-Specify opening size: 18" square, 24" square, 36" square, or 48" square. Hemmed End.-Other sizes available upon request. Maximum size is 72"x48"-Available in stainless steel and galvanized.-If a flange is needed, please see transition pricing.HOOD DeSIGN BASIS:150 fpm capture velocity2 ft. distance from source*Note: minimum L=.75(B). If less, square to square transition will be provided. 7"minimum L allowable for Sq to Rd and 4" L minimum allowable for Sq. to Sq.