Calculation of the number of calories by the Fitbit bracelet or watch

The Fitbit bracelets and watches are the perfect allies for your well-being. Capable of accurately determining a whole host of data related to your daily activity, Fitbit devices give you valuable insights to help you live healthier.

Main indicator calculated by your Fitbit device, the number of calories burned in one day gives you a precise indication of the activity of your body.

This coupled with a monitoring your diet, and you have all the assets on your side to monitor your weight and take care of your health effectively.

But how exactly are these calories burned calculated? Can we trust this device, to what precision?

In this article we will explain in detail how the Fitbit algorithm works to calculate your body's energy expenditure, expressed in calories.

Here is how Fitbit calculates the calories burned in a day. No matter your model, Fitbit Charge, Versa, Ace, Sense, Inspire, and all models, the calculation remains similar.

If you're having trouble with your Fitbit device, here are some troubleshooting articles:

Understanding the calories burned calculated by your Fitbit bracelet or watch

In order to know precisely how your Fitbit is calculated, you should break down the operating mode into sub-parts.

Basis for calculating calories burned: your basal metabolism

Before going into detail in the different activities and the calculation of calories burned according to what you do, it is advisable to understand the concept of "basal metabolism".

What is that ? How important is it for your Fitbit device to determine your calories burned?

What is basic metabolism?

Basic metabolism is the amount of calories burned by your body for your vital functions when you are not active. It is therefore the minimum vital energy that your body needs to live. This therefore includes the respiratory mechanism, blood circulation, the functioning of muscle mass, etc.

This calculation of metabolism is therefore very specific to each. To approach it, Fitbit therefore uses data linked to your identity:

  • Age
  • Weight
  • Size
  • General daily activity

Why is your activity necessary to determine a factor at rest? Because the more active you are on a daily basis, the more athletic you are, the faster your metabolism is said to be at rest.

Many reasons :

  • A more active body has more muscle mass. We saw above that muscle mass, even at rest, consumes energy. This therefore burns calories. Yes, the more muscular you are, the more calories you burn at rest.
  • A body stimulated by sports maintains a livelier metabolism. Greater respiratory capacity, muscle capacity, etc.

This isn't spelled out in detail on your Fitbit device, but there's a good chance Fitbit uses a known formula to determine your basal metabolic rate: The Harris-Bennedict equation.

  • For men : MB = 10 x weight (kg) + 6,25 x height (cm) - 5 x age (years) + 5
  • For women : MB = 10 x weight (kg) + 6,25 x height (cm) - 5 x age (years) - 161

It is therefore the basis of calculation to determine the amount of calories burned by your body in a day.

Then add the different activities that you do on a daily basis.

Specific calorie calculation based on activity

Calories burned while walking or running

First, your device Fitbit records the number of steps you take each day.

It is therefore very important that you have accurately entered the informations requested :

  • The stride size of your walking and running steps

For it, go to this article and make sure the details are correct. Otherwise the calorie burn calculation determined by your Fitbit device will be inaccurate.


Then, when the number of steps is recorded by your Fitbit bracelet, a calculation is made, based on your age, weight, and height, to determine approximately the number of calories burned by this activity.

If you have a problem with the step calculation, go through this article:


Climbing stairs, calculating calories burned

The principle here to determine the calories burned when you go down or up a floor is simple: Your Fitbit watch is equipped with a altimeter. This one is therefore able to analyze if your body goes up or down in altitude.

Fitbit estimate that you go up one floor approximately every 3 meters of staircase. Depending on whether you are going down or up, a coefficient is established, because it is obviously easier, and therefore less energy consuming to descend a staircase than to climb it.

Hence the interest of the altimeter on the Fitbit device. He is able to tell the difference between going up and down.

Finally, your device calculates then always according to your age, your weight, and your height, the number of calories burned by this activity.

And so it will be added to other activities, and your basal metabolism.

Physical activity during the day as the last factor in calculating calories burned

To complete the calculation of calories burned during the day, estimated by your fitbit device, we must therefore take into account the'physical activity.

For this, several criteria are taken into account:

  • The type of activity : For a more precise calculation, it is important to specify what type of activity you are starting. This will bring your Fitbit watch closer to the exact number of calories burned.
  • The change in the heart rate of your heart : When you do your sports activity, your heartbeat will rise during all the effort. This setting, saved by your Fitbit, will allow you to thus determine the number of calories burned.

Once again, depending on your personal characteristics (age, weight, etc.), a calculation is made by your Fitbit.

Calculation of calories burned for a day: Summary

Here is a small summary of everything that has been mentioned above. We have retained many factors that determine the calculation of total calories burned by your body:

  • Your basal metabolism
  • The number of steps taken during the day
  • Specific physical activity
  • The number of floors descended or ascended

All these parameters are therefore added together, allowing you to have a estimate of the number of calories burned in a day.

Are the estimated calories burned by my Fitbit bracelet reliable?

Keep in mind that this is still a calculation, mainly based on data you have about Fitbit. It is by no means a precise data as could be the heart rate, directly given by your pulse.

Care should therefore be taken when using this number. It is estimated on average at more than 80% reliable, whether your personal data is correct.

However, this does not change the fact that knowing roughly how many calories you burn per day is a great way to control, or lose weight if necessary. It is therefore still an excellent indicator for monitoring your health.

If you want to go further in monitoring your well-being, do not miss our Fitbit sleep article: