16 gauge mild steel. Powder coated for indoor or outdoor use. Item does not come with mounting hardware. 12 – 11.5″ x 11.25″. 15 – 14.5″ x 14.25″. 18 – 17.5″ x 17″. 24 – 23.5″ x 23″. 30 – 29.5″ x 28.75″.