Kompell Aire Return Grille listed size 20” wide and 16” tall is for the duct opening measurement. Surface outside dimension is 21.75” wide and 17.75” 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 20" wide by 16" tall, buy a 20x16 grille; If the hole is 16" wide by 20" tall, buy a 16x20 grille.