calculating how many calories you actually need

So I have done a good amount of reading and calucations, and I keeping getting different numbers of how many calories I should take in in a day. Any suggestions? Generally I weight train or do a circuit (some sort of multi muscle cardio eg trx jump squats and some core workouts) in the am and I'm on my feet most of the day (not intense just moving around) then after work more often then not do something else like lane swimming, hula hooping, rock climbing, running or hot yoga. Weekends are normally more circuits and cardio. I follow a workout plan for weight training that switch's my routine every two weeks.
Any suggestions would be appreciated thank you