The nominal diameter of pipes with an outer diameter of 28 mm is 25 pipes and 1 inch pipes. The outer diameter of the pipeline is 28 mm, and the inner diameter is 25 mm. Nominally, the domestic galvanized pipe is marked with the inner diameter, the inner diameter is 25 plus the wall thickness 1.5, and the overall outer diameter is 28 mm. The commonly used water pipes are divided into galvanized iron pipes and plastic pipes, and the metal pipes are nominal according to the inner diameter of the pipeline. Plastic pipes are usually named after the outside diameter of the pipe.