how do smart watches and fitness tracker measure the calories you burnt throughout the day/exercise?

Question

They might tell you, you burnt 2000kcal today or 300kcal during this sport training. But how do they get these numbers?
And afaik studies showed that they are incorrectly measured. How do the experts check that the data is correct? I mean you need the real numbers but for that you used the smart watch (or similar device).

In: 1

Answer 1

They don’t. They make guesses based on the data available to them and the algorithms that were programmed into them.