total height: about 49"
circumference of top chimney cap where it goes over the duct: about 45.5"
bottom of duct: about 47"
So with math, this is chimney cap where it goes over the duct is about = 45.5" / 3.15" = 14.5" diameter chimney cap.
Bottom of duct that touches the floor is about 47" / 3.15" = 15" diameter.
The chimney cap is currently held onto the duct with screws. See pictures.