Are fitness trackers accurate? They have grown immensely over the years. Long-gone are the days when your watch just tracked time – now they’re tracking heart rate, step count, and even sleep.
The problem is that consumers are basing their entire health and programming on these, which requires them to be reasonably accurate. So, are they?
Yes and no – it depends on what variable you’re tracking. What variable? This article will tell you when fitness trackers are accurate and when they can get you in trouble.
What Is A Fitness Tracker?
Remember the days when you would go to the gym or out for a run, and all you grabbed were your shoes? Now, people are grabbing a fitness tracker watch, chest straps, Bluetooth headphones, and ensuring their phone is tracking their workout.
The entire fitness tracker industry has risen immensely in the past few years. Here are some numbers to illustrate this: ¹⁻²
⅕ of American adults admit to using a smart watch or fitness tracker
$62 billion market in 2024
Expected to hit ~$72 billion by 2025 and ~$290 billion by 2032
When we say fitness tracker, we are using a general term but can include a variety of gadgets, including;
Watches
Phones
Chest Straps
This review will primarily be reviewing watches as they are multi-functional tools, and what most people are using.
What Fitness Trackers Measure
Not only has the use of fitness trackers grown immensely over the years, but their technology has as well. While a watch used to just keep time, they now track numerous variables and biometrics.
The most common include;
Steps
Calories burned
Heart rate
Sleep stages
VO2 max and stress (advanced models)
All of these variables can work off very different physiological functions as well as require different measuring mechanisms.
This means that the accuracy of different metrics can vary greatly, from reliable to worthless.
The Accuracy Of Different Metrics On Fitness Trackers
We’re now going to run through how these different metrics measure up, concentrating on 3:
Step Counter
Heart Rate
Calorie Expenditure
1. Step Count Accuracy (2-10% Error During Exercise)
If you’re using your fitness tracker as a step counter, you’re in reasonably good hands. Fitness trackers count steps through two mechanisms;
Accelerometers (Core Technology): Measure movement in 3D space (X, Y, Z). When your wrist swings or your body jolts during walking or running, the device detects a rhythmic acceleration pattern and logs it as a step.
Gyroscopes (Optional): Detect rotation and orientation, helping the device differentiate between actual walking motions and similar non-walking movements like brushing teeth or waving.
The information that is gathered is then run through various algorithms, and bam! You got your steps.
Various studies have found slightly different numbers depending on the setting and placement, but in general, they are accurate.¹⁻²
2-6% for jogging/walking for exercise
5–10% error range in controlled settings
10%+ in a free living environment
The reasoning behind why the accuracy can differ is that during free-living, motion is not fluid. A person may swing their arms around or not at all. In addition, if someone walks too slowly, the device may have difficulty recognizing movement.
Therefore, while your total daily steps may be off, steps during exercise tend to be reasonably accurate.
If you want to increase the accuracy, consider a pedometer or waist-worn device, as these tend to be more accurate.
More importantly, tracking steps, regardless of how accurate the tracker is, is shown to help build consistency and long-lasting habits.³
2. Heart Rate Monitoring (1%–9% Error)
Heart rate monitoring is somewhat surprising. Due to movement and sweat, you’d think these wouldn’t be that accurate, but it seems most are.
The most accurate heart rate monitors are typically chest-worn.
If you’re really concerned about getting the most accurate data, this is what you need. In fact, they can be so accurate that some devices could even qualify for medical use.
This is because they use something similar to an ECG that measures heart rate directly and is accurate to 1–2 beats per minute (BPM) of a medical-grade ECG.⁵
Now, not everyone wants to wear a chest strap every time they train. So what about watches?
Most watches use some sort of optical sensor that measures your pulse. The main problem that occurs is the movement of the sensors, as well as sweat or other debris. However, they’re still surprisingly accurate. Here are some numbers.
A review of 4 popular wrist-worn devices saw an error rate of 1-9%⁴
In a study of 7 devices, the median HR ranges from 1-9% depending on activity²
Walking 3.9%–7.1%
Cycle 6 devices were <5%
All in all, the heart rate monitor on most fitness trackers are reasonably accurate as long as you understand there’s some variability.
3. Calorie Expenditure Estimates (27–93 % Error)
To put it bluntly, calorie expenditure tracking sucks on all fitness trackers. This is largely due to all of the different variables that dictate total energy expenditure, including;
Muscle mass
Hormone levels
Metabolism
Body fat %
This doesn’t even account for the error in actual activity and heart rate. Even with all these variables, fitness trackers still use standardized equations.
As said above, fitness trackers consistently track very poorly. Two results from large reviews clearly show this.
One of the largest studies found a whopping 27-93% in error.
In 2022, a review found all major brands had greater than 30% error.²⁻⁴
In other words, we hope you’re not counting on your fitness watch for your calories.
Are Fitness Trackers Useful?
Although fitness trackers aren’t great for calorie counting, they are surprisingly reliable for HR tracking and step counting.
Still, when looking at calorie counting, while you shouldn’t rely on them 100%, you can use them to pay attention to trends.
For example, let’s say your tracker is saying you burn 1,000 calories every day. However, while you know this is likely overestimating your calories, you’re still losing weight at a healthy rate. 1,000 calories can be a decent gauge to let you know you’re still burning the same amount of calories.
More importantly, consider the fact that multiple studies have found that people are more consistent and successful with their programs when they track their training.
So, does it really matter if your watch says “1,000 calories” when you really burned 600? As long as you don’t reward yourself with McDonald’s, it should keep you on track, but that’s up to you to decide.
The bottom line is that they can be a useful tool and become exponentially more powerful when you pair them with actively tracking your weight and training performance.
References
Evenson KR, Goto MM, Furberg RD. Systematic review of the validity and reliability of consumer-wearable activity trackers. Int J Behav Nutr Phys Act. 2015;12:159. Published 2015 Dec 18. doi:10.1186/s12966-015-0314-1 https://pmc.ncbi.nlm.nih.gov/articles/PMC4683756/#Sec21
Shcherbina A, Mattsson CM, Waggott D, Salisbury H, Christle JW, Hastie T, Wheeler MT, Ashley EA. Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort. Journal of Personalized Medicine. 2017; 7(2):3. https://doi.org/10.3390/jpm7020003
Bassett DR Jr, Toth LP, LaMunion SR, Crouter SE. Step Counting: A Review of Measurement Considerations and Health-Related Applications. Sports Med. 2017;47(7):1303-1315. doi:10.1007/s40279-016-0663-1 https://pmc.ncbi.nlm.nih.gov/articles/PMC5488109
Germini F, Noronha N, Borg Debono V, et al. Accuracy and Acceptability of Wrist-Wearable Activity-Tracking Devices: Systematic Review of the Literature. J Med Internet Res. 2022;24(1):e30791. Published 2022 Jan 21. doi:10.2196/30791 https://pubmed.ncbi.nlm.nih.gov/35060915/
Sartor, F., Gelissen, J., van Dinther, R. et al. Wrist-worn optical and chest strap heart rate comparison in a heterogeneous sample of healthy individuals and in coronary artery disease patients. BMC Sports Sci Med Rehabil 10, 10 (2018). https://doi.org/10.1186/s13102-018-0098-0
