How Smartwatches Track Your Steps and Calories Accurately

Smartwatches have gone from basic timepieces to very health-oriented accessories which we wear daily. We put them on as we go about our day to track our activity from the time we are at the office to when we are out running or in the gym working out. What we see the most of are the step counters and the calorie burn features. Many users check their watch at the end of the day to see the results of how many steps they took and what their calorie burn was. What makes these small devices on our wrist so accurate in that detail? We will break down the science, the sensors, and the smart engineering which makes it all possible.

Step Tracking: Foundations.

At the core of step tracking is motion detection. As you walk, your body goes into a rhythm: your arms swing, your torso moves, and your legs fall into step. A smartwatch uses sensors which in turn use that info to count your steps.

The main component which does the job is the accelerometer, a small device that measures output of movement and direction. As you move your wrist in a motion that represents walking or running, that is detected by the accelerometer, which then records it as a step.

Also, motion by itself is not enough. For instance, when you are sitting and waving your hand, cooking, or even brushing your teeth, your wrist may be moving but you aren’t walking. That is when extra sensors and algorithms are used.

The Role of the Accelerometer

In all directions including side to side, up and down, and also forward and back. As you walk, these patterns of movement repeat themselves, which the watch software uses to determine if what is being detected is in fact a step.

For example, a walking step we see as a subtle up and down movement of the wrist in association with a forward motion. The accelerometer notes this routine pattern. Once the device identifies the rhythm, it includes that into your step count.

Gyroscope and Its Importance

In order to improve accuracy, many smartwatches also use a gyroscope. This sensor measures orientation and rotation. By the use of both accelerometer and gyroscope data, the watch is able to distinguish between random hand movements and actual steps.

For instance, if you are stirring a pot of soup, your wrist may move in a circular pattern that the accelerometer may misinterpret as steps. But with the gyroscope’s input, the watch notes that the movement is not that of walking or running. This cross-check reduces false step counts and improves the accuracy.

Advanced Motion Detection

Today’s smartwatches go beyond tracking up and down motion. They study your gait—that is, your distinct way of walking. What we see is that each person’s stride length, speed, and rhythm is different, but what your smartwatch does is it gets to know your personal walking pattern over time. This is why we see step counts are more consistent on the latest devices as compared to the older fitness bands.

Also, in the case of watches, they consider context. If the GPS reports that you are traveling along a road at a walking speed, and the accelerometer reports rhythmic wrist movement, the watch has that much more reason to determine you are walking.

How Smartwatches Calculate Calories

Counting calories is a bit more complex than counting steps. We are looking at what your body uses for energy during activity and at rest. Smartwatches determine calories by using many data points:

1. Basal Metabolic Rate (BMR): This is what we term the number of calories your body uses for basic functions like breathing, circulation, and digestion at rest. Your Basal Metabolic Rate, which is what we are talking about here, is determined by your age, gender, weight, and height. When you set up your smartwatch and put in this info, what you are doing is providing the base which then the device uses to do its calorie calculations.
2. Activity Tracking: Once your BMR is determined, the watch reports calories burned through movement, which includes walking, running, climbing stairs, or any other physical activity.
3. Heart Rate Monitoring: Today most smartwatches include optical heart rate sensors. What these do is they put out a light which passes through your skin to measure blood flow, which in turn they use to determine how hard your body is working. Also, you’ll see that as heart rate goes up, energy expenditure goes up.

Through the use of BMR along with heart rate data and motion detection, the watch presents you with a custom calorie burn report for the day.

The Science of Heart Rate Sensors

Heartbeat is a major factor in calorie count. As your heart rate goes up, your body uses more oxygen and burns more energy. Smartwatches which have PPG technology use that to track heart rate.

This is done by running green light through your skin. What we see is that blood absorbs green light better than adjacent tissue, which in turn causes the amount of reflected light to change with each heartbeat. The sensor used to measure these changes reports your heart rate in real time.

Through analysis of your heart rate during physical activity, the watch is able to determine which extra calories you are burning above what you burn at rest.

GPS and Activity Recognition

Some smartwatches also have GPS, which improves accuracy. When you are out for a walk or a run, the GPS logs distance and speed. In combination with stride length, it gives you a better picture of what you are burning in terms of energy.

For instance, we see two individuals that logged 5,000 steps each but may have had very different experiences—one was walking at a slow pace and the other was climbing a hill, which was of a faster pace. Without the use of GPS and heart rate monitors, both watches may report the same calorie burn. But with the addition of these features, the smartwatch is able to tell the difference and will adjust the calorie output.

Filtering Out False Data

Accuracy is a result of good software as well as sensors. We see in smartwatches which constantly improve their algorithm to reduce false positives and better calculate step count and calorie burn.

• Idle Detection: If you are moving around but your GPS reports that you are still, the watch will correct that out.
• Sleep Tracking Integration: At night, wearables go into a different mode, which means restless nights in bed won’t count toward your step total.
• Activity Recognition: Some watches track that you are running, cycling, or climbing stairs and adjust their calorie burn stats.

The Role of Stride Length

Stride length is also a factor in step and calorie accuracy. As a rule, taller people have longer strides, which in turn means they cover large distances in fewer steps. On the other hand, shorter people tend to take more steps to cover the same distance. Smartwatches that do this often use height and step pattern to determine stride length. Also, some watches have a feature which allows for manual input to increase accuracy.

When step length is determined, the watch does better at measuring distance. We see also that this is used to improve the report of calorie burn.

Why Accuracy Varies

Although we’ve come a long way with smartwatches today, they still fall short in some areas. Accuracy is an issue that presents itself in many forms:

• Wrist placement: Wearing the watch too loose may affect motion and heart rate results.
• Skin tone and tattoos: Darker skin pigmentation or wrist tattoos at times may affect heart rate sensor results.
• Type of activity: Activities such as cycling or weightlifting may not have the same level of accuracy as walking or running, which in turn causes different movement of the wrists.
• Individual differences: Each person’s energy use is different, which is why formulas only give estimates.

Despite issues at large, most modern devices do in fact perform well enough for daily use and personal fitness tracking.

Continuous Improvements

Every year there are better smartwatch models which we see in terms of improved accuracy. We see sensors which are more sensitive, heart rate monitors that do a better job at what they put out, and software updates which improve on calorie count. Over time, what we notice is that the watches’ estimates are closing in on true energy expenditure.

Some devices use your usage patterns to improve. They compare your exercise routines, heart rate responses, and calorie burn trends, which in turn causes them to fine-tune and better personalize what they do.

Why It Matters

Accurate logs of your steps and calories aren’t just about the numbers, it is about motivation and awareness. When you see how active you have been all day, you are more likely to choose the stairs, go for a run, or complete that final exercise at the gym.

The data we put out there is what holds us accountable. If your watch reports that you have only 3,000 steps, which is your daily step goal, you may go for a walk in the evening to make up the difference. Also, in the case of calorie counting, it gives you a better picture of energy in and out, which in turn guides health choices.

Final Thoughts

Smartwatches present as very put together and basic from the outside, which is true, but what is not true is what is going on in that little package inside. In that tiny form factor are powerful sensors and complex software. For step tracking, we have the accelerometers and gyroscopes which team up to identify walking patterns. For calorie tracking, they use heart rate monitoring, personal info, GPS, and activity recognition.

Although at present we have estimates which aren’t precise numbers, the accuracy is great for such small devices. Also, with each improvement, which is constant, we see smartwatches becoming better at tracking daily movement and energy expenditure. They turn what is invisible to the eye—like your body’s movement and how hard your heart is working—into information you can see and act on.

At present, each time you check in on your step count or calories out of habit, you are seeing the result of great sensor technology and thoughtful design which has been put together to bring your health into the light.