Nominal size in piping refers to a standardized designation used to identify the approximate size of pipes for identification and compatibility — not the exact physical dimensions.
Here’s a clearer breakdown:
Definition:
Nominal Pipe Size (NPS) or Nominal Diameter (DN in metric systems) is a naming convention used to define pipe size. The term "nominal" means "in name only," so the nominal size is not the actual inside or outside diameter.
Examples:
|
Nominal Pipe Size (NPS) |
Actual Outside Diameter (OD) |
Wall Thickness Depends on Schedule |
|
1 inch |
1.315 inches (33.4 mm) |
Varies by schedule (e.g., Sch 40, Sch 80) |
|
2 inches |
2.375 inches (60.3 mm) |
Varies by schedule |
So, a 1-inch pipe doesn’t have a 1-inch internal or external diameter — it’s just a label. The actual OD stays constant, but the wall thickness (and therefore the inner diameter) changes depending on the schedule (wall thickness classification).
Why Use Nominal Size?
Ensures standardization across manufacturers
Helps with interchangeability of components (fittings, valves, etc.)
Simplifies communication and design across projects
If you're working in metric systems, "DN" (e.g., DN25, DN50) is the equivalent, representing approximate internal diameters in millimeters — though still nominal.
