Documentation checked: 21 October 2026. Garmin and WHOOP change their apps, plans and models often.
How Garmin and WHOOP measure HRV
Both read your pulse with light, through an 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, and both take HRV from your sleep. They summarise different parts of the night.
Garmin
Compatible Garmin watches calculate HRV through the night. In the morning you see the average for your whole sleep period, a chart of how it changed in 5-minute windows, and the highest 5-minute value of the night (Garmin, HRV Status). Garmin’s HRV Status page describes 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), but it does not state outright that the watch shows RMSSD. The Dial study authors report that the Fenix 6, like every device they tested, reports RMSSD. Garmin’s manual says to wear the watch while sleeping for the best results.
WHOOP
WHOOP shows HRV in milliseconds with each Recovery, and its developer documentation names the field for RMSSD (WHOOP developer documentation). The Dial study quotes WHOOP describing it as a “dynamic average during sleep” weighted towards your last slow-wave (deep) sleep stage. WHOOP does not publish the weights. Its accuracy page says 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).
Why your Garmin and WHOOP numbers differ
HRV changes with sleep stage. In a sleep-lab study of 15 healthy young men, median RMSSD was 67.3 ms in slow-wave sleep and 79.7 ms in REM sleep (Herzig and colleagues, 2018). A value weighted towards deep sleep can sit apart from a whole-night average, and each company filters the signal its own way. Garmin warns that differences in the timing and length of a measurement make device-to-device comparisons hard (Garmin, HRV Status). Expect a steady gap between the two apps, in either direction.
The model below draws one simulated night and shades the hours each device reads.
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
- Garmin
- 47.1 ms RMSSD
The average of 5-minute windows over the whole sleep period the watch detects, brief wakes included.
Same night, same heart: Garmin 47.1 ms and WHOOP 54.6 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. Garmin: 47.1 ms RMSSD. WHOOP: 54.6 ms RMSSD. Same night, same heart: Garmin 47.1 ms and WHOOP 54.6 ms, because each device averages different hours.
How Garmin HRV Status works
WHOOP feeds each night’s HRV into a daily score. Garmin first sets it against your own history. After seven nights you see a 7-day average; after about three weeks of nights the watch has a 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 range and shows a status (Garmin, blog). Garmin says three weeks is the minimum, and that a few months of data can firm the range up (Garmin, HRV Status).
- Balanced: your 7-day average is inside your baseline range.
- Unbalanced: it is above or below the range.
- Low: it is well below the range.
- Poor: your baseline has dropped below Garmin’s age-based standard, and the watch stops showing the range.
The simulation follows those rules: a 7-night average from night 7 and a baseline band from night 21. Press “New device” or “Firmware shift”: a change in the measurement alone can turn the status orange, or even red, for a week or two.
How a baseline builds
A teaching model, not a measurement
Overnight HRV (RMSSD) 30 to 60 ms
Nothing happens. The 7-night average appears on night 7, the baseline band on night 21, and the status reads balanced on every one of the 36 nights that have one.
The buttons work when JavaScript is on.
A chart of 56 simulated nights of overnight HRV in milliseconds. A thin grey line joins the nightly values, which bounce up and down. A thicker line shows the 7-night average from night 7 on; it is much smoother. A shaded band appears from night 21 and marks the baseline range. Under the chart a strip colours each night by its status: green for balanced, orange for unbalanced, red for low and pale grey for no status, with a legend of the four colours. Three buttons change the scene from night 22: a new device makes the nights read lower and noisier, a firmware shift lowers them by a fixed amount, and a reset wipes the band. A sentence under the chart says how many nights are flagged and when the status settles.
A Balanced week does not mean you are rested, and an Unbalanced one is not a diagnosis. What unbalanced HRV status means goes through each status and its usual causes, and should I reset my watch explains why a reset rarely helps.
Is Garmin or WHOOP more accurate for HRV?
In the one study that tested both overnight against the same reference, WHOOP 4.0 agreed more closely with a chest-strap ECG than the Garmin Fenix 6.
The 2025 home study
Dial and colleagues had 13 healthy adults wear several devices at home, with a Polar H10 chest-strap ECG as the reference, for 536 nights in total. For overnight RMSSD:
| Garmin Fenix 6 | WHOOP 4.0 | |
|---|---|---|
| Nights compared | 150 | 289 |
| Concordance (CCC) | 0.87 | 0.94 |
| Average error (MAPE) | 10.52% | 8.17% |
| Average error in ms | 5.29 | 4.17 |
| Authors’ rating | Poor | Moderate |
Overnight RMSSD against a chest-strap ECG (Dial and colleagues, 2025). A concordance of 1 means perfect agreement.
The authors report that the Fenix 6’s percentage error was significantly higher than WHOOP’s; the gap in milliseconds was not statistically significant. In a second analysis that compared each night with the person’s own usual values, WHOOP’s error was again significantly lower. The researchers could not copy WHOOP’s deep-sleep weighting, so they compared WHOOP’s value with the ECG’s all-night average.
Overnight RMSSD against a Polar H10 chest strap
Dial and colleagues, 2025
13 adults, 536 nights at home. The authors report the Garmin Fenix 6’s percentage error as significantly higher than WHOOP 4.0’s; their average error in milliseconds did not differ significantly.
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. The authors report the Garmin Fenix 6’s percentage error as significantly higher than WHOOP 4.0’s; their average error in milliseconds did not differ significantly.
Other studies of Garmin and WHOOP
These studies tested other models in other settings, so their numbers do not rank the two brands.
| Study | Device | Setting | Result |
|---|---|---|---|
| Miller and colleagues, 2022 | Garmin Forerunner 245 Music | 53 young adults in a sleep lab; a 3-minute reading 30 to 60 minutes before lights out, awake | ICC 0.24; 22.4 ms low on average |
| Miller and colleagues, 2022 | WHOOP 3.0 | Same night, whole sleep period; WHOOP supplied beat data | ICC 0.99; 4.5 ms low on average |
| Theurl and colleagues, 2023 | Garmin vivoactive 4 | 263 people, most after a heart attack or stroke, 30 minutes lying down; researchers computed HRV from the beat intervals the watch produced | RMSSD concordance 0.66, rated moderate |
| Bellenger and colleagues, 2021 | WHOOP 2.0 | 6 adults, last slow-wave sleep, in a sleep lab | 1.66% bias in log RMSSD lnRMSSD The natural logarithm of RMSSD, with no unit. Taking the log evens out the spread of RMSSD values, which makes day-to-day changes easier to compare and analyse. More about lnRMSSD with the best filter |
ICC is intraclass correlation, where 1 is perfect agreement.
The 2022 study read Garmin’s number while people were awake, and the authors suggest that Garmin devices that sample during sleep may agree better. Its authors also state that their group receives funding and equipment from WHOOP, and that WHOOP was not involved in the study.
Limits of the evidence
Each study was small, or limited to one night, one setting or one group of patients. None tested a current Garmin watch’s overnight HRV, the CIRQA band, the WHOOP 5.0 or the WHOOP MG. Both companies use private algorithms that can change with an update; see can a software update change my HRV. The device validation tracker lists every study we have checked.
HRV Status and Training Readiness vs WHOOP Recovery
HRV Status uses HRV alone. Because it follows a 7-day average, one bad night rarely moves it.
Training Readiness (1 to 100) combines HRV Status with last night’s sleep score, recovery time, acute load, sleep history and stress history (Garmin owner’s manual). Body Battery (5 to 100) draws on heart rate variability, stress, sleep quality and activity.
WHOOP Recovery is calculated when you wake, from 0 to 100%. Its inputs are 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 (WHOOP developer documentation).
What is in a readiness score?
From the makers' own documentation
- HRV : named by the maker HRV status (the seven-day average against your baseline range), and the stress history of the last 3 days, which itself comes from HRV.
- Resting heart rate : not named Not named by the maker.
- Breathing rate : not named Not named by the maker.
- Sleep : named by the maker Last night’s sleep score and the sleep history of the last 3 nights.
- Temperature : not named Not named by the maker.
- Activity or load : named by the maker Acute load (recent training) and recovery time.
Garmin's Training Readiness names 3 of the six inputs: HRV, sleep and activity or load. The manual lists the six factors. It does not say how they are weighted.
- 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.
A list of scores, one per device, with six inputs for each: HRV, resting heart rate, breathing rate, sleep, temperature and activity or load. Garmin's Training Readiness names 3 of the six inputs: HRV, sleep and activity or load. The manual lists the six factors. It does not say how they are weighted. 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.
Neither company publishes its weights. A Training Readiness of 60 and a Recovery of 60% are not the same thing, and either can fall on a night your HRV rose, so read the HRV behind the score. Recovery scores explained lists each score’s inputs.
Garmin CIRQA vs WHOOP
Garmin announced the CIRQA Smart Band on 21 July 2026 as its first screen-free smart band (Garmin Newsroom). It is a small module on a fabric strap, and the data appears in the Garmin Connect app.
- HRV: Garmin lists HRV in CIRQA’s sleep tracking, along with HRV Status and Training Readiness (Garmin Newsroom). The manual’s HRV Status card shows the 7-day average of your overnight HRV (CIRQA owner’s manual, home settings).
- Placement: on the wrist, above the wrist bone, or on the upper arm above the elbow, with the module on the outside of the arm and not where you lie on it. You tell the app which, because the band uses the wear location to work out its health metrics (CIRQA owner’s manual).
- Battery: up to 10 days; charging by cable takes up to 90 minutes. Battery Saver turns off heart rate, so leave it off at night if you want HRV (CIRQA owner’s manual).
- Subscription: none required for CIRQA’s features, Garmin says (Garmin Newsroom).
In format CIRQA is close to WHOOP: a band for the wrist or the upper arm, read in a phone app. The model behind it differs. WHOOP sells the band only with a membership and gives a nightly HRV inside Recovery; CIRQA is bought outright and uses Garmin’s HRV Status.
We found no published study of CIRQA’s HRV, and no Garmin statement on whether it processes HRV the same way as Garmin’s watches. The Fenix 6 results above do not carry over to it.
Garmin CIRQA vs WHOOP covers the band in more detail, with its cost over three years.
Practical differences for tracking HRV
Placement and screens
WHOOP asks for a snug fit one finger-width above the wrist bone; if you can see the green sensor lights, it is too loose. For lifting or boxing it suggests WHOOP Body apparel on the bicep, torso or calf (WHOOP Support, accuracy). A Garmin watch shows HRV Status on its screen and in Garmin Connect; with CIRQA you read it in the app. Keep the same placement every night, so that a move from wrist to arm does not show up as a change in your HRV.
Charging and missed nights
Both take the nightly HRV only from sleep, so a night without the device leaves a gap, and Garmin needs consistent sleep data to build and keep a baseline. WHOOP quotes 14 or more days of battery for the WHOOP 5.0 and MG (WHOOP, membership), and its battery packs slide onto the sensor to charge it while you wear it; the wired Basic Charger, which comes with the One plan, is not made to be worn (WHOOP Support, charging; WHOOP, membership). CIRQA charges through a port on the back of its module, so it comes off your arm (CIRQA owner’s manual, charging); charge it while you are awake.
Subscription
Garmin Connect is free. The Garmin Connect+ subscription, as Garmin’s CIRQA manual lists it, adds AI insights, LiveTrack extras, live workout data, nutrition logging, training guidance, a performance dashboard and social features; HRV Status is not on the list (CIRQA owner’s manual, Connect+). WHOOP sells its device only as part of a membership, and every plan includes a device, a charger and Recovery (WHOOP, membership). Prices change, so we do not list them.
Getting your data out
- Garmin: Garmin Connect can export a day of wellness data, including HRV, as FIT files, and a full account export by email (Garmin Support). The RR converter turns a Garmin activity file that logged beat intervals (with HRV logging on, or a chest strap worn) into a plain RR file.
- WHOOP: an in-app export sends CSV files, including one with Recovery, resting heart rate and HRV for each day (WHOOP Support, export).
Daytime readings
Both also use HRV during the day. Garmin’s stress level (0 to 100) comes from heart rate variability while you are inactive (Garmin owner’s manual); WHOOP’s Stress Monitor, on the Peak and Life plans, compares heart rate and HRV with your baseline (WHOOP Support, Stress Monitor). Keep these apart from the overnight value; is Garmin stress the same as HRV explains the difference.
Can you wear Garmin and WHOOP at the same time?
Yes. Put them on different wrists, or the band on your upper arm; the Dial study had people wear several devices at once through the night. You will see two numbers:
- Pick one as your main trend. Do not average them.
- Compare direction, not size. Both should rise and fall on roughly the same days.
- With two Garmin devices, such as a watch and CIRQA, choose a primary wearable in Garmin Connect; it sets which device feeds daily health metrics like sleep (CIRQA owner’s manual, unified training status).
If you switch from one to the other, start a new baseline. Garmin needs about three weeks before HRV Status returns; how long a baseline takes lists the waiting times.
Which is better for tracking HRV?
Neither is better for everyone. For HRV alone, the trade-offs are:
- Agreement with an ECG. The 2025 study favours WHOOP 4.0 over the Fenix 6, on older models. Nothing yet covers CIRQA or current Garmin watches.
- How the number is read. HRV Status gives a slow, baseline-relative signal built only from HRV. WHOOP gives a nightly value and a daily Recovery that mixes in other inputs.
- Format. A Garmin watch has a screen; CIRQA and WHOOP are bands you can move to the arm.
- Subscription. HRV Status needs none; WHOOP comes only with a membership.
A chest strap, the reference in the Dial study, reads the heart’s electrical signal; the chest-strap guide explains how. For each device on its own, see Garmin HRV Status and WHOOP HRV; WHOOP vs Oura compares WHOOP with a ring, and normal HRV explains what published values can and cannot tell you.
When to talk to a doctor
Neither device diagnoses anything, and neither HRV Status nor Recovery is a medical test. Talk to a doctor if low HRV or a Low status comes with feeling unwell, lasting fatigue, palpitations, dizziness or chest pain, whatever the number.