How many calories a day will it take to gain 5 lbs in one month?

In general, to gain 1 lb of body fat you need to create a calorie surplus of 3500 Calories. For example, if you intake 2500 calories a day and expend 2000, that's a surplus of 500 calories a day. At that rate you will gain 1 pound a week (7 days x 500calories/day = 3500 Calories) You need to consider your natural body metabolism and how much you exercise, and compare that to how much you eat.