What Is Flange Nominal Pipe Size (NPS)?

In the world of piping systems and industrial components, understanding technical terminology is essential for ensuring proper design, compatibility, and performance. One of the most frequently encountered terms is Nominal Pipe Size (NPS) — especially when dealing with flanges. Whether you are sourcing materials for a pipeline project or specifying dimensions for industrial fittings, knowing what NPS means and how it affects flange selection is crucial.

In this article, we’ll explain what flange nominal pipe size (NPS) is, how it is used, and why it matters when choosing the right flange for your application. If you’re in the business of industrial piping, power generation, chemical processing, or any field that involves pressure-rated piping systems, this guide will help you make more informed decisions.

What Does Nominal Pipe Size (NPS) Mean

Nominal Pipe Size, abbreviated as NPS, is a standardized dimensionless number used in North America to identify the size of a pipe. Despite the term “nominal,” which suggests an approximate measurement, NPS is a specific designation that determines the diameter and wall thickness of a pipe and, by extension, the dimensions of the corresponding flanges.

It is important to understand that the NPS does not exactly match the actual inside or outside diameter of the pipe, particularly for sizes NPS 14 and below. Instead, it serves as a convenient label used in industry standards.

NPS and Pipe Dimensions

NPS is used in conjunction with the Schedule (SCH) of a pipe, which indicates wall thickness. Together, NPS and Schedule determine:

Outside diameter (OD)
Inside diameter (ID)
Wall thickness

For example:

NPS 2 does not mean the pipe has an exact 2-inch inside diameter
A Schedule 40 NPS 2 pipe has a 2.375-inch outside diameter and a wall thickness of 0.154 inches

Since flanges must match the pipe’s OD, understanding NPS ensures that the flange fits properly and seals effectively.

Why NPS Matters for Flange Selection

Flanges are essential components in piping systems, used to connect pipes, valves, pumps, and other equipment. The flange must match the NPS of the pipe to ensure alignment and pressure integrity.

Each flange size is manufactured according to the ASME B16.5 or B16.47 standards, which specify dimensions based on NPS and pressure class. These standards ensure that components from different manufacturers are interchangeable as long as the NPS and pressure rating are the same.

At sasaflange, we produce a wide range of flanges in standard NPS sizes that meet global industry specifications. Whether you need weld neck, slip-on, socket weld, or blind flanges, selecting the correct NPS ensures safe and efficient operation of your piping system.

Common NPS Ranges for Flanges

Small bore sizes: NPS 1/2 to NPS 2 are commonly used in instrumentation, small-diameter process lines, and residential plumbing
Medium bore sizes: NPS 3 to NPS 12 are found in industrial processing plants, refineries, and water treatment facilities
Large bore sizes: NPS 14 and above are used in power generation, shipbuilding, and oil and gas pipelines

Each NPS size comes with standardized flange dimensions such as bolt circle diameter, bolt hole size, thickness, and raised face height, all of which must be matched for a secure connection.

NPS vs DN: Understanding the Global Perspective

Outside North America, the equivalent standard to NPS is DN (Diameter Nominal), used in Europe and other international markets. While NPS is based on inches, DN uses millimeters. Though the numbers are not exact conversions, standard references exist to match NPS to DN sizes.

For example:

NPS 2 corresponds roughly to DN 50
NPS 6 matches DN 150

This cross-reference is essential when working with global supply chains. At sasaflange, we support both NPS and DN standards to serve customers across North America, Europe, Asia, and the Middle East.

Selecting the Right Flange Using NPS

When choosing a flange for your project, make sure to specify:

Nominal Pipe Size (NPS)
Pressure class (e.g., 150, 300, 600)
Type of flange (weld neck, blind, slip-on, etc.)
Material (carbon steel, stainless steel, alloy, etc.)
Facing type (raised face, ring-type joint, flat face)

Getting the NPS wrong can lead to mismatched components, potential leaks, and even system failure. Always refer to ASME standards or consult with your flange supplier to ensure proper compatibility.

Final Thoughts

Understanding flange nominal pipe size (NPS) is essential for designing and maintaining high-performance piping systems. It is the foundation upon which proper pipe and flange matching is built, ensuring leak-proof connections, structural integrity, and system efficiency.

NPS may not represent the actual diameter of the pipe, but it plays a vital role in standardizing piping components, simplifying design, and facilitating global trade. Whether you are managing a refinery, building a power plant, or engineering a municipal water system, selecting the correct NPS is key to operational success.

For top-quality flanges that match industry-standard NPS sizes, rely on sasaflange — your trusted partner in precision flange manufacturing and supply.