Can You Trust Your Watch’s Calorie Burn?

Weight loss boils down to calories in vs calories out, so it makes sense to try and measure how many calories you’re burning. If you can control your calorie intake AND calorie burn, then you can guarantee that you will not only lose weight but also the rate at which you will lose it. One way many are trying to do just that is by their measuring their calorie burn on their fitness watches, such as the Apple Watch and Fitbit, but how accurate are they? Can you trust what they’re telling you?

Wearable fitness trackers, such as fitness watches, have come a long way over the past decade with technology improving immensely. Studies have shown they have a high level of accuracy for measuring data such as heart rate and step count (1) (2), however, their ability to measure energy burn during exercise and throughout the day hasn’t come as far. This has left many people frustrated with their lack of weight loss and scratching their heads as to why they’re not making progress when their fitness watch tells them they’ve burned far more calories than they’ve consumed.

Calorie Burn During Exercise Accuracy

Surprisingly, one study in 2020 found that not a single fitness watch could accurately measure calorie burn within acceptable limits more than 13% of the time. Worse yet, less than 10% could even measure calorie burn within acceptable limits at all! (1) Fitness watches tend to over- and underestimate calorie burn, with the accuracy of their readings dwindling as the intensity of exercise increases. Below is a list of which watches tend to do what:

Fitness watch EE during exercise table.jpg

Overall, most watches tend to underestimate calorie burn during exercise activities, which means they tend to tell you you’ve burned fewer calories than you have. Low-intensity activities (such as household chores and walking) tend to have the lowest degree of error. The degree of error increases as exercise intensity increases.

Total Calorie Burn Accuracy

Okay, so fitness watches aren’t great at telling us how many calories we’ve burned during a workout, but what about how many we burn throughout the day? After all, it’s not how much you burn in a workout that really matters when it comes to losing, maintaining or gaining weight, but the total amount you burn for the day. Well, it turns out that they’re not great at measuring that either.

Another study in 2020 showed that fitness watches were very inaccurate at measuring total daily energy burn (2). Watches had large differences in estimated calorie burn compared to what indirect calorimetry and doubly-labelled water (two more accurate methods of assessing calorie burn) had reported, with the watches tending to under-estimate calorie burn. Only the Fitbit Flex had a small error, while the Garmin Vivofit, Epson Pulsense and Withings Pule O2 had the largest errors.

Fitness watch TEE table.jpg

How To Determine How Many Calories You're Burning

Without expensive laboratory equipment, the only way to determine how many calories you need to lose, maintain or gain weight is to go by how your body is actually responding. Calorie calculators and wearable fitness trackers, such as watches, only estimate calorie burn – they cannot calculate precisely how many calories your body burns during exercise or throughout the day. Monitoring how your body weight changes over time will give you a clear indication of whether you're in a caloric deficit or surplus or in balance. By knowing how many calories you're consuming and doing some maths, you can then calculate how many calories your body is expending each day or week on average, which can be extremely powerful for achieving your goals.

Key Take-Aways

  1. Fitness watches tend to have significant errors in estimating calorie burn
  2. They tend to underestimate calorie burn during exercise and throughout the day
  3. They should not be used to try to precisely balance calorie intake with calorie burn
  4. The only way to truly know how many calories you need to lose, maintain or gain weight is to monitor how your body actually responds over weeks and months.

Take The Guess Work Out Of Fat Loss

It’s no secret that losing body fat can be confusing; your body weight can do weird things, there is so much conflicting information online, what works well for one person doesn’t work well for the next, and so on. Thankfully, our coaches are highly qualified and experienced in helping people lose body fat and can take all of the guesswork out of it for you. Put your nutrition and training on autopilot with our fat-loss coaching programs!


  1. Reliability and Validity of Commercially Available Wearable Devices for Measuring Steps, Energy Expenditure, and Heart Rate: Systematic Review. Fuller, Daniel, et al. 9, s.l. : JMIR mHealth and uHealth, 2020, Vol. 8.
  2. How well do activity monitors estimate energy expenditure? A systematic review and meta-analysis of the validity of current technologies. O'Driscoll, Ruairi, et al. 6, s.l. : British Journal of Sports Medicine, 2020, Vol. 54.

About the author

Daniel Brack

BSc., Pn1, CFT, SEN

From commercial pilot to fitness coach, Daniel changed careers to pursue his passion for health & fitness. He is the founder of Recalibrated Bodies and co-founder and Head Coach of Titan Fitness Algarve. He has a Bachelor of Science in Strength and Conditioning and is a Certified Fitness Trainer and Sports and Exercise Nutritionist. Daniel was an international IFBB men's physique competitor and is a father to two daughters. His passion is helping others become more educated and skilled to reach their health and fitness goals.

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