 2 app value agreed less closely (0.63, 10.2 ms low on average) (Miller and colleagues, 2022).
That comparison was not like for like. WHOOP gave the researchers its beat-to-beat intervals for the whole night, and the researchers worked out HRV themselves; Oura’s number came from its app, and the study notes that the window the app used was not specified. The authors state that their research group receives funding and equipment from WHOOP, and that WHOOP was not involved in the design, conduct or reporting of the study.
Limits of the evidence
Both studies were small and in healthy people: 13 adults at home, or 53 young adults for one night. Neither tested the WHOOP 5.0, WHOOP MG or Oura Ring 5. Both companies use private algorithms that can change with an update; can a software update change my HRV covers what that does to a trend. The device validation tracker lists the peer-reviewed studies we have checked, with their numbers.
Recovery vs Readiness: how each score uses HRV
WHOOP Recovery is calculated when you wake, as a percentage from 0 to 100%. WHOOP’s developer documentation lists the inputs as resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate and HRV, plus blood oxygen and skin temperature for WHOOP 4.0 members.
Oura Readiness sums several contributors, including resting heart rate, body temperature, sleep, recent activity and HRV Balance (Oura, Readiness Score Readiness score A single number, often out of 100, that a device builds from your HRV, resting heart rate, sleep and other inputs to suggest how ready you are for the day. The weighting is not published. More about Readiness score ). HRV Balance compares a rolling two-week average of your nightly HRV, with more weight on recent days, against your baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline over the past three months (Oura, HRV Balance). What is Oura HRV Balance works through an example.
What is in a readiness score?
From the makers' own documentation
- HRV : named by the maker Heart rate variability; the API field for it is named hrv_rmssd_milli.
- Resting heart rate : named by the maker Resting heart rate.
- Breathing rate : not named Not named by the maker.
- Sleep : not named Not named by the maker.
- Temperature : named by the maker Skin temperature, for WHOOP 4.0 and later.
- Activity or load : not named Not named by the maker.
Also: Blood oxygen (SpO2) on WHOOP 4.0 and later. Sleep and breathing rate are not named in the developer documentation.
WHOOP's Recovery names 3 of the six inputs: HRV, resting heart rate and temperature. WHOOP lists the measurements that "factored into the score" but not how they are combined.
- HRV : named by the maker HRV Balance: "How has my HRV changed over the past few weeks?" (a two-week average against three months).
- Resting heart rate : named by the maker Resting Heart Rate: "Was my RHR substantially higher or lower than usual?"
- Breathing rate : not named Not named by the maker.
- Sleep : named by the maker Sleep, Sleep Balance, Sleep Regularity and Recovery Index (hours of sleep after the heart rate reached its low).
- Temperature : named by the maker Body Temperature: "Is my body temperature higher or substantially lower than usual?"
- Activity or load : named by the maker Previous Day Activity and Activity Balance (recent load).
Oura's Readiness Score names 5 of the six inputs: HRV, resting heart rate, sleep, temperature and activity or load. "The sum of these daily Readiness Contributors make up your Readiness Score." No weights are published.
A list of scores, one per device, with six inputs for each: HRV, resting heart rate, breathing rate, sleep, temperature and activity or load. WHOOP's Recovery names 3 of the six inputs: HRV, resting heart rate and temperature. WHOOP lists the measurements that "factored into the score" but not how they are combined. Oura's Readiness Score names 5 of the six inputs: HRV, resting heart rate, sleep, temperature and activity or load. "The sum of these daily Readiness Contributors make up your Readiness Score." No weights are published.
Neither company publishes how it weighs the inputs, a gap the Dial study authors also point out. A Recovery of 60% and a Readiness of 60 are not the same thing, and either score can fall on a night when HRV rose, so read the HRV behind the score. Recovery scores explained lists each score’s inputs, and which recovery score should I trust compares them.
Practical differences for tracking HRV
Fit and placement
WHOOP asks for a snug fit about one finger-width above the wrist bone; if you can see the green sensor lights, the band is too loose. For lifting or boxing it suggests WHOOP Body apparel, which holds the sensor on the upper arm, torso or calf. Cold, tattoos, skin tone and body hair can affect the optical signal (WHOOP Support, accuracy).
Oura recommends the index finger, with the middle or ring finger also working well, and a ring that feels snug, not tight; its sensors work best in close contact with the underside of the finger (Oura Member Care, ring size). The Ring 5 can be worn on the same three fingers (Oura, Ring 5).
Keep the same placement every night. A ring moved to another finger, or a band moved from wrist to arm, starts a new series.
Missed nights and charging
Both take the nightly HRV only from sleep, so a night without the device has no HRV value. WHOOP marks a Recovery as unscorable when there was not enough data for the night (WHOOP developer documentation). One missing night barely moves Oura’s two-week HRV Balance, but it leaves a gap in both trends.
The Oura ring comes off your finger to charge, so charge it at a time when you are awake. WHOOP’s battery packs slide onto the sensor and charge it while you wear it; WHOOP’s wired Basic Charger is not made to be worn (WHOOP Support, charging).
Daytime readings
Neither puts a daytime value in your nightly HRV, but both offer a daytime stress reading built partly on HRV. Oura’s Daytime Stress compares heart rate, HRV, motion and body temperature with your own baseline while you are awake and fairly still; it needs a Gen3 or later ring and an active membership (Oura Member Care, Daytime Stress). WHOOP’s Stress Monitor turns heart rate and HRV, compared with your HRV baseline and adjusted for movement, into a Stress Score on the Peak and Life plans, not WHOOP One (WHOOP Support, Stress Monitor). Keep these readings apart from the overnight number; overnight vs morning HRV explains why.
Getting your data out
- WHOOP: an in-app export sends CSV files, including one with Recovery, resting heart rate and HRV for each day (WHOOP Support, export). The developer platform gives apps you connect each Recovery with its HRV value (WHOOP developer documentation).
- Oura: all your data can be exported as CSV from the Membership Hub, with or without a membership. The Oura API needs an active membership (Oura Member Care, membership). Its sleep data includes the night’s average HRV and the series of HRV samples behind it (Oura API).
Membership
WHOOP sells its device only as part of a membership. Each plan includes a device and a charger (the WHOOP 5.0 on One and Peak, the WHOOP MG on Life) and Recovery (WHOOP, membership).
Oura sells the ring separately. Without an active membership, its support page says, you see only the three daily scores (Readiness, Activity and Sleep), the ring’s battery, basic profile information, settings and Explore content, a list that does not include the nightly HRV number. Gen2 rings need no membership (Oura Member Care, membership). Prices change, so we do not list them.
Can you wear WHOOP and Oura at the same time?
Yes. The Dial study had people wear a ring and wrist devices on the same nights. You will see two different numbers:
- Pick one as your main trend and base decisions on it. Do not average the two; averaging two different measurements does not make a better one.
- Compare direction, not size. Both should rise and fall on roughly the same days, with a steady offset between them.
If you switch from one to the other, start a new baseline rather than joining the new device’s numbers to the old trend. Oura’s HRV Balance needs about a week of data before it appears and builds its baseline over three months; how long a baseline takes lists the waiting times, and the lesson on building your HRV baseline shows how to read the first weeks.
Which is better for tracking HRV?
Neither is better for everyone. For HRV tracking, the trade-offs are:
- Comfort. A ring is less in the way at night; a band can move to the upper arm if you dislike rings or lift weights.
- Detail. With an active membership, Oura’s API gives the HRV samples behind each nightly average; WHOOP’s export and Recovery data give one HRV value per day.
- Agreement with an ECG. The 2025 study favours Oura’s rings slightly (typical error 3.9 against 4.2 ms), on models that came before the Oura Ring 5 and WHOOP 5.0. New hardware may change that.
- Membership. WHOOP comes only with one; without one, Oura shows only the three daily scores.
You do not need either for HRV. A chest strap, the reference in the Dial study, reads the heart’s electrical signal: the chest-strap guide explains how, and the chest-strap test takes a reading in your browser. A phone camera can measure HRV at rest too (can a phone camera measure HRV, camera test). Keep a morning reading in its own series, apart from any overnight average.
For each device on its own, see WHOOP HRV and Oura HRV; HRV by device covers the other brands, and normal HRV explains what published values can and cannot tell you.
When to talk to a doctor
Neither device diagnoses anything, and neither score is a medical test. Talk to a doctor if low HRV or low scores come with feeling unwell, lasting fatigue, palpitations, dizziness or chest pain, whatever the number.