The Harris Benedict Formula provides a means of calculating your daily calorie requirements for maintaining your current weight, based on your Basal Metabolic Rate (BMR) and your level of physical activity.
Little/no exercise: BMR * 1.2 = Daily Calories
Light exercise: BMR * 1.375 = Daily Calories
Moderate exercise (3-5 days per week): BMR * 1.55 = Daily Calories
Heavy exercise (6-7 days per week): BMR * 1.725 = Daily Calories
Very heavy exercise (very active, every day): BMR * 1.9 = Daily Calories
There are five different levels of daily physical activity to choose from, so it that way it is a very good guide, rather than an absolute definitive answer. For example, perhaps you would rate yourself somewhere in between two of the activity levels. However, you could easily make an adjustment and mutiply your BMR by a figure somewhere between your two activity levels to get a reasonable guide for daily calorie intake.