First thing to know about size selection: take the vent cover off the wall or ceiling. Measure the duct opening where the vent can fit in. Use the measurement to determine right size to buy. This product has listed size 10“w X 8”h is for duct opening measurement only, actual outsize overall dimensions is 12“ wide by 10” high. If your duct opening size is 10” x 8”, this is the right size and will fit in well. If you measure outside or surface size, you will get a large one and it will not fit.