Kompell Aire Return Grille listed size 16” wide and 12” tall is for the duct opening measurement. Surface outside dimension is 17.75” wide and 13.75” tall. Size Selection Guide 1. Measure the size of the air duct opening(the size of the “hole” ). 2. Select the listed size that match the duct opening size. 3. The outside or overall size is 1.75” larger for both width and length so it can cover the “hole”. 4. Do not use outside measurement for size selection or it would be too bit to fit. Width vs HeightThe first measurement in a return grille size is the width(horizontal measurement), the second measurement) is the height(vertical measurement). For example: if the hole on the wall is 16" wide by 12" tall, buy a 16x12 grille; If the hole is 12" wide by 16" tall, buy a 12x16 grille.