The world of tech wearables has exploded in recent years and, by far, the most popular option has in the realm of smartwatches and activity trackers. You are probably wearing one right now, or at least have someone close to you who wears one. Smartwatches can perform various tasks, but the key feature for many people is how they can help us reach our fitness goals.

In particular, their ability to track our activity and measure how many calories we have burned. But have you ever sat back and wondered how exactly Apple, or Fitbit, or Garmin are managing to calculate the calories you are using? It is not quite as simple a task as it can first appear.

What are calories?

A calorie is a unit of measurement. Specifically, it is a unit of energy. There have been a variety of methods used to define a calorie over the last century. Still, generally, a calorie is the amount of heat that’s needed to raise the temperature of a gram of water from 14.5 degrees celsius to 15.5 degrees.

It is known as a small calorie and is different from the calories you see on your food labels. They are 1,000 of these little calories each and so are sometimes referred to as kilocalories.

In any case, calories measure energy. Every piece of food you eat contains calories, and every part of your body uses calories to function. So you are undergoing a constant energy transfer from your food to your body.

Why are calories important for weight loss?

The focus on calories in weight loss comes about because excess calories are the driving force behind weight gain. Whenever you consume more calories than you use, that energy has to go somewhere. And what your body cleverly does is store it as fat so that it can be used at a later date. When you regularly consume too many calories, this can lead to weight gain.

So, if you are trying to lose weight, you will need to keep track of the number of calories you are taking in from your food and the number of calories you are using up. You can easily track the calories you eat by using a calorie tracker, but a smartwatch can be very useful in tracking the number of calories you have used.

How do smartwatches measure calories?

Now we have covered why it is essential to measure the calories you use, let’s look at how smartwatches achieve it. Whenever you create your account for your smartwatch or activity tracker, it will ask you for some necessary information. It usually includes:

  • height
  • weight
  • age
  • sex
  • usual activity level

The company behind your smartwatch isn’t just collecting this information for fun. It is using it to calculate your base metabolic rate (BMR). Remember that everything your body does uses energy. Absolute everything. This includes breathing, digestion, and blinking. So even if you are sat on your couch watching TV, you are burning calories. The number of calories you burn when you aren’t moving around varies quite a lot, depending on those factors you input into your account. For example, men tend to have a higher BMR than women, and larger people have a higher BMR than smaller people.

Once your smartwatch has your BMR, it can then calculate your active energy expenditure. It is the amount of energy you use when you do anything other than sitting and doing nothing. It can be anything from walking up the stairs to housework to a HIIT training session.

Smartwatches tend to use their inbuilt accelerometer to measure your movements (this is also how they count your steps). They will also use their heart rate monitor to see how fast your heart is beating (the faster the heart rate, the more calories you burn on average).

The exact formula that is used varies depending on the company and is generally kept pretty secret. For example, Fitbit has stated that they use AI, machine learning, and a database full of biometric data from a wide range of people to design their calorie calculator algorithm. Similarly, Apple conducted a study using 10,000 people’s metabolic profile.

Apple measured the study participants’ calorie expenditure using metabolic gas measurements (which is precise) and then correlated that with their heart rate and movement data. Their watches are then trained using this data to calculate calorie burn as accurately as possible.

How accurate are smartwatches calorie data?

The calorie burn estimate you get from your fitness band, or standalone smartwatch will only ever be that – an estimate. It would be impossible for your smartwatch to be 100% correct in how many calories you have burned during your activity because they don’t ever measure calorie burn directly. They can only make an estimate based on the accelerometer and heart rate data and their algorithm. Certain factors aren’t usually considered by smartwatches that can impact how many calories you use.

For example, people with a higher muscle tone tend to have a higher metabolic rate than people with fewer muscles, and they tend to burn more calories while they are active. Most companies don’t yet take this into account when they estimate the number of calories you have used.

The good news is that in most cases, the absolute accuracy shouldn’t matter too much. The exact number that pops up on your screen often isn’t the most important part of the calorie information. The essential thing is comparing the number of calories burned from one day to the next and from one activity to the next. Doing this can help you learn about improving your fitness level or losing weight by burning calories and helping keep you motivated to stay as active as possible. Plus, it’s fascinating!

Final thoughts

Learning about how many calories you use in a day or how many you use performing particular activities is one of the big draws of smartwatches and activity trackers. The different manufacturers use their own algorithms to calculate the exact numbers of calories used. Still, they all rely on their accelerometer and heart rate monitor data to make that calculation.

While this will never be 100% accurate, it can give you an excellent idea about what activities are suitable for burning calories and can help you keep track of how active you are on a day-to-day basis.