Chim-A-Lator 482020 20" x 20" Stainless Steel Spark Arrestor with 3/4 Mesh is the perfect way to help a chimney function at a optimum level. This Chimney Cap keeps out all of the debris and protects the chimney from any disruptive weather conditions. The outside dimensions have a width from 18" to 20.25" and the length measures from 18" to 20.25".