RV and Home White Stamped Steel Floor Diffuser/Register with Damper 12" X 2" (Duct Opening Size) Outside Dimension 13.5"x 3.75"

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 12“w X 2”h is for duct opening measurement only, actual outsize overall dimensions is 13.75“ wide by 3.75” high. If your duct opening size is 12” x 2”, 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.