This is a very broad number so I would take the number from this formula and undershoot it to make sure you are going to lose weight. The great thing about figuring out this number is that you will know how much to eat if you work an office job all day and don't have time to work out all the time. This is just one way to calculate the BMR which is your Base Metabolic Rate.
BMR =For Men: 66 + (6.23 x your weight(lbs)) + (12.7 x height(inches)) - (6.8 x age(yrs))
BMR =For Women: 655 + (4.35 x weight(lbs)) + (4.7 x height(inches)) - (4.7 x age(yrs))
For an automated way please visit http://www.bmi-calculator.net/bmr-calculator/
The BMR is the base to your end calculation which is to figur how many calories you need based on your lifestyle. You can do this by simply taking your BMR and times it by 1.2 (sedentary - no exercise), 1.375 (exercise 1-3X), 1.55 (exercise 3-5X), 1.725 (6-7X). These numbers are based on how many times you workout in a week.
So putting it all together you simply need to count calories everyday to lose weight. Calories in and calories out. If you create a deficit you will lose weight, just don't create too much of a deficit. For example if I am to consume 3000 calories with my lifestyle I wouldn't cut back anymore then to 2500 calories a day. After awhile you will get to a point where you won't need to look at the back of boxes to figure out the calories. Good luck to you.