Here is the Harris-Benedict Formula:
weight in pounds x 11
IF you are under 30 years old:
multiply by 1.3 if you are fairly inactive, have fairly high body fat, or are new to lifting
multiply by 1.4 if you are moderately active, have a job where you are not sitting at a desk but are up and walking/carrying/doing physical tasks.
multiply by 1.5 if you are very active, participate in sports on top of your workouts, have been weight training for at least a year
(regularly
), or have pretty low body fat.
If you're 30 to 40, subtract 0.05 from these values, so you multiply by 1.25, 1.35 or 1.45 for sedentary, moderately active, and very active respectively.
If you're over 40, subtract 0.1 from these values, so you are using 1.2, 1.3 or 1.4 for your multiplication values.
For maintenance, keep the numbers you get. If your goal is weight loss, subtract between 500 to 1000 calories per day. If your goal is weight gain, add up to 500 calories per day.
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You'll also find VERY knowledgeable people here:
http://forums.jpfitness.com/cgi-bin/ultimatebb.cgi