You weight determines the number of calories you need to maintain that weight. With that number, you can calculate how many calories you need to gain or lose weight, too.
Simple multiplication will give you the magic number. Multiply your current weight by 15 calories per pound if you are moderately active. This will be the number of calories you need to maintain your weight.
A 115-lb. person needs approximately 1,752 calories a day to maintain weight. If you are relatively inactive, multiply by 10 calories per pound. This gives you 1,150 calories needed. To lose or gain 1 to 2 lbs. a week, you will need to decrease or increase your calories per day by 500 to 1,000. Women should not go below 1,200 calories a day, nor men below 1,500 calories a day, unless under medical supervision.
You don't have to consume 1,752 calories every single day to maintain weight. Your weight naturally fluctuates from day to day, and as long as you average 1,752 calories per day over the week, your body weight will remain about 115 lbs. For example, if you splurge and have 2,000 calories one day, consume fewer calories the next day or over the rest of the week.