DN (Diameter Nominal) in mm to Inches

**Nominal pipe size (NPS)**, defined by the**American standard ANSI / ASME**is based on**inches**.**Nominal diameter (DN)**, defined by the**European standard ISO 6708**is based on**millimeters**

In daily practice; uses of measuring units depend on preference.

Some use metric and some imperial measuring systems.

NPS (Nominal Pipe Size); is defined according to American standards like ANSI / ASME

In many technical documents; inches won’t be mentioned. Instead ” shall be added after the size. For example 4″, is the same as 4 Inches.

As a general practice, DN is always written in front of digits. For example, DN25, DN100, etc. as written 25DN, could be confusing to others.

## DN to inches flange conversion

From DN6 to DN 90

DN (Diameter Nominal in mm) | NPS (Nominal Pipe Size in Inches) |

6 | 1/8″ |

8 | 1/4″ |

10 | 3/8″ |

15 | 1/2″ |

20 | 3/4″ |

25 | 1″ |

32 | 1-1/4″ |

40 | 1-1/2″ |

50 | 2″ |

65 | 2-1/2″ |

80 | 3″ |

90 | 3/2″ |

DN to inches flange conversion; From DN100 to DN600

DN (Diameter Nominal in mm) | NPS (Nominal Pipe Size in Inches) |

100 | 4″ |

115 | 4-1/2″ |

125 | 5″ |

150 | 6″ |

200 | 8″ |

250 | 10″ |

300 | 12″ |

350 | 14″ |

400 | 16″ |

450 | 18″ |

500 | 20″ |

600 | 24″ |

For some flanges; you can easily calculate flange size in inches.

As 1 inch = 25.4 mm. Like DN 25 = 1″ Flange (but you can see there is a slight error of 0.4 mm)

This error increases as the flange size is bigger. For example, 24″ * 25.4 = 609.6mm but we use DN600 for 24-inch flange. It is always recommended to refer size conversion chart when assigning flange size.

In the next post; we shall discuss about recommended water flow rate from the flange/pipe size.

You may check our cooling tower tonnage calculation