Fitbit heart rate monitoring explained

We discuss features, accuracy and applications of the PurePulse sensor
Fitbit heart rate monitor guide
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Tracking heart rate is a huge feature of Fitbit's smartwatches and fitness trackers – and drives the data we demand from our wearables. But how does Fitbit track heart rate – and is it accurate?

Thanks to the in-built heart rate monitor found in the Inspire HR, Fitbit Charge 4, Fitbit Versa 2 or Fitbit Inspire HR (or any of the rest of the Fitbit clan) you're able to gauge not only your heartbeat activity during workouts, but also tap into wellness features and some of the best sleep tracking in the business.

We get stuck into how it works, and why you'd want to try it out:

Why does Fitbit track your heart rate?

By tracking your heart rate, Fitbit can better establish facts about your body and offer better insights. Fitbit uses its heart rate sensor for:

1. Real-time 24/7 heart rate
2. Resting heart rate
3. Average heart rate
4. Calorie burn information
5. Heart rate via sleep for Sleep Stages
6. VO2 Max via Cardio Fitness Score
7. Breathing via Relax app

How does Fitbit measure heart rate?

1. Fitbit Heart Rate - How does it measure

Similar to how we covered the tech for the Apple Watch heart rate monitor, let's give a quick breakdown to how pretty much all wrist-based heart rate monitoring works.

Photoplethysmography essentially works upon one simple premise: blood is red because it reflects red light and absorbs green light. So by using green LEDs and pairing them with photodiodes, Fitbit uses its own in-house PurePulse technology to detect the amount of blood flowing through the wrist.

When your heart beats, this flow, and as a result the green light being absorbed is greater. These lights are then flashed hundreds of times per second in order to gain the most accurate BPM (beats per minute) data.

Naturally, while every company dabbling with this optical sensor technology is working from the same blueprint, but the accuracy of the readings will come down to how each company's algorithms interpret the data.

How accurate is Fitbit heart rate data?

Like with any optical heart rate solution, Fitbit's PurePulse technology is solid but not without its issues.

Fluctuating between high heart rate and low heart rate during interval training can often prove to be problematic for the heart rate monitor to keep up with. As we say though, Fitbit's trackers aren't alone in this problem.

If you sense your Fitbit device isn't quite tracking your heart rate correctly, there are a couple of things to keep in mind.

Firstly, a good tight fit is extremely important as light leaking in from the sides of your watch will disrupt the signal.

When you're not exercising, wear your device a finger's width below your wrist bone. And when you are exercising, consider wearing the device slightly higher on your wrist for more accurate readings, since some exercises will cause your wrist to move frequently.

Naturally, as with any wrist-based monitor, you'll also need to make sure the back of the watch is touching the skin at all times, while also ensuring it isn't strapped too tightly onto your wrist.

How to view your heart rate on a Fitbit

2. Fitbit Heart Rate - How to View Your Data

Below we'll get into the intricacies of the heart rate monitor, but it's worth pointing out how you can actually view your heart rate data, too.

Depending on the device itself, you should be able to see your current BPM on the home screen or by swiping. For detailed info from your exercise or on resting heart rate, you'll need to head to the Fitbit app's dashboard and tap through to the Heart Rate section and select the day you want to view. The same also applies to the Fitbit web app.

Resting heart rate while you sleep

Heart rate tracking isn't just about exercise. Fitbit's trackers are also able to keep track of your resting heart rate — a metric which refers to the heart rate measurement when you're awake, calm and have not recently exerted yourself.

In order to estimate this, Fitbit interprets data taken from when you're awake and when you're asleep, meaning that those who take their device off before bed won't receive the most accurate results.

Typically, your resting heart rate is higher than your heart rate while you're asleep, so don't start panicking if you notice that the figure is higher than the lowest number you see in your graphs.

Active Zone Minutes

Fitbit's new metric is called Fitbit Active Zone minutes, which is now live on the Charge 4 and will be coming to other devices in due course.

Active Minutes have always been a fixture of your daily goals, where you needed to aim for 30 minutes of brisk walking, or more, per day. But Active Zone Minutes take things to the next level.

The premise is simple – the World Health Organisation and American Heart Association recommend 150 minutes of exercise per week, or 75 minutes of vigorous exercise.

Fitbit now uses heart rate zones to award these minutes to a goal: if you get into the fat burning zone for 1 minute you get 1 minute awarded.

Previously, this discriminated against more active people, who would get the same reward for a HIIT workout than someone for a brisk walk. So if you get into the Cardio or Peak zones, you get 2 minutes awarded for every minute of exercise.

What's more, your zones are personal. That's because they are calculated from your resting heart rate and your max HR (based on age - 200), and this has two benefits: first, your goal should be tailored to your fitness level. Secondly – as you get fitter you'll have to work harder to hit the goal.

Read our full guide to using Active Zone Minutes.

VO2 Max for serious fitness data

While VO2 Max - the metric which essentially calculates the maximum amount of oxygen that an individual can utilise during intense exercise - has made its way onto a large number of sports watches and even hearables, Fitbit has a different name for it - Cardio Fitness Score.

To access your score, simply head to the heart rate section of the Fitbit app and swipe the graph across. The higher your score, the better your fitness, and Fitbit will help determine where you sit on the scale by looking at your age, gender and resting heart rate. For more on understand the metric, read our VO2 Max guide.

Fitbit heart rate zones

3. Fitbit Heart Rate - Heart Rate Zones

Heart rate zones are essentially groupings, which allow you to adjust the intensity of your training based upon how long you spend in certain stages.

When you set up a Fitbit account you'll be assigned your maximum heart rate, which is configured by the usual method — 220 BPM minus your age — and from there your heart rate zones are also established.

Essential reading: How to train with heart rate zones

At the top is what Fitbit labels as your Peak Zone, which means your heart rate is above 85% of its maximum. Next below is your Cardio Zone, which covers the ground betwee 70% and 84%, before scaling down to the the Fat Burn Zone when your heart is pumping at between 50% and 69% of its maximum. Anything below 50% of your maximum heart rate is considered Out Of Zone.

How to customise Fitbit heart rate zones

It's all well and good knowing about your heart rate zones, but it's also nice to have a bit of control over them. And there are two ways to customise them.

If you're working from the Fitbit app, head to Account from the app dashboard and scroll down to Heart Rate Zones in settings. From there, you're able to add a custom zone and also adjust your max heart rate.

To change the same things from the Fitbit web app, head to the dashboard, select the gear icon, select Settings and then Personal Information.

Using heart rate tracking to keep calm

4. Fitbit Heart Rate - How accurate is Fitbit heart rate?

Heart rate variability, which is calculated by looking at the time in between heartbeats, is another measurement used by Fitbit to keep a track on your heart. However, since this is focused on tracking fluctuations, things like a user's age, body position, the time of day and health status can all affect readings.

Fitbit's Relax app, available on the Charge 3, Ionic and Versa, measures the beat-to-beat changes in order to recommend a personalised breathing pattern during each guided breathing session.

However, these are more of a real-time way to get you to focus on relaxing, as opposed to something you view in the companion app and keep a track of in the long term. You're able to choose a session lasting either two or five minutes, and all you need to do is follow the circle on the screen for inhaling/exhaling. Sparkles will show after around 20 seconds if you're aligned to the device.

You can read more in our heart rate variability guide.

Working out calorie burn

Firstly, your Fitbit device will take into account your basal metabolic rate (BMR), which is the calories that burn by maintaining necessary body functions like breathing, heartbeat, and brain activity.

This BMR accounts for around half of your daily calories, with this estimated from the gender, age, weight and height you enter when setting up your device. And with the tracker resetting the stats at midnight, this is why you'll notice calories already burned when you wake up - your still burning as you sleep. Naturally, Fitbit will also be adding on the calories you burn through activity.

If you're looking for more on this, we've gone into detail about how calorie burn estimates actually work.

Fitbit heart rate data by device

Fitbit heart rate monitor guide: Tracking your beats from the wrist

Naturally, with Fitbit's heart rate tech involved in both fully fledged smartwatch and fitness trackers with smaller screens, what you see depends on which device you have on your wrist.

The devices which actually offer heart rate monitoring are the new Fitbit Inspire HR, Fitbit Ionic, Fitbit Versa, Fitbit Charge 3, and the Fitbit Alta HR. And while the PurePulse tech is consistent throughout, meaning the results should be the same no matter which tracker you wear, things are displayed slightly differently. For example, heart rate zones display a little differently on the live readout across devices.

Fitbit SpO2 sensor explained

The newest Fitbit devices - the Charge 3, Versa and Ionic - come with an SpO2 sensor for tracking blood oxygen levels.

This data has just been enabled in the Fitbit app, and blood oxygen levels are now read in the Estimated Oxygen Variation graph. You can read our explainer of what that is - and how to use it here.

The first place we'll be able to make use of it is with Sleep Score, Fitbit's new way to analyse your sleep data.

This is currently in beta, but when it rolls out it will use the SpO2 sensor to track any breathing disturbances through the night. Eventually, it will let Fitbit alert users if it thinks they might be suffering from sleep apnea.