If you don't eat enough your body goes into starvation mode. Your metabolism will drop and you won't lose weight.
Here is an about.com link with information on how to calculate how many calories one should eat to maintain and/or lose weight.
Women:
655 + (4.3 x weight in pounds) + (4.7 x height in inches) - (4.7 x age in years)
Men:
66 + (6.3 x weight in pounds) + (12.9 x height in inches) - (6.8 x age in years)
Please note that this formula applies only to adults.
Calculate Activity
Step two: In order to incorporate activity into your daily caloric needs, do the following calculation:
* If you are sedentary : BMR x 20 percent
* If you are lightly active: BMR x 30 percent
* If you are moderately active (You exercise most days a week.): BMR x 40 percent
* If you are very active (You exercise intensely on a daily basis or for prolonged periods.): BMR x 50 percent
* If you are extra active (You do hard labor or are in athletic training.): BMR x 60 percent
Add this number to your BMR.
The result of this formula will be the number of calories you can eat every day and maintain your current weightt. In order to lose weight, you'll need to take in fewer calories than this result.
As you lose weight, you can re-calculate the formula to assess your new BMR.