Measuring a Diamond shape vent can be confusing to some.
It is just a square turned at a 45 degree angle, right?
So, why not measure it as a square?

Mainly because it is not a square.  It is a diamond.
And as such, it may measure out slightly wider or taller when measured correctly.
Squared measures will not show where the difference is.

Whether cutting a new opening or replacing an old vent, first is gaining access to the hole in the wall.
Measuring can be done from the outside of the wall.
Or if the attic access is enough to get to the back of the vent area, it can be done from inside the attic.

The true width of this opening is right across the middle.
Place the tape measure inside the opening to get the available space between the corners.

The height is the same right up the center.

This is called "opening size" on the order form.
For opening size, the vent will be reduced 1/4" per side dimensionally.
This is not 1/4" per measure.
This dimensional reduction will allow the back of the vent to slide into the opening for installing.

Common are the front flange and the no flange for "opening size".

Measuring the diamond vent itself is usually if the hole will be cut after the vent arrives.
The width is shown in red. 
These vent measures should be only of the vent.
Do not include any flange in the measures.

The center height is the same.
Hook the measuring tape on a corner and measure to the tip of the opposing corner.

These are called "tip to tip" measures since they are from tip to tip of the actual vent.
More commonly used for the back flange and the J channel vents.

The depth of the vent is determined by how much space there is behind the wall that is free of framing.
If there are studs in the way, they may not need removed for structural integrity of the wall.
In this case, a 1" or 2" deep may be all the wall will allow.

Where there is not obstruction to the open area, the 4" deep (3.5") and 6" deep (5.5") can be used.

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