Chim-A-Lator 351818 18" x 18" Black Galvanized Square Spark Arrestor with 5/8 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 16" to 18.25" and the length measures from 16" to 18.25".