Take out the existing floor register and measure the length width of the opening in inches all registers will be listed in inches.
Proper size floor registers.
In order for your forced air furnace to work properly you have to have the right sizes of duct vents in each room expelling air.
The overall size of the house the size of the room and the numbers of window in the room all play a part in determining the size of the overall air conditioning unit and the number of registers.
The register shown above is a common aluminum adjustable curved blade one way 10 6 ceiling register.
It is an industry standard to measure the actual hole opening in the floor to get the correct size of floor register or grille that you will need.
The duct vent size will depend on the size of the furnace and the size of your home so you know how many vents you need in each room.
Air will move straight through registers and grilles.
How to measure floor registers and grilles.
Registers have the same inlet neck and outlet face size.