How do fitness trackers work? How accurate are they really?

December 4, 2020

Technology makes it easier than ever to stay healthy, but how do fitness trackers work and how accurate are the measurements they take?


A woman using her fitness tracker during a workout  
Fitness trackers have become the go-to accessories over the last several years, and it seems that everyone is wearing some sort of smart device to help them reach their health goals and maintain healthy habits. These devices range from smart watches to purpose-built health trackers, and they are capable of tracking a variety of data that allows users to better understand their own habits and activities. But, how do fitness trackers work? Our guide explores their various functions and the accuracy of these devices.

How do fitness trackers measure calories burned?


If you are on a mission to count calories, then a fitness tracker can be one of the most useful tools at your disposal. However, you may be wondering how they actually track the calories that you consume and burn each day. Here are a few factors that your fitness tracker will take into account when measuring how many calories you have burned during a specific time period:

Your basal metabolic rate


When you first set up your fitness tracker, you will enter basic information about yourself, including your age, gender, height and weight. This information will be used to calculate your personal basal metabolic rate, and then that rate will be used in your device's algorithm to calculate the number of calories you are burning each day.

Your activity level


Your smart watch or fitness tracker will have an accelerometer installed in it, and this will track your activity level each day. The data collected by the accelerometer will be used by the algorithm when calculating the number of calories burned.

Your manual entries


The accuracy of your fitness tracker will depend largely on the information that you manually enter into it. That's why it's essential to be as detailed as possible when inputting information, such as the food that you have consumed or the activities you have participated in throughout the day.

The algorithms that are used to calculate the number of calories burned will vary based on the fitness tracker that you purchase, so you will want to read up on the specific device you are considering before you make your final purchase.

How do fitness trackers measure heart rate?


Have you ever noticed a strobing green light on the underside of your smartwatch or fitness tracker? It is this small green LED light, in combination with a light sensor, that helps measure and calculate your heart rate through a process called photoplethysmography (PPG). 

Because your blood actually absorbs green light, your tracker is able to determine your heart rate by measuring the difference in light absorption at any given time. While this is a pretty useful trick, the technology is not always the most accurate way to measure heart rate - more on that in a moment.

How do fitness trackers count steps?


Those who are avid fitness tracker wearers can often be heard talking about how many steps they have taken in a day or what their step goal is for the next day. For many, it's a great way to stay motivated, and may result in friendly competitions with their friends, coworkers or family members. So, since counting steps is such an important function of the fitness tracker, it's important to understand how these devices actually count the number of steps that you take in a day.

Every fitness tracker, regardless of its make or model, includes an accelerometer. This innovative device tracks your movements throughout the day, and it interprets those movements based on the initial data that you provide when you set up your fitness tracker. The accelerometer not only counts your steps, but it also will record the pace of your steps and estimate the number of calories you burned while taking those steps.

How do fitness trackers know when you are asleep?


Another hallmark feature that many fitness trackers promote is their ability to tell you how long you slept — and even how well you slept. This often begs the question, how does a device know when I am actually sleeping or when I have entered into a state of deep sleep?

Again, the fitness tracker uses its accelerometer to track your movements; it detects when you have bee laying still for a period of time and begins counting that as sleep. 

While some of them claim to track the various stages of sleep, it is difficult to do this with the accelerometer alone - you don't move any more or less in the various stages of sleep. Sleep stages are actually a phenomenon that takes place primarily in the brain, and so accurately measuring the different stages requires devices that measure brainwaves or monitor eye movement in the case of REM sleep. 

Some trackers use heart rate monitors in addition to the accelerometer, claiming that different stages of sleep correlate with different heart rate patterns. However, this claim is disputed by some sleep scientists

With that in mind, you should be wary when relying on your fitness tracker to help you manage your sleep or detect sleep disorders. They can be helpful for giving you a general sense of how much sleep you're getting in a given night, but little else beyond that.

How accurate are fitness trackers?


The accuracy of your fitness tracker is going to depend on several factors, including:

The model you purchase. Different fitness tracker models use various algorithms to calculate their data, so the accuracy of your data may depend on the model that you purchase.

The data you provide. Most algorithms rely on you to input accurate data about yourself. With that in mind, the more accurately you can answer the questions it asks, the more accurate the measurements will be. 

How accurately do fitness trackers count calories burned?


As we discussed, your fitness tracker measures the calories you burned based on two things: The measurements taken with its internal accelerometer and your basal metabolic rate, which is determined by algorithms and the data you provide.

All that is to say that they rely solely on the information you give them and movement to estimate the number of calories you've burned. While these can give you a good general idea of the calories you burned, there are better, more accurate ways to do so. Measuring your skin temperature, respiratory rate, heart rate, and galvanic skin response (how much you sweat) can provide a much more exact measurement. Apart from your heart rate, these measurements are simply not yet available on current fitness trackers. 

How accurately do fitness trackers count steps?


Fitness trackers are fairly good at counting steps. However, the best location to place a step counter is actually your hip - not your wrist. Smartphones in your pocket are actually much more accurate for measuring steps. Of course, you don't always want to have your phone in your pocket, so a tracker on your wrist is the next best thing. 

Beyond that, people with slower movements, or an uncommon gait when walking, might have their steps undercounted. To test how accurate your fitness tracker is at counting steps, dedicate one day to an experiment: Wear both your fitness tracker and your smartphone (in your pants pocket if possible) and compare your steps at the end of the day. The reading from your phone should be closest to your actual steps, so you can determine how accurate your wrist device is based on that.

Lastly, in order to give your fitness tracker the best chance at accurately counting your steps, it is recommended that you wear it on your non-dominant wrist. This will help prevent the tracker from confusing arm movement with actual steps.

How accurately do fitness trackers measure heart rate?


When it comes to measuring heart rate, your fitness tracker works best when you are at rest. However, when you're taking part in strenuous activity and start to sweat, it can begin to all out of sync. 

If you look at the heart rate on your fitness tracker in any given moment, it may not be right on the mark. But if your tracker, or the associated applications, keep track of your heart rate over a longer period of time, this average tends to be fairly close.

Understanding accuracy and what you can expect from your fitness tracker


Measurements are tricky. While it's easy to think of them in a binary way (i.e. they are either right or wrong) accuracy refers to how close to the "true" value a measurement is, and it is rare that any measurement is 100% accurate. That's why it's important to view any given measurement in context and determine what an acceptable level of accuracy looks like based on what you're hoping to achieve. 

Rather than asking whether or not fitness trackers are accurate, the better question is, are they accurate enough for your purposes? What are you hoping to get out of your fitness tracker? For someone who is hoping to get a general idea for their progress towards a more healthy lifestyle, fitness trackers are great. However, if you are an elite athlete trying to get specific data points, then a standard commercial fitness tracker probably isn't reliable enough. Some people hope that their fitness tracker can help them spot complex health problems, like cardiovascular issues or sleep problems. While some can certainly point you in the right direction, you should always consult a physician if you believe you have reason for concern. 

With a better understanding of the capabilities of fitness trackers and their overall accuracy, you can choose the smart device that is best for your lifestyle. When you select the right fitness tracker for you, such as a Fitbit, an Apple watch or a Samsung watch, you can start taking the next steps toward meeting your health goals.

A fitness tracker displaying heart rate

Since 1896, Germania Insurance has been the Insurance Texans Trust for great coverage and outstanding customer service. But don’t take our word for it! Hear what our customers have to say

For more information about our insurance products, request a free quote online, or reach out to one of our trusted agents today!


Read more: Exercise is important, but getting to the gym can be a hassle. Check out our blog to learn how to put together a home gym!

by Geoff Ullrich

About the Author

Geoff Ullrich is a writer and Content Marketing Specialist at Germania Insurance.