How Fitness Trackers Count Calories
Firstly, lets be clear that Fitness Trackers only try to count the Calories that you BURN, not those that you eat*.
Secondly, Fitness Trackers do not directly measure Calories burned but provide an estimate based on the information you give them about your height, weight and sex, plus your level of activity that it has measured. Databases have been built up over time from experiments where many varied test subjects have exercised under laboratory conditions and their calorie burn versus effort closely measured. Its this data that is used by Fitness Trackers to estimate the calories that you have burned.
Fitness Trackers that measure Heart Rate, should be more accurate in their Calorie estimation than those that do not measure Heart Rate as they are able to directly see how hard you are working.
Some of the Apps that are supplied with Fitness Trackers allow you to search food databases and build up a picture of the Calories you have eaten. There are also websites that do this that are not associated with a Fitness Tracker manufacturer. Click here for a Google list of such sites (we do not vouch for them, merely present the link).
All day Calorie burn.
Your body burns calories all the time, regardless of whether you are working out or asleep. Some Fitness Trackers will incorporate this into their calculations, to give you a better idea of whether you are burning more calories than you are eating (meant to be good if you are trying to lose weight) or eating more than you are burning.
*A device was said to be able to track calories that you eat but it seems to have been a fake (that took in the BBC!). Read the BBC article on the calorie tracker here, and the story about the scam here.