I'm not even close to being Cathe, but this is what I know.
To determine your resting metabolic rate, that is, the amount of calories you need to simply breath, pump blood, and be alive: Multiply your weight by 10 calories per pound (or 22 calories per kilogram).
For example, if you weigh 120 pounds, you need approx. 1,200 calories to simply do nothing all day except exist.
Add more calories for general daily activity apart from your exercising. If you are moderately active throughout the day, add about 50% of your resting metabolic rate. If you are sedentary, add a little less than 50%; if very active, a little more.
For example, a moderately active 120 pound woman who requres 1,200 calories for her resting metabolic rate needs about another 600 calories for activities of daily living. This totals 1,800 calories per day - without exercising.
Add more calories for your purposeful exercise. If you are running, the general rule of thumb is 100 calories per mile, but more precisely, this depends upon your weight. If you weigh more than 130 pounds, add a few more calories. If you weigh less, subtract a few, or go by these guidelines:
120lb - 95 calories per mile
140lb - 110
160lb - 125
180lb - 140
Hence, if you are a 120-pound woman who runs five miles per day, you burn about 475 calories while running. This brings you to about 2,275 calories per day to maintain your weight.
If you want to lose weight, subtract 20% of your total calorie needs
I hope this helped. It helped me. I got it out of Nancy Clark's Food Guide for Marathoners, but it has a lot of good information about everyday needs also.
Good luck to you,
Christi