What is a drop in vent grille? A drop in vent is designed to be inserted after the flooring has been installed. Simply cut out a hole where the duct is located and insert the vent. This particular vent is made for ducts sized at 4 inches by 12 inches. For precise measurements please take a look at the diagram above. Unlike a vent register, a vent grille is not adjustable and cannot be closed. Please keep that in mind when making your purchase. This drop in vent in the Carbonized strand bamboo style is also available in the 4 inch by 10 inch size here.