It is best to check the dirt level in the bag before each use. For best performance, it is recommended that you change your bag when it is 1/2 to 2/3 full or when the outside of the bag feels firm to the touch. When accumulated, small dust particles can keep the air from circulating, this can reduce the suction in your vacuum. You may need to change your bag more frequently when vacuuming fine dust and debris, pet hair or new carpet.