1. Determine your Resting Metabolic Rate (RMR) – the energy you need for basic life functions. Multiply your body weight by ten. So, if you weigh 150 pounds, 150 x 10 = 1,500. Your RMR is 1,500.
2. Determine your overall activity level (not including exercise). If you're very active, add 60% - 70% to your RMR. If you're moderately active, add 50%. And if you're generally sedentary (desk job), add 30% - 40%. Since you're saying your moderately active, we’ll use the middle of the range. So 1500 + (1,500 x 50%) = 1500 + 750, or 2,250.
3. Add your workout calories. Figure 100 calories per mile (running or walking) -- this just is an approximation, but should be fairly accurate regardless of your pace. If you're running four miles a day, then 4 x 100 = 400, and 2,250 + 400 = 2,650. So, on days that you're running 4 miles, you need about 2650 calories to maintain your weight and properly fuel your workouts. Obviously that will change if you're running more or less.
More Information About Daily Calorie Requirements
If you're hoping to lose weight with running, keep in mind that a pound equals 3,500 calories. So you would need to create a 3,500 calorie deficiency with exercise or calorie cutting. A safe, healthy weight loss rate is about one to two pounds per week. If you lose weight faster than that, you may be losing muscle in addition to fat.
To average one pound lost per week, you need to create a 500 calorie deficiency, either through exercise or calorie cutting, per day. So, in the above example, if the runner continues with running 4 miles a day and limits her calorie intake to 2,150 calories, she'll lose about a pound a week.
Keep in mind that if you do lose weight, you'll need to recalculate your daily calorie needs since your lower weight means you won't need as many calories.