What is DN? DN stands for ‘Diameter Nominal’. Put simply, it is a rough translation of mm from imperial sizes, assuming that an inch is 25mm. We refer to a 12″ flange as DN300, when in fact it is 304mm. Please use the table below for your reference and if you have any other questions, please ask the Just Valves team. ½” DN15 ¾” DN20 1″ DN25 1¼” DN32 1½” DN40 2″ DN50 2½” DN65 3″ DN80 4″ DN100 5″ ..