8mm diameter. 635/BOX - used for inhalation of substances and are a safer alternative to pipes made of plastic, pop cans or copper which can release toxic fumes or burst/crack when heated or burned. For use with ¼ “ x 3/8” Small Diameter Plastic Mouth Piece/Vinyl Tubing or 5/16” x 7/16” Medium Diameter Plastic Mouth Piece/Vinyl Tubing.