Shoemaker 425 Series 16" x 16" White Step Down 4-Way Diffuser with Damper (For 16" x 16" Hole) Actual overall outside dimensions measure 20-1/8" x 20-1/8." Steel construction; for evaporative coolers or central blower systems. Modern step-down design with 4-way air diffusion. Countersunk mounting holes for flush appearance with color matching screws.