How Do Smartwatches Work? (Complete Guide, Different Sensors & More)

You may be considering buying a smartwatch, but just how do they work? Smartwatches are far beyond analog watches. They have the availability to show you your calls and text, tell you what direction to walk in, play your favorite music, track your steps, and show you the weather forecast, but just how is this possible?

On this page, I’ll tell you how smartwatches work, all of the features along with the biometrics that give you such valuable health data.

What is a Smartwatch?

A smartwatch is an internet-connected, otherwise known as Internet of Things or IoT, watch that is connected to your smartphone. I like to think of it as a wearable extension of your smartphone that communicates with it directly via either Bluetooth technology or a cellular plan, just like your phone itself.

To connect my Google Pixel Watch 3 to my Android, for example, I had to make sure I had Bluetooth enabled on my phone, then download the Wear OS app and follow the instructions and pair the devices. From there, I could use my smartphone for a ton of actions, including:

• Calculator
• Calls and texts
• Fall detection
• Fitness data
• Health data
• Music
• Navigation
• Payments
• Reminders
• Sleep data
• Weather
• Voice assistant

Depending on the smartwatch, it could actually replace other devices, like a medical alert system with fall detection or a fitness tracker that keeps track of your steps.

I love taking my smartwatch with me on runs so I don’t have to carry my bulky phone and can track my steps and play music, hands-free. It also comes in handy when I need to find my way around a new town, or to check the weather. And if there are any important calls or texts, I’ll still get notified.

How Different Parts of a Smartwatch Work

Smartwatches might look small, but they pack in a lot of smart technology. Here’s how everything comes together behind the scenes.

The Basics

• Smartwatches run on a small rechargeable battery — usually lithium-ion. A built-in chip called a power management IC controls how the battery charges and powers different parts of the watch.
• The main chip inside (called a System-on-Chip, or SoC) handles everything — from running apps to processing data from sensors.
• Your smartwatch connects to your phone through Bluetooth Low Energy (BLE). The first time you use the watch, you pair it using a companion app on your phone. After that, the watch and phone talk to each other in the background — sending data, syncing notifications, and keeping settings up to date.
• Your phone sends notifications (texts, calls, reminders) to the watch using Bluetooth or Wi-Fi. The watch displays them instantly. Some watches even let you reply directly using voice, prewritten replies, or an on-screen keyboard.
• The watch has a basic operating system (like watchOS or Wear OS). It can run small versions of apps—fitness trackers, weather, alarms, timers, music controls, and more. Some watches support voice assistants like Siri, Google Assistant, or Alexa.
• Software updates are usually sent to the watch through the phone app. These updates can add new features or fix bugs. Your data is also protected Bluetooth connections are encrypted, and pairing is secured using digital keys.

How a smart watch tracks your movement

The watch uses two main sensors to understand your movement:

• Accelerometer: Detects steps and movement by measuring how fast the watch moves in different directions.
• Gyroscope: Detects rotation, like when you tilt your wrist to check the time.

Together, these sensors help count steps, track workouts, and even detect falls in some models.

How a smart watch measures your heart rate

• The watch shines a green LED light into your skin.

• A light sensor measures how much light is reflected back.

• Since blood absorbs green light, the watch can detect changes in blood flow and use that to estimate your heart rate.

This technique is called photoplethysmography (PPG).

How GPS Tracking Works

GPS watches include a tiny chip that listens for signals from satellites.

By calculating how long signals take to arrive, the watch figures out your location, speed, and distance — especially useful for outdoor workouts like running or biking.

How Blood Oxygen (SpO2) Sensing Works

Similar to heart rate tracking, the watch uses red and infrared light.

It looks at how much light passes through your skin to estimate how much oxygen is in your blood. This data can help spot issues with breathing or sleep.

How a smartwatch measures your sleep

While you sleep, the watch uses its accelerometer to detect how much you move during the night. It also monitors your heart rate and sometimes your blood oxygen levels.

By combining movement and heart rate patterns, the watch estimates when you’re in light, deep, or REM sleep, and when you’re awake. Some models use machine learning to improve accuracy based on your personal patterns over time.

How a smartwatch counts calories

Smartwatches estimate calories burned using data from your heart rate, age, weight, height, and gender.

When you’re moving, the watch tracks how active you are (e.g., walking, running) and how intense your effort is based on your heart rate. It also calculates your resting metabolic rate (RMR) — the calories your body burns just to stay alive — and adds your activity-based calories on top of that.

How a smartwatch knows what exercise you’re doing

The watch analyzes your movement patterns, pace, heart rate, and sometimes GPS data. If you’re swinging your arms and moving steadily, it might recognize walking.

Faster, rhythmic motion with elevated heart rate might trigger running or cycling. Some watches support automatic workout detection, and others let you manually choose the activity from a list.

Over time, the watch gets better at identifying your workouts by learning your movement and heart rate patterns during each type of activity.

Are these biometrics accurate?

We’ve gone over the many sensors that detect biometrics, but just how accurate is this data? The answer is, it depends heavily on the make and model. But what is the exact range of accuracy?

Fortunately, there have been many recent studies on this exact topic. A 2024 study published in Global Cardiology Science & Practice looked at the ability of smartwatches to accurately monitor oxygen saturation, heart rhythm, and pulse.

Combining other studies, it found that, of 400,000 participants who wore Apple Watches that detected the heart’s electrical activity, over half received notifications that they had irregular pulses.

Of the percentage of people who returned their ECG (electrocardiogram) patches, 84 percent had atrial fibrillation (AF), a type of heart arrhythmia, meaning that the sensors were right eight out of ten times.

Another study found that Apple Watches detected 96 percent of AF, while another study of Samsung devices found it to be 94 percent sensitive to AF.

Another study published in March 2024 by the Mayo Clinic looked at patients in the hospital due to COVID-19, seeing if smartwatches could detect significant hypoxia, insufficient oxygen at the tissue levels, using SPO₂ sensors.

Out of 750 pulse oximetry measurements from 200 patients, the Apple Watch had an accuracy rate of about 85 percent, while the Withings ScanWatch had a lower rate of about 79 percent. Overall, smartwatches weren’t accurate enough to replace the medical technology doctors use to detect hypoxia.

In sum, biometrics can vary between devices, and you should never use a smartwatch as a substitute for medical care.

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