Measure the inside pipe diameter to determine your correct size.
The 6" - 8" round caps have an outside bottom collar diameter of 12 1/2" The 9" - 11" round caps have an outside bottom collar diameter of 15 5/8". The 12" -14" round caps have an outside bottom collar diameter of 19". This bottom collar is what pushes down onto the top of the chimney pipe as you are sliding the inside collar (matches the inside diameter of your chimney) into the inner pipe of the chimney.
Ecommerce Solutions by Mountain Media; "ICP Weathershield Solid Pack Metal Chimney Caps")