How does Fitbit measure heart rate?


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Page last updated: 28/09/2022

How does Fitbit measure heart rate?


In this guide, we will answer the question “How does Fitbit measure heart rate”, some additional considerations on how the Fitbit may potentially calculate or estimate heart rate and why it could be important.

How does Fitbit measure heart rate?

If you wonder ‘How does Fitbit measure heart rate?’ we have the answer for you. Fitbit has an optical heart rate monitor, also known as photoplethysmography. Unlike EEG or Electrocardiogram, which reads electrical activity, an optical heart rate monitor detects your pulse by shining a light through your skin to measure blood flow.  However, some might argue that it is not as accurate as using other kinds of devices.

However, some factors may affect the measurement, such as physiology, location of the device (how you wear it) and even movement. This is why Fitbit recommends users wear its products snuggled up a couple of inches above the wrist to get the most accurate readings but let’s face it, no device is perfect or is said to measure what they measure with a 100% accuracy. 

But, How does Fitbit detect heart rate? “When your heart beats, your capillaries expand and contract based on blood volume changes. To determine your heart rate, the optical heart-rate sensor in your Fitbit device flashes its green LEDs many times per second and uses light-sensitive photodiodes to detect these volume changes in the capillaries above your wrist”. 

Subsequently, your device calculates how many times your heart beats per minute and the optical heart-rate sensor can detect a range between 30 to 220 bpm when you sleep and while you are awake is said to detect a range between 40 to 220 bpm. However, on the Fitbit Versa 2, the range varies from 30 to 250 bpm.  

How can I check my Heart rate?

Simply swipe up on your clock face to see your current heart rate measurement and either your heart-rate zone or your resting heart rate. However, to see your heart rate details tap (on Fitbit Sense and Versa 3) or swipe (all other devices).

If your device has an Exercise app, you have the option to check your heart rate in real-time and the heart rate zone when you are working out. However, not that your device doesn’t track your heart rate while you are swimming. 

Another common question regarding heart rate monitoring is what’s my resting heart rate? Well, your resting heart rate corresponds to the number of times your heart beats per minute when you are well-rested and relaxed. Typically, resting heart rate ranges between 60 to 100 bpm, but this rate can vary depending on age and fitness condition. 

Why should I track my resting heart rate?

Tracking your resting heart rate can be important because it may indicate not only your fitness level but also your overall cardiovascular health. In general, people who have an active life have a lower resting heart rate because their heart muscle is in better shape or condition than those who don’t. 

Finally, to check your current resting heart rate on your Fitbit, simply swipe from the clock face to see the data for the past 30 days and tap the Heart Rate tile on the app. Fitbit premium members can also see their resting heart rate data in the ‘Health Metrics’ option. These are the instructions:

  • In the morning, tap the Today tab  > Health Metrics.
  • See a graph of your resting heart rate.
  • For details, tap Learn More above the graph.
How does Fitbit measure heart rate?

Are there any factors that can affect my resting heart rate measurement?

According to, “Several factors can affect resting heart rate: stress, alcohol or caffeine intake, or fever usually raise resting heart rate, while regular exercise or meditation can lower it. Air temperature and certain medications can also affect resting heart rate.”

What are heart-rate zones?

Knowing this information is very important to people who would like to start their weight loss journey. But, what are heart-rate zones? Simply put, they are percentages of your maximum heart rate that can allow you to determine the intensity of your current workout or activity.

Your Fitbit will use a formula to calculate your heart-rate zones however they may vary from one device to the other. For the Fitbit Charge 4, Inspire 2, Iconic, Sense and Versa series:

“Fitbit personalizes your heart-rate zones using your heart rate reserve, which is the difference between your maximum heart rate and your resting heart rate”. The heart rate reserve is an indication of your overall cardiovascular fitness and your maximum heart rate is generally determined by your age, while your resting state can be lowered by increasing your fitness level. 

Heart Rate Variability (HRV)

Your Heart Rate Variability is the variation in time between every heartbeat. If your heart beat is 70 bpm, your heart doesn’t necessarily beat once per second so it is your autonomous system which determines the timing of each heartbeat. Moreover, HRV can vary from one person to the other and some studies have even shown that people with higher HRV have better cardiovascular fitness and might be more resilient to stress (

Moreover, if you experience a significant drop in HRV, it may indicate that your body is experiencing illness, stress or depression. But why should you track HRV? “Trends in HRV data can help you be more aware of how stress and lifestyle choices, such as nutrition and exercise, can affect your body”.

How does my Fitbit track HRV?

Fitbit uses a formula called RMSSD to estimate your HRV considering your recent heart-rate data. In the graph, your latest HRV measurement reflects the data logged from your most recent sleep session that lasted at least 3 hours.

To see your HRV, wear your device the entire day, including at night when you are sleeping and then check your status in the morning by opening the Fitbit app and tapping on the Today Tab> Health Metrics. Here you can see the graph of your nightly average heart rate variability in milliseconds. For more details, tap on ‘Learn More’ above the graph.

What factors can affect my HRV?

Several factors can affect your HRV such as your age, sex, sleep quality, stress, and your current lifestyles (habits) such as the food you eat and physical activity. If your Fitbit detects that your heart rate is outside of your high or low thresholds while you appear to be inactive for 10 minutes, it will send you a notification. 

If you receive a notification, tap Open and then tap the notification in the App to learn more about your data and complete a survey where you log any potential factors, medications or symptoms. You can use this data and share it with your healthcare provider if needed. 

Why is this blog about How does Fitbit measure heart rate important?

As we have discussed on ‘How does Fitbit measure heart rate’, measuring your heart rate can be beneficial and your Fitbit is equipped with an optical heart rate monitor, also known as photoplethysmography to measure heart rate. However, we need to consider some factors that could affect your heart rate and why it is necessary to take a deeper look into the measurements should a concern arise from them. 

Please feel free to leave any comments or thoughts about the content of this article!

Frequently Asked Questions (FAQs) about How does Fitbit measure heart rate

Are Fitbits accurate for heart rate?

A study on consumer wearable heart rate measurement showed that the Apple Watch 3 and Fitbit Charge 2 were generally highly accurate across the 24-hour condition. Specifically, the Apple Watch 3 had a mean difference of −1.80 beats per minute (bpm), a mean absolute error percent of 5.86%, and a mean agreement of 95% when compared with the ECG across 24 hours. 

How does Fitbit measure resting heart rate?

Fitbit measures resting heart rate by counting the number of times your heart beats in 30 seconds while you continue to rest and relax. Additionally, it multiplies the result by 2 to get your resting state rate. If you wear a Fitbit or any other similar device, this metric is measured automatically for you every day. 

How often does Fitbit measure heart rate?

Fitbit measures heart rate continuously in a variety of wrist-based bands thanks to PurePulse technology. This means whenever you are wearing your Fitbit, your pulse is automatically tracked 24 hours a day, seven days a week.

Why won’t my Fitbit read my heart rate?

If your Fitbit isn’t reading your heart rate it is possible that your heart rate tracking is off so go on your device, open settings> Heart Rate> tap to turn the setting On. On Fitbit Alta HR and Fitbit Charge 2, you must turn on heart rate tracking in the Fitbit app. Tap Heart Rate, and tap to turn on the setting to Auto.

Which Fitbit has the most accurate heart rate?

It is believed that the Fitbit Charge 3, of all the trackers tested, is the simplest to use and the most accurate for measuring steps and heart rate. It reliably senses and identifies workouts such as running, walking and biking in a period of 10 minutes after starting to move.