This is probably the easiest shaped vent to measure.
Just measure in a few places to ensure the opening is squared.
Most won't be perfectly square.
Start with the hole in the wall where the vent will go.
By checking the width measure in 3 places, tells how square the rectangle opening is.
For the width, use the smallest measure taken.
It is a good idea to treat the height measure the same.
Take two or three measures and choose the smallest for the height.
These measures should be used as "opening size" on the order form.
We will reduce the vent 1/4" per side to ensure a good fit.
And to allow for expansion and contraction of the wall.
When using "tip to tip", the measures will be of the vent itself.
As the picture shows, do not include the flange in the measures.
This works for the square and rectangle shaped vents.
1 inch deep is 1" thick.
2 inch deep is 2" thick.
4 inch deep is 3.5" thick.
6 inch deep is 5.5" thick.
The 4" and 6" are known as commons for standard stud walls.
This is why they measure 1/2" smaller than their name.