ICP Weathershield Air Insulated Metal Chimney Caps

Measure the inside pipe diameter to determine your correct size.
All of these caps have a lid and outside collar diameter of 16".
This outside collar is what pushes down onto the top of the outer chimney pipe as you are sliding the inside collar (bottom of the chimney cap and matches the inside diameter of your chimney) and squeezes into the inner pipe of the chimney while pushing it down into the pipe.