How does Fitbit calculate fat burn?


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Page last updated: 28/09/2022

How does Fitbit calculate fat burn?

In this guide, we will discuss “How does Fitbit calculate fat burn”, heart rate, what are the maximum heart rate scores per zone, target heart rate training and some additional considerations when you are starting your fitness journey.

How does Fitbit calculate fat burn?

If you wonder, ‘How does Fitbit calculate fat burn?’ we need to consider the following three heart rate zones that will show up in your Fitbit app:

  • Fat Burn Zone. In this zone, your heart is beating 50-69% of your Maximum Heart Rate. 
  • Cardio Zone. In this zone, your heart is beating 70-84% of your Maximum Heart Rate. 
  • Peak Zone. In this zone, your heart is beating at least 85% of your Maximum Heart Rate.

Many users would like to make sure how much fat they are burning as a result of a training, workout or physical activity. However, we all know that it is not enough just to consider how the Fitbit calculates fat burn but also think about what is required for our transformation. For instance, changing our daily habits and gradually increasing our fitness intensity. 

Consequently, spending hours at the gym without a plan or goals, paying attention to the calories burned won’t actually help us make a change. However, it is important to gradually increase the intensity and avoid exceeding ourselves from the beginning. Imagine you were to run a 5k marathon without the proper training. Most probably you wouldn’t even get too far before your body telling you it is too much. 

Heart rate

In simple words, your heart rate is how many times your heart beats per minute. When you see something like 70 bpm it means that your heart was beating 70 times over a period of 1 minute and knowing your heart rate can help you in several ways, for example:

  • Identifying if there could be a problem. You may have noticed how, when you go to your doctor for a checkup, they check your heart rate. If they consider it is too low or too high then they will start investigating the root cause.
  • Helping measure your fitness journey. We know a heart is a muscle and as you increase your fitness level, your heart starts to get stronger. Your heart rate ends up being an indication of the progress you have made.
  • Losing weight. This is probably one of the most important topics for many people, as part of your fitness goals. This means that the intensity of your activity will indicate which heart rate zone you are in and will allow you to lose weight. 

Is there a maximum heart rate?

This is a very important question since it is one of the most important things you need to know. As indicated by, “It’s a simple math equation. Subtract your age from 220. The answer is your Maximum Heart Rate. Even better, your Fitbit actually does this math for you”.

Moreover, since your heart is a muscle, it loses some capacity over the years as we age. The heart of someone who is 20 years old can have an expected Maximum Heart Rate (MHR) of 200 while someone who is 80 may drop their MHR to 170. 

How does Fitbit calculate fat burn?

Fat Burn Zone

Your heart will be beating 50 to 69% of your MHR in this zone and it is considered low to medium intensity. If your heart is in this zone, it means you are burning fat and it will help you lose weight. However, as good as it is, you may not be burning as many calories as the next zone.

Cardio Zone

Your heart will be beating 70 to 84% of your MHR and it is considered medium to high intensity, the goal for most people who workout. You will be burning more calories than the Fat Burn Zone and you will also feel this one is more demanding than the Fat Burning Zone. 

Peak Zone

Your heart will be beating at least 85% of your MHR and this zone can have short bursts of very high-intensity exercise. This is mostly associated with exercises like High-Intensity Interval Training or HIIT. The goal here is to briefly push your heart into this zone during timed intervals, but this is not the zone you’d like (or need) to start your fitness journey. 

Target Heart Rate Training

Target Heart Rate Training may sound very complicated but it isn’t. It just means you track your heart rate during your workouts and try to keep it within a set intensity zone or range that is in tune with your fitness goals.

However, it is important you make sure to know your heart rate zones. The Fitbit app can show you a graph depicting how much time you have spent in any of the three heart rate zones. The purpose of knowing about them can help you make the most out of your training and reach your fitness goals, all at the same time.

As indicated on, “Target heart rate zones are calculated using maximum heart rate—the highest number of times your heart can safely beat in one minute. To keep things simple, Fitbit uses the common formula of 220 minus your age to estimate your maximum heart rate. So a 35-year-old would have a max heart rate of 185 (220-35=185)”. 

Once your Fitbit has identified your maximum heart rate, it will simplify things by organizing the information. Each of the zones will represent an intensity level or percentage of your max heart rate. 

How we think Fitbit can Help You

We have discussed the benefits of tracking your heart rate and as long as you have it configured in the Fitbit app correctly for your age, it will calculate it in real-timereal time telling you in which zone you are in. The heart rate technology that Fitbit uses is really good, it involves LED lights on the back of your device. 

“Because your blood is red it absorbs green light. Each heartbeat produces a mini-surge of blood that absorbs more green light. In between each beat less green light absorbed (”

The green lights you see flashing multiple times every second calculates how much is absorbed and the amount of time between each surge, resulting in your heart rate measurement. 

Why is this blog about How does Fitbit calculate fat burn important?

We have discussed how does Fitbit calculate fat burn as the result of your heart rate measurements and how it gets categorized into cardio, fat burn or peak zones considering your maximum heart rate scores. However, before starting your fitness journey, it is important to set realistic goals and not just simply try to go into the fat burn zone if you are just wanting to lose weight all of a sudden. Your body needs to adjust to the changes in intensity so it needs to be increased gradually.

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

Frequently Asked Questions (FAQs) about How does Fitbit calculate fat burn

Is it better to be in cardio or fat burn?

It is better to be in cardio when you exercise, since you will burn more glycogen or stored carbohydrates as your main energy source, using less fat, nevertheless, your total caloric burn is much greater. Calories burned is what results in weight loss and in contrast, exercising at low intensities, the fat burn zone, you have no afterburn. 

How does Fitbit calculate calories burned?

How does my Fitbit device calculate calories burned? Fitbit devices combine your basal metabolic rate (BMR)—the rate at which you burn calories at rest to maintain vital body functions (including breathing, blood circulation, and heartbeat)—and your activity data to estimate your calories burned.

Does a Fitbit show calories burned?

Fitbit can show calories burned as the approximate amount of how many calories you have burned throughout the day or during a given exercise and can create a snapshot of your overall daily and weekly expenditure. 

What exercise burns the most fat?

The exercise that burns the most fat is said to be aerobics since cardio is considered to be the best fat burning exercise by excellence. Cardio is meant to increase your blood flow and help you work off those extra calories. In contrast, weight-training will turn fat cells into muscle.

What’s the difference between burning calories and burning fat?

The difference between burning calories and burning fat is that when you start burning calories through exercise or physical activity then it turns into burned fat (carbohydrates in our system). In order to burn calories from your stored fat, your body requires the presence of oxygen.

Why does my Fitbit say I’ve burned so many calories?

Your Fitbit will estimate the amount of calories burned throughout the day and night until midnight when it resets automatically. This means that the number you see on your tracker or Fitbit app first thing when you wake yo is an estimate of the calories burned during the night.