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