One of the most important aspects of any wearable sleep or fitness tracker is its accuracy. Without reliable data, these devices range from gimmicky to useless. So for this article, I wanted to take a closer look at WHOOP — the wrist-worn fitness tracker I’ve been using since 2019 — to find out just how accurate it really is.
In a nutshell, my research and first-hand testing shows that WHOOP is highly accurate in the following areas:
- Heart rate variability (HRV)
- Respiratory rate
- Resting heart rate
- Sleep quality and stages of sleep
However, you might experience mixed results with heart rate tracking during certain types of exercises and activities, especially when they involve lots of wrist movement.
If you’re new to WHOOP and wondering whether it’s the right fitness tracker for you, I encourage you to read my hands-on WHOOP review (or watch the video review below) for more background on the device’s features, capabilities and limitations.
WHOOP Heart Rate Sensor Accuracy
The best way to determine the accuracy of a wrist-worn heart rate sensor is to simultaneously use a three-lead electrocardiogram (ECG) and then compare the two sets of data. The second-best way is to use a chest strap, which doesn’t rely on photoplethysmography and instead measures the electrical signals emitted by your heart as it beats.
One thing that’s worth mentioning is that both the sensor and the open-source algorithm used by WHOOP to interpret the raw data and convert it into “heartbeats” are pretty standard across the industry.
Wrist-worn devices such as WHOOP rely on light-emitting diodes (LEDs) that shine light through your skin and onto your blood vessels. Interpreting the light signal as it bounces back from your skin is an analog process, and is thus subject to interference and signal noise.
A study funded by the Australian Institute of Sport (AIS) found that WHOOP was 99.7% accurate in measuring heart rate and 99% accurate in measuring heart rate variability when compared to the gold-standard electrocardiogram-derived (ECG) metrics.
While that sounds pretty good, it’s worth noting that those measurements were taken under conditions ideal for wrist-worn devices that might not reflect your circumstances.
For example, a dark tattoo can absorb some of the emitted light, thus altering how much of the light is reflected back to the sensor. Similarly, dark skin has more pigments (such as melanin) which absorb more of the laser light, potentially leading to less-reliable readings.
Factors that can negatively influence the accuracy of WHOOP and similar wrist-worn devices include:
- A loose strap that has insufficient contact between the sensor and the skin.
- Wrist movement that temporarily breaks contact between the sensor and your skin.
- Body motion that the sensor might interpret as heart contractions.
- The ambient temperature and your body temperature, both of which influence blood flow.
- Variables that can interfere with the reading of the reflected light (e.g., your skin color or tattoos).
These factors make it difficult for WHOOP to accurately detect your heart rate in less-than-ideal conditions. That’s particularly true in situations where your heart rate changes a lot, like during CrossFit or other types of high-intensity interval training (HIIT).
Factors That Can Negatively Impact WHOOP’s Heart Rate Tracking
So let’s talk about each of these to see how they influence WHOOP’s accuracy, and explore any potentially mitigating factors.
WHOOP’s sensor needs to be in close contact with your skin at all times. Otherwise, it won’t be able to accurately measure your heart rate or other biometrics.
That’s why it’s so important to use a strap that provides a secure fit. It should be tight enough that you can place your pinky finger between the strap and your skin with no excess space (as shown in the photo above).
Be sure not to over-tighten the strap, because doing so can restrict your blood flow and cause issues.
WHOOP seems to have realized that strap fit is a crucial factor in ensuring accuracy, as the company released its SuperKnit strap together with the WHOOP 4.0. SuperKnit is an incredibly comfortable and infinitely adjustable strap that makes it easy to keep the WHOOP sensor in close contact with your skin at all times.
If you do CrossFit or similar exercises that involve gloves, gymnastic grips, dumbbells or a lot of wrist movement, WHOOP Body (a special line of activewear that enables you to wear the sensor on your arm or hip) might actually be the better choice.
Skin Color and Tattoos
WHOOP’s optical heart rate sensor relies on the reflection of light that’s emitted by the sensor’s LEDs. Anything that can interfere with the frequency of the emitted light or its reflection can introduce noise and render the signal less accurate.
Examples of this include ambient light that reaches the sensor due to an improper fit or tattoos.
Note that WHOOP 4.0 includes a pulse oximeter that uses red and infrared light, both of which are less prone to interference from skin pigments or tattoos. However, WHOOP only uses the pulse oximeter for sleep tracking, because it’s prone to interference by movement.
Any time you move your wrist, you risk the sensor temporarily losing contact with your skin. If that happens, you’re likely to get some inaccurate heart rate readings. When that occurs during a time when your heart rate changes a lot (e.g., dumbbell interval training), you might see a huge gap between your actual HR and what WHOOP recorded.
The best way to mitigate issues related to wrist movement is to wear the sensor on another part of your body by using WHOOP Body.
One thing you might not have known is that WHOOP (and other optical HR sensors) also rely on the accelerometer to detect the contractions of your heart and the force of the blood flowing through your blood vessels.
As you can imagine, the accelerometer picks up a lot of “noise” during exercises, such as steps taken, jumps, and any other movement. WHOOP has to distinguish between those different input sources and filter out the ones related to your heartbeat.
While differentiating between the motion induced by continuous movements — such as jogging or walking — and your heartbeat is relatively easy, it’s much more difficult for fast-paced movements like CrossFit.
But I’ve also heard reports that even monotonous movements like biking can throw off an optical HR sensor, because the way the muscles contract during biking can be misinterpreted as a contracting heart.
Obviously, the same holds true for other wrist-worn devices, such as the ones from Fitbit, Biostrap, Polar, Garmin and other brands.
The best way to mitigate that issue is to move the sensor to your hip by wearing WHOOP’s special underwear or shorts (part of the WHOOP Body line)
Ambient and Body Temperature
Most wrist-worn sensors seem to do better in warmer temperatures. That’s because if it’s too cold, the body restricts capillary blood flow, making it more difficult for the sensor to get a good reading.
So if it’s cold outside, I recommend wearing extra layers or warming up sufficiently before you begin measuring your heart rate.
WHOOP Calorie Burn Accuracy
Accurately tracking how many calories you burn is incredibly difficult without first establishing your basal metabolic rate (BMR), which involves wearing a face mask that measures how much oxygen you inhale and how much carbon dioxide you exhale.
That’s not something you can easily do at home without the proper equipment.
Instead, what most fitness trackers do is try and predict your BMR based on height, weight, age, and biological sex. While doing so often yields satisfactory results, predicting the active burn is much more difficult.
WHOOP uses a heart-rate-based formula to estimate active burn, which utilizes a model developed in South Africa in 2005. You can read more about the challenges of predicting BMR and active calorie burn on the WHOOP blog.
The bottom line is that a wrist-worn device can’t accurately predict your absolute calorie burn, so don’t get hung up on the numbers reported in the WHOOP app.
However, WHOOP can accurately record relative changes in calories burned, which can be helpful if you want to compare rest days vs. workout days, or the impact of different types of workouts, on your calorie burn.
A few years ago, WHOOP announced a partnership with PNOE, a company that makes hardware and software to calculate basal metabolic rate (BMR). I was hoping they’d announce an integration shortly thereafter, but so far nothing has come out of this partnership.
WHOOP Strain Tracking Accuracy
The strain metric in the WHOOP app gives you insight into how much load you’ve placed on your cardiovascular system through exercise or other daily activities that might elevate your heart rate.
As you can imagine, the strain score is highly dependent on an accurate heart rate reading. As a result, certain types of physical activities cause slightly inaccurate strain scores.
What many WHOOP users don’t understand is that the strain score doesn’t impact the recovery score, because the latter is predominantly influenced by your heart rate variability, resting heart rate, and quality of sleep.
WHOOP Sleep Tracking Accuracy
Since sleep tracking relies on an accurate heart rate reading (along with other biometrics), how is it possible that WHOOP can do that so accurately?
The reason is simple: your movement pattern is vastly different while sleeping vs. while exercising. In fact, during the restorative phases of sleep, you might not move at all. That makes it incredibly easy for WHOOP to take accurate heart rate measurements.
Considering the excellent results shown in various validation studies, it’s safe to assume that the hardware sensors packed into WHOOP can accurately track your biometrics while you sleep.
One thing I’ve seen other people complain about is WHOOP’s supposedly unrealistic recommendations on when to go to bed and how long to sleep.
Based on my understanding of how the “sleep need” figure is calculated, and my experience with the “sleep coach,” I don’t think the recommendations are unrealistic at all. What most people don’t understand is that sleep debt is cumulative.
So if you’ve been sleeping an hour less than your body needs over the course of three weeks (for example), you can expect a higher sleep need for the next few days or even weeks. Sleep debt doesn’t just go away because you want it to.
If you’d like to learn more about the technology behind WHOOP’s sleep tracking capabilities, check out my list of the best sleep trackers and scroll down to the WHOOP section.
How Accurate Is the WHOOP Recovery Score?
The recovery score is so accurate and useful that I’d be wearing WHOOP even if that was the only feature it offered.
Many WHOOP users, and even other reviewers, don’t fully understand how the recovery score is calculated. What’s important to recognize is that the previous day’s strain score has zero influence on your recovery the next day.
Instead, the recovery score is a reflection of how your nervous system is doing. To find that out, WHOOP measures your HRV during the last phase of deep sleep. That’s the time when your brain is most “disconnected” from the rest of your body, preventing movement or other influencing factors.
We already know that WHOOP is incredibly accurate when the conditions are right, and deep sleep (also called slow-wave sleep) is the perfect condition to take RHR and HRV measurements.
Why I Don’t Care About Activity Tracking
Now that you know everything there is to know about WHOOP’s accuracy, let me tell you why the areas where WHOOP shows weaknesses are irrelevant to me.
I’m no longer a professional athlete. I’m a serious fitness enthusiast and I wear WHOOP to help me improve my overall health and well-being. Having insights into the quality of my sleep, including what factors influence it and the condition of my nervous system, helps me make better decisions throughout the day.
What doesn’t help me is knowing how many calories I burned, or if my strain on a given day was 13 or 14. Those are nice parameters to know, but they don’t significantly influence my behavior.
Of course, there are some exceptions to this. For example, if my strain is consistently high despite not having exercised, I might have to work on reducing my stress (which is a common cause of an elevated strain score). But beyond that, I care more about my recovery score, my HRV/RHR trends, and how much restorative sleep I get.
These are factors that WHOOP can accurately report on. And they’re the ones that I can directly influence based on my lifestyle choices.
If you’re a professional athlete and you keep tabs on strain and calories burned to plan your meals and workouts, then you might need a separate chest strap to measure your heart rate during exercise. Unfortunately, WHOOP doesn’t currently allow the use of third-party HR sensors.
Despite the fact that I don’t really care about activity tracking, I wanted to find out how WHOOP compares to a chest strap heart rate monitor in order to share that information with you.
My WHOOP vs. Chest Strap Experiment
I decided to do a little experiment and wear a Wahoo chest strap during a brisk outdoor walk and a typical CrossFit workout involving various movements that caused my HR to change rapidly.
Shoot me an email If you want to find out exactly how I set up the test. In a nutshell, I connected the chest strap to my Apple Watch, enabled real-time HR broadcast in the WHOOP app, and recorded WHOOP’s HR data using the Wahoo Fitness app. Both sources stored their data in Apple’s Health app, which is where I extracted it from.
The results were pretty much as I expected.
Brisk Outdoor Walk
WHOOP was relatively accurate in comparison to the chest strap during my brisk outdoor walk. On average, WHOOP was only off by 3.8 beats per minute.
Here’s a better representation of how close WHOOP’s HR data was to the chest strap. I should point out, however, that the Wahoo app only recorded one HR event every four seconds, whereas the chest strap/Apple Watch combo recorded an event every second.
|Within 25 BPM||100%|
|Within 10 BPM||95%|
|Within 5 BPM||74%|
|Within 3 BPM||52%|
You can see those discrepancies in the graph below — particularly in the areas where my HR rapidly changed.
As expected, the HR deltas were wider during a high-intensity workout that involved dumbbells, push-ups, thrusters and box step-overs with rest periods in between rounds.
In other words, the WHOOP sensor and algorithm had to deal with a rapidly changing heart rate, a lot of irregular motion, and wrist movement. It’s kind of a nightmare scenario for any wrist-worn fitness tracker.
|Within 25 BPM||96%|
|Within 10 BPM||81%|
|Within 5 BPM||58%|
|Within 3 BPM||40%|
As you can see in the table above, there’s a noticeable spread between what the chest strap recorded and what WHOOP thought my heart rate was.
WHOOP Accuracy FAQs
Based on my testing, WHOOP 4.0 is more accurate than WHOOP 3.0 as far as heart rate tracking during exercise and sleep tracking are concerned. That’s likely because of the additional sensors that are included in the new version, and the ability to wear the device on your body (via WHOOP Body garments). You can learn more about the difference between the two versions in my WHOOP 4.0 review.
While the device was designed to be worn 24/7, doing so is not always necessary, depending on your tracking goals. For example, sleep tracking isn’t negatively impacted if you don’t wear WHOOP during the day. Vice versa, you can track your workouts during the day without wearing WHOOP for sleep. However, to get the most complete picture of your daily activities, including strain, recovery and sleep, I recommend wearing WHOOP 24/7.
Yes! For WHOOP to capture accurate heart rate data it’s important for the sensor to remain in close contact with your skin. Certain exercises that involve wrist movements, including kickboxing and CrossFit, make it difficult for the sensor to remain in contact with your skin, thus reducing WHOOP’s accuracy. As a result, I recommend considering wearing WHOOP on your arm or hips (using the bicep strap or WHOOP Body garments) to improve accuracy.
WHOOP accurately reports certain biometrics right away, including strain and sleep. However, certain metrics, such as recovery or your daily health report, take a bit longer to become meaningful because WHOOP has to establish a baseline first. Depending on the specific biometric, WHOOP needs somewhere between a week and 30 days to establish a meaningful baseline.
Depending on the particular biometric you want to verify, I recommend a chest strap heart rate monitor or a sleep lab to identify inaccurate data. Since most of us don’t have easy access to a sleep lab, I usually gauge WHOOP’s sleep tracking accuracy by comparing it to other sleep trackers and based on how I feel in the morning.
The best thing you can do is ensure that the WHOOP sensor remains in close contact with your skin by tightening your strap or wearing the sensor on your arm or hip via WHOOP Body garments.
Final Verdict: WHOOP Is Very Accurate in the Areas That Matter
Based on my use cases and requirements, the WHOOP strap is accurate in the areas that matter most to me, including recovery and sleep tracking. In other areas, WHOOP has the same limitations as other wearables, and it might not always track your heart rate as accurately as a chest strap or an ECG.
So if you’re a professional endurance athlete who relies on super-accurate HR data for training and competitions, you will have to use a chest strap. There’s no way around that.
However, a chest strap doesn’t provide any of the other benefits of the WHOOP strap, including detailed sleep stage and recovery tracking. So you might end up with both a chest strap and a WHOOP.
I’m a hobby CrossFitter and usually don’t care about tracking my workouts on my Apple Watch or WHOOP. I just want to know how recovered I am and how I slept. Those are the factors that determine how I plan my day.
If you want to give WHOOP a try, use this link and get an extra month added to your new membership.
I’m a healthy living and technology enthusiast.
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