1. Remove existing vent. Measure the actual opening in the wall, ceiling or floor. (width is always expressed first, followed by the height.) It is very important that measurements are accurate.
2. If the rough opening measures 20 1/2" wide x 30 1/2" tall, then the correct size piece to order would be 20" x 30".
3. The actual vent or register's interior dimensions will be slightly smaller so the piece's frame fits into the recessed opening. The vents outer dimensions or flange will be larger. This will ensure the flange will fit flush on the finished surface and will completely cover the rough opening. The outer flange needs a 1 1/2" clearance around the roughed in opening.