How many calories a day should I eat to lose weight?

There are a ton of calorie calculator websites out there. I have an app for my IPad to track my weight loss. No matter where I look I get a different answer on how many calories I should consume per day to lose weight. Anywhere from 1,000-2,000. I want to lose 2lbs per week and be safe about it but I don't know who to trust. Is there a good medically backed website I can get the info from?