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How Do Fitness Trackers Work? Your Questions Answered

Your fitness tracker sits there on your wrist every day, collecting data and relaying it to you. It tells you about your pace, your steps, your stride length, your heart rate. It maps your route. Higher-end models calculate your ground contact time, your vertical oscillation, your VO2 Max, and how effective your training is. If you already have one, you probably take your little tech marvel for granted. If you’ve been thinking about getting a fitness tracker, you might be wondering about all the intricate details. How exactly do they work? If so, this article is for you.

We’ll cover what fitness trackers do, how they use sensors and algorithms to track metrics, and how accurate they are. We’ll cover some other fitness tracker questions that you might have, too.

By the end, you’ll know exactly how they work and how you get health data that is so valuable in helping you to meet your goals to live a physically fit lifestyle.

What Do Fitness Trackers Do?

While fitness trackers can do a wide variety of things related to health and exercise, most people are interested in fitness trackers because they count steps. In order to get to the magic number of 10,000, you need a device that will count for you.

But they can do so much more than count steps. Fitness trackers can also monitor your sleep, tell you your heart rate, give you how many calories you’ve burned, list how many miles you’ve walked, keep track of your cycle (if you’re a female). The list goes on.

Basically, anything that you can think of that would help you achieve your health and fitness goals a fitness tracker can do. You can even put in food you’ve eaten and glasses of water drank to address that area of your well-being.

When Were Fitness Trackers Invented?

Fitness trackers were first invented in 1965 by Dr. Yoshiro Hatano, who was trying to figure out how to address obesity. He came up with the 10,000 steps number in order to balance calories consumed with calories that burned through exercise.

Since then, they have become increasingly popular. In the 1980s, Polar watches started to add heart monitors. Nowadays, you can sync fitness trackers to your phone and have all the information at your fingertips. Over 120 million fitness trackers were sold in 2019.

What Sensors Are in Fitness Trackers?

In order to gather information, various sensors are built into fitness trackers, which vary by product. These are the more common ones.


Technically, acceleration means a change in speed or direction. And that’s exactly what an accelerometer does: measures movement based on changes in speed and direction. The accelerometer will measure any direction that you move, so most of your steps will be recorded by this sensor. Typically, fitness trackers use a 3-axis accelerometer.


A gyroscope is used to measure rotation and orientation. When a 3-axis gyroscope is used, that means that a fitness tracker can get 6 degrees of motion. This is helpful in accounting for workout motions that are more complex.


The altimeter measures altitude, as the name states. You might think that’s a little weird to have in a fitness tracker if you aren’t going mountain climbing, but it’s useful to account for climbing stairs or those uphills you work so hard on.

Optical Heart Rate Monitor

This sensor measures your pulse by using light on your skin. By knowing the rate at which blood is being pumped, it can measure your heart rate. The faster it is being pumped (i.e., the faster your pulse), the higher your heart rate.


GPS stands for global positioning system, and this sensor measures distance based on GPS satellites. It’s done by triangulation and getting signals from at least three GPS satellites. As we know from using GPS in our cars, it’s great for directions and tracking where we have gone in relation to other places.

Other Sensors

Other sensors that fitness trackers may have include a temperature sensor, which can help your tracker estimate how challenging a workout was depending on how much your body heats up, or a bioimpedance sensor, that can collect heart rate data.

How Do Fitness Trackers Use These Sensors?

The raw data you get from these sensors is often data that you might want or need to know. Your device, though, translates the data into helpful information. Here are some examples:

How Do Fitness Trackers Measure Steps?

The fitness tracker inputs the information received from the accelerator and gyroscope. A personalized algorithm detects what you are doing, from standing still to walking to running fast.

The device usually tells you not only how many steps you’ve taken, but also at what speed. A couple extra calculations give you pace and your calorie count.

How Do Fitness Trackers Monitor Sleep?

First, you’ll probably have to put your fitness tracker in sleep mode so that your device knows that you’re asleep. Then it will be able to use the accelerometer (and gyroscope) to record movements you make when you sleep.

Then the device will use the programmed algorithms to try to make sense of the information recorded by the sensors. Taking into account heart rate and movement, it will describe how well you slept. Light sleep, deep sleep, and wakefulness all have their signatures. Your fitness tracker can show you everything once you wake up.

How Do Fitness Trackers Measure Calories?

This is probably the least accurate component of a fitness tracker because how many calories you burn is so specific to an individual. Typically, when you get a new fitness tracker, you’ll have to input information like your gender, height, weight, age, and some other lifestyle questions.

From this information, the tracker uses algorithms to figure out how many calories you burn resting as well as how many you’ll burn from exercise. The problem is that it’s less accurate for less intense activities like walking, as one meta-analysis found.

How Do Fitness Trackers Count Floors Climbed?

If your fitness tracker has an altimeter, it will detect when you’re going up in elevation. When you climb about 10 feet at one time, a fitness tracker typically registers that you have climbed one floor.

However, a true altimeter actually measures changes in barometric pressure. Some use GPS data and maps, but both methods have their limitations. If you use a stairmaster for example, it won’t register that you climbed any stairs. That being said, if you walk outside when the barometric pressure is changing (like a storm), it might say that you climbed stairs when you actually didn’t. In some cases, GPS-based elevation data fails to account for things like bridges over river gorges, and follows the land (or even river beds) instead.

How Accurate Are Fitness Trackers?

This can vary so widely depending on the type of fitness tracker you get and how personalized you’re able to make it. At the very minimum, a fitness tracker will give you a good idea of how many steps you’re taking, how many floors you’ve climbed, and so forth.

The easiest way to figure out the accuracy of your fitness tracker is to measure it against information that is known (like the distance from your house to a mile away) or counting calories on your own. You can compare and see how much it matches up.

Counting Steps

Counting steps tends to be one of the more accurate components of a fitness tracker. This is especially true if you’re wearing your phone in your back pocket or wearing a tracker on your hip. 

Otherwise, if it’s on your wrist, moving your wrist just the right way might be classified as a step. This is even true if it’s on your non-dominant wrist. I’ve experimented with the accuracy of fitness trackers myself and sometimes things like a push-up would be classified as steps.

Measuring Calories

As we hinted at above, calories burned vary so much from person to person. Your tracker will need to know accurate information about you in order to produce the most accurate data about calorie burn possible. 

If you have a fitness tracker that also uses heart rate and perspiration to figure out the number of calories burned, your fitness tracker will likely give you a more accurate measurement of calories.

Measuring Sleep

Sleep information can give you a broad overview. It likely is not going to be incredibly useful, though. It can be interesting to know how much time you spent in a particular sleep cycle. And it certainly can point to sleep problems. But you probably don’t want to rely on this information too much.

Your device provides a snapshot of what things are like when you’re sleeping. It can tell you how much you move, for example, and you’ll know if you were asleep or awake. This could be helpful information to know.

Fitness Tracker Questions

Now that you have a broad overview of fitness trackers, you might be wondering about the bells and whistles. We’ll answer those questions here.

Are Fitness Trackers Waterproof?

Some fitness trackers are waterproof, but this is definitely not the case for every model. Double-check that yours is one of those if you need waterproofing. For example, if you want to go swimming, some fitness trackers are designed to withstand water pressure.

That being said, a good number of fitness trackers are water-resistant, which means that you don’t need to worry if you get caught outside in the rain with your fitness tracker on. It just isn’t going to work for swimming.

Can Fitness Trackers Play Music?

Some fitness trackers can play and store music. Depending on what fitness tracker you get, you might need your phone nearby in order to listen to your tones. So you’ll need to look into specific fitness trackers to figure out which one is best for you.

A side note that you should keep in mind is that you’ll need to purchase Bluetooth headphones in order to be able to listen to your music. If you have onboard storage, you should be able to get 500+ songs on your tracker. If it’s Bluetooth only, you’ll stream music.

Should I Buy a Fitness Tracker With GPS?

If you want to make sure that your fitness tracker is the most accurate in terms of tracking distance, get a fitness tracker with GPS. Although it is less accurate on cloudy days or with tree cover, typically it gives you very accurate information on distance.

If you’re planning to use your fitness tracker as a serious runner, you’ll want GPS. That’s the reason I upgraded my Fitbit Charge to a GPS watch. I wanted to know exactly how far I had run.

Final Thoughts

Fitness trackers are a great tool in achieving your fitness goals. They encourage you to get out there and exercise. They may even remind you to move when you’ve been sitting down for too long. And the data provided can give you insights into your fitness journey.

While they are great for giving you a broad overview on your health, there might be some measurements that you shouldn’t take as fact. Calories burned, quality of sleep, and other metrics need to be taken with a grain of salt. Even distance covered can be variable day-to-day.

That being said, because fitness trackers are getting increasingly more complex and providing you with more information, they are becoming more and more personalized in giving you information that you can actually use.

Succeeding in exercise routines is all about seeing progress, and your fitness tracker will give you that every single day. Here’s to getting in those 10,000 steps every day!

Rachel Basinger
The Wired Runner