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How to Size Your Register Box, by Ryan from


Ryan at
July 25, 2013
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It can be confusing when you go to cut in your new diffuser for your HVAC system. What size Ceiling box do you buy? What size grille do you buy? What size should the hole be? How do you connect the air conditioner duct?

For my example I'm going to use an 8" x 8" x 8" sheet metal fitting called a C Box just to layout the dimension you'll need. The first two dimensions are always the length of the sides of the duct fitting, and the round collar is always the third dimension. 

The plaster flange is going to add about one inch to each side. This flange is supposed to go above the sheet rock. If you're installing the C box or B box snug against a rafter, it's best to nail the box directly to the framing. from the inside. Also, don't forget to spray paint the inside of your box black before installing. This makes it so you can't look up through the register and see shiny sheet metal.

The diffuser is also sized off the dimension of the hole. In other words, an 8" x 8" diffuser will fit a 8" x 8" x 8" C Box. The visible side of the diffuser is purposely oversized to hide the hole more completely. Below Pictures are of a 8" x 8" diffuser.

Thanks for Reading!