Have you ever glanced at your smart watch after a workout and wondered, “How does it actually know how many calories I’ve burned?” It seems almost magical—this tiny device on your wrist claims to measure the energy your body has used, without ever looking inside. But there’s real science behind that number, and it’s not just a random guess.

At its core, calorie tracking on a smart watch is all about estimation. Unlike lab equipment that measures oxygen intake or carbon dioxide output, your watch relies on sensors and algorithms. It starts with your personal data: age, gender, height, and weight. These numbers help calculate your Basal Metabolic Rate—the calories your body burns just by existing. Then, the watch adds movement data.

Most watches use an accelerometer to detect motion. Some also include a gyroscope and heart rate monitor. When you walk or run, the watch tracks steps and pace, and estimates calorie burn based on known averages. But here’s where heart rate changes the game. By measuring your pulse, the watch understands how hard your body is working. A faster heart rate usually means more energy being spent, especially during activities like cycling or swimming where steps don’t tell the full story.

Still, these are estimates, not facts. Studies show that most smartwatches are reasonably accurate for steady-state cardio like running, but less reliable for weight training or intermittent sports. Sweat, loose wristbands, and skin tone can all affect heart rate sensor accuracy. And no watch can measure muscle fatigue or metabolic efficiency—two things that influence real calorie burn.

So, should you trust the number? Yes—but with perspective. Your watch isn’t a laboratory, but it is a consistent tool. It may not be 100% precise, but it gives you a reliable baseline to track trends over time. Use it to compare one workout to another, not to justify that extra slice of cake.

In the end, your smart watch doesn’t just count calories. It counts commitment. And that’s worth more than any number on the screen.