It is best to check the dirt level in the bag before each use. For best cleaning performance, it is recommended that you change your bag when it is ½ to ⅔ full. When accumulated, small dust particles can keep the air from circulating, which can reduce the suction in your vacuum. You may need to change your bag more frequently when vacuuming drywall dust, pet hair, other fine particles or new carpet. Some bagged units also offer a check bag indicator to make it easier for you to know when to change the bag.