How does Fitbit track heart rate?

In this brief guide, we will answer the question “How does Fitbit track heart?” and provide you with details on how Fitbit processes information for different heart rates.

How does Fitbit track heart rate?

The Fitbit uses an optical sensor to measure your heart rate. The medically used device for measuring heart rate is a pulse oximeter. It measures both the blood oxygen saturation levels and the heart rate.  

The most accurate way to find heart rate activity would be an ECG (electrocardiogram). An ECG reads electrical activity whereas an optical sensor detects pulse by shining a light through the skin to see blood flow. Fitbit suggests that the users wear their wrist bands a couple of inches above the wrist to get the most accurate results for heart rate.

The heart pumps blood to the whole body. When the heart beats, the blood capillaries expand and contract based on the changes in blood volume. The blood is red in colour which means it reflects red light and absorbs all other light. 

The optical sensor makes use of green LEDs. The green LEDs are used because the blood absorbs the highest amount of green light. The LEDs shine light hundreds of times in a minute. Photodiodes are small devices that detect light changes, and are used in conjunction with LEDs.

As the blood volume increases and decreases between consecutive heartbeats, the amount of light absorbed and reflected back also changes. The light-sensitive photodiodes detect the change in light intensity reflected onto it. 

With the help of algorithms, variation of light intensity detected is changed into digital signals and transformed into heart rate reading. The optical heart rate sensor detects a heart rate reading in the range of 30-220 BPM (Beats per minute).

What Is Resting Heart Rate? 

Another important number to know is your resting heart rate. You can get an accurate resting heart rate after you’ve been sitting quietly for 30 minutes or when you wake up in the morning. Resting heart rate is the number of times your heart beats per minute when you are still and well-rested. This is an important indicator of your fitness level and overall cardiovascular health. Athletes and fitness experts tend to have lower heart rates.

For adults, the typical resting heart-rate range is 60 to 90 beats per minute. Radical changes to this number can signify health issues and should be discussed with a doctor.

Fitbit logs your heart rate data when you are both awake and asleep to estimate your resting heat rate. To get the most out of your fitbit, wear your device to sleep. This will also give you a better reading of your resting heart rate.

Now that you are aware of heart zones, you can work at the right intensity to achieve your fitness goals. Start a workout as you normally would. As you begin moving, you’ll see a visual representation of your zones on your fitbit tracker.

If you want to work on speed? Pick up the pace and reach your peak heart rate. Looking to lose fat? Ease up to fall back into your “Fat Burn” zone. Trying to build stamina, work in the cardio zone of your heart rate. When you adjust your pace on the move, your Fitbit tracker responds instantly.

What is the heart rate zone?

Another feature that comes with the Fitbit is that it shows the percentage of heart rate zones. Based on your age the Fitbit uses the common formula of 220 minus your age to calculate the maximum heart rate. So for example a 30-year-old would have a max heart rate of 190.

Fitbit categorizes heart rates into three heart rate zones: Peak, cardio, and fat burn. These zones are based on your maximum heart rate. The picture shows these three zones. As you can see in the picture your heart rate varies between the various heart rate zones during an exercise routine.

  • Peak zone

Intensity: Vigorous ( 85 to 100 percent of your maximum heart rate)

This is the maximum heart rate your heart can achieve. This is the point of workout where you are pushing yourself at the limit. This zone is short-lived due to its strenuosity. It is useful in enhancing performance speed, for example increasing your cycling time and building your stamina.

  • Cardio zone

Intensity: Hard (70 to 84 percent of your max heart rate)

During the cardio zone, your heart rate is at 70 to 84 percent of your max. It is where the hard work is done when you’re training for a big event. You can usually sustain it for about 30 minutes. This is where you build your cardiovascular fitness and muscle strength.

  • Fat burn zone

Intensity: Moderate (50 to 69 percent of your max hr)

This is where your body builds aerobic endurance and learns to burn fat as fuel. This is where you will usually find your heart rate during a yoga class. Below this heart rate is considered your resting heart rate which is usually taken as a value between 60-90 beats per minute.

If there’s another heart rate zone that you want to track, you can create a custom zone on Fitbit. Find the “HR Zones” setting in your account profile and turn on “Custom Zone.” You can enter the upper and lower limits you want.

Create a custom heart rate zone

Instead of using these 3 heart-rate zones, you can even create a custom zone to target a specific heart rate :

  1. In the Fitbit app, tap the Today tab  and tap your profile picture.
  2. Tap Heart Rate Zones.
  3. Enter your custom zone.

What is Heart Rate Variability?

Heart rate variability or HRV is the variance in time interval between consecutive heartbeats. Much like the difference between speed and acceleration, the Heart Rate Variability (HRV) is a measure of how the heart rate changes between different activity levels. Normally a heart does not have an even ticking interval like a clock. If looked closely, there is constant variation in between heart beats in the order of milliseconds.

HRV varies from person to person. Studies have shown that people with a higher HRV have better cardiovascular fitness and are more resilient to stress. Mindfulness, meditation, sleep, and physical activity can help improve your HRV. A significant drop in HRV may be an indicator of your body experiencing illness, stress, or depression or anxiety.

How do I view my Heart Rate Variability in the Fitbit app?

Its advisable to wear your device for a full day, including to sleep at night to get a better understanding of your HRV. To check your stats

  1. open the Fitbit app and tap the Today tab  > Health Metrics.
  2. See a graph of your nightly average heart-rate variability in milliseconds.
  3. For details, tap Learn More above the graph.


In this brief guide, we answered the question “How does Fitbit track heart?” and provided you with details on how Fitbit processes information for different heart rates.


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