Unless you’re in a metabolic chamber (basically, a huge calorimeter that people can live inside), there is no accurate way to measure the calories you burn. The various calculations we use are all estimates that may be accurate for some people in some ranges of activity, but they get wildly wrong really quickly. That doesn’t mean calorie counting your exercise is useless, though. You can use it to measure your relative exertion over time (for example, seeing if you’re keeping up with your workout regimen week after week, or if you’re working harder one day than another). In this way, calories are as good (and bad) as any other metric you apply consistently to the same activity, whether it’s number of fitbit steps, number of miles jogged, number of minutes spent swimming, etc.