top of page
  • Writer's pictureSubir Biswas

How do Smartwatches Measure Calories | Tech-Knowledge

Updated: Aug 22, 2022

A very common use of any smartwatch or fitness tracker is to track fitness or health. There are various ways developed by the phone manufacturers by a combination of different sensors and all. But on the top, basically measuring the steps and the calories burned during various exercises. But the biggest questions are how these sensors are working and used to calculate that information to show real life data.

What are calories?

A calorie is a unit of energy. The amount of energy in an item of food or drink is measured in calories. Our bodies need the energy to keep us alive and our organs functioning normally. When we eat and drink, we put energy into our bodies. Our bodies use up that energy through everyday movement, which includes everything from breathing to running.

The calorie content is often given in kcals, which is short for kilocalories, and also in kJ, which is short for kilojoules. A kilocalorie is another word for what's commonly called a calorie, so 1,000 calories will be written as 1,000kcals. Kilojoules are the metric measurement of calories. To find the energy content in kilojoules, multiply the calorie figure by 4.2.

Why are calories important?

It is directly linked to health, mostly body weight. When we eat and drink more calories than we use up, our bodies store the excess as body fat. If this continues, over time, we may put on weight. To maintain a stable weight, the energy we put into our bodies must be the same as the energy we use through normal bodily functions and physical activity.

How do Smartwatches Measure Calories | Tech-Knowledge

As a guide, an average man needs around 2,500kcal (10,500kJ) a day to maintain healthy body weight. For an average woman, that figure is around 2,000kcal (8,400kJ) a day. However, these values can vary depending on age, size and levels of physical activity, among other factors.

There's a wide range of online calorie counters for computers and mobile phones. Many of these can be downloaded and used for free. But the smartwatch or fitness tracker can help you know how much energy you used in an entire day or after any physical activity.

How do smartwatches measure calories?

Now jump into the main topic of this article, "How do smartwatches measure calories"?

Let’s find out how this digital and smart gadget achieves this. In general, a smartwatch or fitness tracker cannot hold the data for a longer period (in general, it holds the data for 1 day to 1 week maximum). Most of the historical data is stored in the mobile phone application (app) with which the smartwatch or fitness tracker is linked. Generally, the phone application is linked with cloud which stores data for entire life. However, now a days smartwatches are coming with eSIM which can store the data directly to cloud but at the end user is going to check the historical value either in phone app or computer application. Whenever you long into the fitness application of the mobile phone, it will ask you for a few necessary information like height, weight and age (in general the app calculates from date of birth), gender, activity level (Physical activity is anything that makes you breathe faster and feel warmer) etc. This information is very important to measure the BMI (Body mass index) or BMR (Base metabolic rate).

However, for a note, everything your body does uses energy which includes breathing or blinking of the eyes. But the issue is, these can't be countable without equipment connected to our bodies. So, those basic calculators ask only for physical activities like walking, running, cycling, swimming, dancing, an active job, playing outdoor games, etc. However, this is not the case for the smartwatch, as it can detect heart rate. Smartwatches use their inbuilt accelerometer to measure your movements. They will also use their heart rate monitor to see how fast your heart is beating. The concept is that the faster your heart rate, the more calories you burn.

During exercise, your muscles must burn calories to fuel their contractions. The conversion of calories from their stored nutrient state to the form that can be burned by your muscle cells is achieved, during aerobic exercise, through the process of cellular respiration, which requires oxygen, and the delivery of oxygen through your bloodstream to your active muscle cells is directly related to your heart rate. It is this relationship that allows you to predict your energy expenditure (i.e., calorie burn) from your heart rate. Essentially, with increased exercise intensity, your muscles must burn more calories, and so your heart must beat faster to provide the oxygen necessary to convert those calories to the form of energy that can be burned by your muscles.

According to Shapesense, the formula below used for energy calculation is below. Besides, if you know your VO2max you might want to try the calculator below this one, as it is slightly more accurate. The formula can only be used if the heart rate is between 90 and 150 beats per minute. If you calculate with a heart rate out of that range, the calculation will not be correct.


Calories per minute = (-55.0969 + (0.6309 * HR) + (0.1988 * W) + (0.2017 * A)) / 4.184
Calories per minute (considering VO2max) = (-95.7735 + (0.634 x HR) + (0.404 x VO2max) + (0.394 x W) + (0.271 x A))/4.184


Calories per Minute = (-20.4022 + (0.4472 * HR) - (0.1263 * W) + (0.074 * A)) / 4.184
Calories per Minute (considering VO2max) = (-59.3954 + (0.45 x HR) + (0.380 x VO2max) + (0.103 x W) + (0.274 x A))/4.184

HR = Heart rate (in beats/minute)

W = Weight (in kilograms)

A = Age (in years)

VO2max = Maximal oxygen consumption (in mL/kg/min)

Check out the table below to get an idea of how many calories are burned as heart rate increases. This was counted on a person of 30 years and for 60 minutes of physical activity.

​ Heart Rate

​Burned Calories for 65 Kg Female

​​Burned Calories for 75 Kg Male

​90 bpm



100 bpm



110 bpm



120 bpm



130 bpm



140 bpm



150 bpm



The exact formula that is used varies depending on the company and is generally kept pretty secret as it is their USP (unique selling proposition). The smartwatch owners did many trials and based on a lot of data they estimated and calculated the calories.

Apple Watch Series 3 or later can record an estimate of your VO2max using the heart and motion sensors during an outdoor walk, run, or hike. Apple Watch supports a VO2 max range of 14-60 mL/kg/min that is validated for users 20 years or older.

How accurate is calorie data measured by smartwatches?

The data measured by the smartwatches are basically estimations, not 100% accurate as it is not measuring the calorie burn directly. It can only make an estimate based on the accelerometer and heart rate data and its own algorithms. This means the basics are the accelerometer and heart rate sensor. If these measurements are not accurate, the calorie values will not be accurate at all. However, big manufacturers (Apple, Fitbit, Samsung etc.) use AI (artificial intelligence) to compare lots of data to reduce the percentage of error. And on top of that, the ML (machine learning) methods help to improve the logic for the individuals by comparing the data from day to day.

And one more point you may have noticed is that there are different settings for each type of exercise. This is intentional as there are some different rates to reading the pulse and some other factors required. But as I said, it is a property of the manufacturer, which they developed day by day with tonnes of data.

Moreover, this is really a motivation for the individuals to improve their fitness by losing or balancing their body weight. Obviously, this is needed to remember that health is wealth. Have a smartwatch or fitness tracker and start monitoring your health.


Keytel LR, Goedecke JH, Noakes TD, Hiiloskorpi H, Laukkanen R, van der Merwe L, Lambert EV. Prediction of energy expenditure from heart rate monitoring during submaximal exercise. J Sports Sci. 2005 Mar;23(3):289-97.

Swain DP, Abernathy KS, Smith CS, Lee SJ, Bunn SA. Target heart rates for the development of cardiorespiratory fitness. Med Sci Sports Exerc. January 1994. 26(1): 112-116.

9,252 views3 comments

Related Posts

See All


Jan 17, 2022

This is a very interesting post to read. I have always wondered how smartwatches measure calories burnt. Thank you for sharing.

Subir Biswas
Subir Biswas
Jan 17, 2022
Replying to

Thank you for your time to read this article. I glad that it is useful for you. You can find similar articles in the website.


Jan 04, 2022

Good information to read 📚. Thanks for sharing.

Post: Blog2 Post
bottom of page