The Accuracy of Fitness Watches

The fitness watch is like a tracking bracelet for fitfam: it makes us easy to identify, and keeps us on the straight and narrow. You may be considering purchasing one, have one already or just have some concerns about how accurate they are. We’re going to run through some common misconceptions and delve into just how accurate they are.

Not all fitness trackers are created equal, and some offer different functions. In general, most offer the ability to track the wearers’ step count, hours sleep, heart rate and calories burned throughout the day and during exercise.

Fitness Watches in Health

Wearable fitness trackers (for the purpose of this article we will discuss fitness watches only) are becoming increasingly researched in literature. From a behavioural perspective, it is well-documented in the research that merely the act of wearing one encourages adults to be more physically active[1]. Wearers reported feelings of improved health[2].  It is important to note here that these are all subjective feelings reported by wearers, and not objectively quantifiable markers. Obviously, simply the wearing of a fitness tracker does not immediately half your body fat percentage. What is more likely, is that the constant visual reminder of your daily movement patterns encourages the individual to be more active.

The fitness tracker is the central hub for recording levels of physical activity and certain health markers.

The Accuracy of Fitness Watches

Not all fitness watches are created equal, and the accuracy of fitness watches differs greatly between models. This is important in health and fitness, particularly as inaccuracy can affect outcome; if you think you’re burning more calories than you are (and increase energy accordingly) this could affect your bodyweight for example.

Step Count

The accuracy of the step count varies between models, and some are more accurate than others. Watches range from being 99.5% (Apple Watch) to 79.8% accurate (Samsung Gear 2) at recording an individuals’ step count[3]. Regardless of the model, fitness watches are generally accepted as accurate methods of gauging steps taken per day/distance covered[4].

Heart Rate: At Rest & During Exercise

The gold standard in medicine for monitoring heart-rate is an electrocardiogram (ECG). An ECG is an snapshot of heart rate over a designated time period. It is obviously not practical for the novice fitfam member to walk around hooked up to an ECG. The fitness watch is a much more viable option. Generally, fitness trackers are considered relatively accurate at measuring the heart rate at rest[5]. When compared against the ECG, fitness trackers vary in accuracy during exercise. In general, fitness watch accuracy is greatest during treadmill-based activity and poorest on the cross-trainer[6].

This inaccuracy at recording heart rate has direct implications for the accuracy of “calories burned” reported by fitness watches after training. With such variations between watches and modes of exercise, I don’t think we can rely on the calories expended as reported by fitness watches as accurate.

Calories Expended

This is likely the one all you fitfam care the most about: does my Fitbit accurately tell me how much I have burned? The answer sadly, is a big fat no. Height, weight, age, fitness level, pre-existing conditions and muscle mass are just some of the factors that influence how much energy we use to carry out daily tasks. Different watches use different variables to estimate caloric output. Literature is generally conclusive that fitness watches are not accurate at estimating energy expenditure[7]. A systematic review found that trackers generally tend to underestimate caloric output[8]. Another study found that some watches (Fitbit Surge and Actigraph) greatly overestimated caloric output[9]. Some devices can be inaccurate by up to a whopping 93%[10]. So literature is unsure whether activity is over or underestimated, but either way it’s not accurate.

With such variation between studies in literature, it is extremely unlikely that your fitness watch will give you an accurate estimate of how many calories you burn a day. Anecdotally speaking, I can back this up. At peak bulk, my Fitbit told me I was burning approximately 3,700 calories a day. I was only eating roughly 3,000 calories a day, and I was gaining weight. If the watch was to be believed, I would be stage lean by now!

Hours Slept

As you have no doubt gathered by now, the fitness watch is relatively inaccurate for most of the physical markers. And sleep is no different. Fitness trackers in general exhibit low validity in accurately assessing sleep duration and quality[8]. The gold standard for accurately tracking sleep is polysomnography, and involves the patient be wired to within an inch of their life, which fitness watches obviously do not do.

Fitness watches typically use movement and heart rate to estimate sleep, and this is not as accurate as the gold standard. Furthermore, as we discussed above, watches aren’t too accurate at assessing movement and heart rate either! Fitbits in general have a tendency to overestimate sleep time and quality[11].

Key points from this section:

  • Accurate for counting steps.
  • That’s about it.


So, much like the skinny teas of a few years ago, the fitness watch is all a lie. On the positive side, it doesn’t come with the laxative effects the teas do. Fitness trackers aren’t all that accurate. Does that mean we can completely discount them? I don’t think so.

We can use these glorified pedometers to get a rough ballpark of where we’re at. We can loosely know how many steps we take per day, sleep in general and calories burned per day. It is important that we don’t rely on them for any degree of accuracy. For example, if you are trying to lose weight, it would be unwise to eat 2,200 calories to lose weight if your watch says you burned 2,400 that day.

Furthermore, given that they are proven to encourage activity amongst individuals, their accuracy might be questionable, but their role in encouraging healthier habits is indisputable.

The fitfam takes the L this time, but I’ll still be wearing my Fitbit anyway!


  1. Mercer, K., Li, M., Giangregorio, L., Burns, C., Gindrod, K. (2016) ‘Behavior Change Techniques Present in Wearable Activity Trackers: A Critical Analysis’, JMIR Mhealth Uhealth. Available at:
  2. Lunney, A., Cunningham, N.R., Eastin, M.S. (2016) ‘Wearable fitness technology: A structural investigation into acceptance and perceived fitness outcomes’, Computers in Human Behavior. Available at:
  3. El-Amrawy, F., Nounou, M.I. (2015) ‘Are Currently Available Wearable Devices for Activity Tracking and Heart Rate Monitoring Accurate, Precise and Medically Beneficial?’, Healthcare Informatics Research. Available at:
  4. Pobiruchin, M., Suleder, J., Zowalla, R., Wiesner, M. (2017) ‘Accuracy and Adoption of Wearable Technology Used by Active Citizens: A Marathon Event Field Study’, JMIR Mhealth Uhealth. Available at:
  5. Shcherbina, A., Quiroz, J., Lang, M., Innominato, P. (2019) ‘Accuracy of Consumer Wearable Heart Rate Measurement During an Ecologically Valid 24-Hour Period: Intraindividual Validation Study’, JMIR mHealth uHealth. Available at:
  6. Gillinov, S., Etiwy, M., Wang, R., Blackburn, G., Phelan, D., Gillinov, A.M., Houghtaling, P., Javadikasgari, H., Desai, M.Y. (2017) ‘Variable Accuracy of Wearable Heart Rate Monitors during Aerobic Exercise’, Medicine & Science in Sports & Exercise. Available at:
  7. Maher, C., Ryan, J., Ambrosi, C., Edney, S. (2017) ‘Users’ experiences of wearble activity trackers: a cross-sectional study’, BMC Public Health. Available at:
  8. Evenson, K.R., Goto, M.M., Furberg, R.D. (2015) ‘Systematic review of the validity and reliability of consumer-wearable activity trackers’, International Journal of Behavioral Nutrition and Physical Activity. Available at:
  9. Kirk, S. (2014) ‘Comparison of the Apple Watch, Fitbit Surge and Actigraph GT9X in Measuring Energy Expenditure, Steps, Distance and Heart Rate’. Available at:!etd.send_file?accession=csu1462375247&disposition=inline.
  10. Shcherbina, A., Mattsson, C.M., Waggott, D., Salisbury, H., Christle, J.W., Hastie, T. , Wheeler, M.T., Ashley, E.A. (2017) ‘Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort’, Journal of Personalized Medicine. Available at:
  11. Haghayegh, S., Khoshnevis, S., Smolensky, M.H., Diller, K.R., Castriotta, R.J. (2019) ‘Accuracy of Wristband Fitbit Models in Assessing Sleep: Systematic Review and Meta-Analysis’, Journal of Medical Internet Research. Available at:

Published by Michelle Carroll

I am an online coach (MSc Sports & Exercise Nutrition, EQF Level 4 Personal Trainer, PN Level 1) and radiographer (BSc). I believe in empowering others to make better choices for their health through education. I think that the fitness industry has created a disconnect between best practices and “evidence-based” practices. I hope by chronicling my experience as a healthcare professional and my education as a fitness professional I can assist others on the path to bettering themselves.

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