Here is a cool tool I found.This tool does not only calculate your daily calorie requirements, but also it can estimate the amount of proteins, carbohydrates and fats you should eat a day to lose weight(or to maintain your current weight or to gain weight, depending on what you choose as your "goal")
Please note that The American College of Sports Medicine (ACSM) suggests that calorie levels should not drop below, 1200 calories per day for females and 1800 calories per day for males.