** shamelessly stolen from an explaination given to me at my fave diet site **

Your Basal Metabolic Rate (BMR) is the amount of calories your body (given normal metabolic conditions) burns in a day, given that you do nothing more than exist. No eating, no walking, no getting out of bed. These are your ‘existence’ calories. To calculate your BMR, use these formulas:

Men BMR = 66 + (6.3 x Body Weight in lbs) + (12.9 x Height in inches) – (6.8 x age)
Women BMR = 655 + (9.6 x weight in kilos) + ( 1.8 x height in cm) – (4.7 x age)

YOU SHOULD NOT AIM TO EAT LESS THAN YOUR BMR!

Now that you’ve got your BMR, you’ve got a good starting point for how your body (at your current weight) burns calories. Now it’s time to use that information and figure out how many calories you burn during a normal day. You know, when you crawl out of bed and actually do something. Take the BMR, and multiply it by the Activity Multiplier below that best describes your average day:

Activity Multiplier
Sedentary = 1.2 (little exercise, desk job)
Light Activity = 1.375 (light exercise/sports 1-3 days/wk)
Moderate Activity = 1.55 (moderate exercise/sports 3-5 days/week)
Very Active = 1.725 (heavy exercise/sports 6-7 days/week)
Extreme Activity = 1.9 (heavy exercise/sports, physically demanding job)

Now, if you’re trying to decide where you fall – say you’re thinking you may be between light and moderate activity – then pick a number in between. Remember, this isn’t exact, so don’t stress too hard. So now you have a rough estimation of how many calories you use during the day, courtesy of the Harris Benedict method.

To figure out how many calories you want to take in every day, you need that first number from earlier, the one about lbs/week. If you want to lose 1lb a week, you need to cut 1lb worth of calories per week. A pound of fat is equivalent to about 3500 calories. Luckily, that divides nicely by the number of days in a week, so subtract 500 cals from your Harris-Benedict number, and that’s what you need to take in. It’s that easy. Again, even if it means having a deficit of less than 500, don’t eat less than your BMR.

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