How do sports watches count calories burned and how do you know which ones to trust?

Many people wear smartwatches or sports watches not only to count steps or measure their heart rate, but also for the information that is often the most motivating: how many calories did I burn today? But – how accurate are these numbers? Can you trust them, or is it more of a rough estimate?

In this article, we'll look at how watches count calories , what goes into the calculation , why brands can vary, and how to get the most out of this data.

What affects the calculation of calories burned?

Smartwatches don't guess a number "out of thin air." They use complex algorithms that take into account:

• Basic user data – gender, age, height, weight

• Heart rate – the higher the heart rate, the higher the energy expenditure

• Type and duration of activity – running, walking, strength training, cycling, etc.

• VO₂ max or fitness level – for more advanced models

• Heart rate variability (HRV) – for more accurate stress assessment

These factors are then processed by a computational model that tries to estimate the total energy expenditure as accurately as possible, i.e. not only the basal metabolic rate (BMR) , but also the energy burned during activity ¹ .

What technologies do the watches use?

1. Heart rate sensor

Most watches use an optical sensor (PPG) on the wrist, which uses light to measure changes in blood flow to the skin. From this, it determines the current heart rate , which is the main indicator of exercise intensity . More advanced models also allow connection to a chest strap , which measures the pulse of the electrode directly from the heart - this is more accurate, especially during more demanding or interval training ² .

2. Accelerometer + GPS

Motion sensors (accelerometer, gyroscope) record movement and speed. If you have GPS , route, pace, and elevation data make the calculation even more precise – for example, running uphill burns more calories than running on the flat.

3. Metabolic equations

Based on the data above, the watch uses various equations – such as the Harris-Benedict equation for basal metabolic rate, or complex models such as Firstbeat Analytics , which is used by brands such as Garmin. These models connect data on heart rate, training intensity and the user's physiology ³ .

Can I trust the data from the watch?

It depends on the brand, model and type of activity. For example , calm cardio (running, brisk walking) is usually measured by a watch quite accurately - with a deviation of about ±5–10% compared to laboratory reality. For strength training or activities with variable intensity (HIIT, circuit training), the deviation is usually larger - often up to ±20%.

The quality and placement of the sensor also matters. Wrist measurements can be affected by sweat , cold, or a loose strap. If you want accuracy, it's ideal to add a chest strap to your watch ² .

Why do calories vary between brands?

Each brand (Garmin, Polar, Apple, Fitbit…) uses its own calculation algorithms and different approaches to measurement. Some rely more on GPS, others on VO₂ max or exercise indices . This means that two different models can show you different calorie expenditure for the same activity – and yet both numbers can be considered correct within their calculation system.

What does "active" vs. "total" calorie expenditure mean?

• Total calories = calories burned throughout the day (including sleeping, sitting, walking, training)

• Active calories = only those calories you burn beyond the scope of normal life – e.g. during exercise

When planning weight loss or adjusting your energy intake, it's good to know whether you're working with total or active expenditure ¹ .

How to get the most out of data?

• Have your personal information set correctly – age, weight, gender, height

• Don't overestimate the numbers when it comes to strength training – take them as a guide.

• Use GPS + chest strap when running (if you have one)

• Focus on the long-term trend , not one specific number.

• Calories aren't everything – also track heart rate, VO₂ max, HRV and recovery

Conclusion

Sports watches won't give you the absolute truth, but they will offer a very good estimate. The better the model and the better the input data, the better the accuracy. The greatest power of this data is not in a single number, but in long-term monitoring of trends . If you want to improve your performance, lose weight or optimize your training, a watch is a great helper - if you know how to use it.

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