What Does Nominal Size Mean

Nominal Size is a standardized designation that does not represent actual dimensions. In the steel pipe industry, communicating using actual dimensions can be complicated. Different standard sizes vary slightly, and pipes with the same outer diameter may have different wall thicknesses and inner diameters. Therefore, the steel pipe industry has adopted a unified designation for communication: Nominal Size. For example, pipes labeled as NPS 4″ may differ in inner diameter, weight, and pressure rating, but their fittings (flanges, pipe fittings) are fully compatible.
Common size designations in the steel pipe industry include:
NPS: Designated size
OD: Actual outer diameter
WT: Actual wall thickness
ID: Actual inner diameter
Therefore, when purchasing pipes, it is necessary to confirm the NPS/DN, Schedule, and standard with the customer. Quotations must specify: OD/ WT/ LENGTH.