Correct way to measure header collector?

[lodnirgs]

What is the correct way to measure the header collector flange? They are offered in 2 1 /2", 3" and 3 1/2". Is this the diameter of the opening? Is it the bolt diameter? I have headers now and need new collectors and gaskets but I am uncertain of the size I have. Thank you for your help.

 

[broncnaz]

It goes by pipe size

 

[TJK74]

Diameter of the tube/pipe opening, Not the bolt size.