Steel Pipe Schedule Chart: Dimensions & Specifications

Introduction

In steel pipe procurement, specifications serve as the core metrics determining product compatibility, cost, and project feasibility. Among these, Schedule stands as a critical parameter directly influencing key factors such as wall thickness and pressure rating. This article provides an in-depth analysis of primary specification parameters including NPS, DN, and Schedule, alongside a detailed Schedule conversion chart, offering an effective guide for your steel pipe procurement.

What are DN and NPS?

DN – Nominal Diameter

The metric system standard for indicating pipe diameter is widely used in Europe, China, Russia, and most countries adopting ISO standards. The number following DN is an integer that does not directly correspond to the pipe’s inner or outer diameter but serves as an identification and matching code. It ensures that all pipe materials (such as fittings, flanges, valves, etc.) with the same DN value can be interconnected.

NPS – Nominal Pipe Size

The Imperial system standard, also used to denote pipe diameters, is widely employed in the United States, Canada, and their affected regions, serving as the most common standard in international energy projects. When the NPS value is greater than or equal to 14, the NPS number corresponds to the actual outside diameter in inches. When the NPS value is less than or equal to 12, the NPS number does not correspond to an actual outside diameter but instead corresponds to a fixed outside diameter value. Therefore, NPS also serves as a designation and matching code for ensuring compatibility between pipe materials.

Application and Transformation in Business

For clients in Europe or China, DN is primarily used.
For other regions such as North America, the Middle East, or large international EPC projects, NPS is predominantly used.
There exists a non-exact correspondence between DN and NPS; they are not entirely equivalent.

Although DN and NPS are related, they should not be used interchangeably in most cases. This is particularly true for pipe fittings such as flanges and pipe components, which must be manufactured and supplied according to the customer’s specified standard system (e.g., ISO/EN or ASME/ANSI) and corresponding standard dimension tables. This ensures perfect compatibility of interfaces such as threads, bevels, and bolt holes.

What is Schedule？

In steel pipe specifications and dimensions, Schedule (SCH) denotes wall thickness, a critical factor determining the pipe’s pressure rating and strength. The Schedule number does not directly indicate the pipe’s wall thickness dimension but corresponds to NPS. The same Schedule corresponds to different wall thickness dimensions under different NPS. The number following Schedule more directly represents the pipe’s pressure-bearing capacity. A higher Schedule number indicates thicker walls, greater pressure resistance, and enhanced corrosion resistance.

Common Schedule

• Schedule 10: Features thinner walls, suitable for low-pressure applications.
• Schedule 40: Common standard wall thickness, applicable to most scenarios.
• Schedule 80: Features thicker walls, suitable for medium-to-high pressure applications.

What is ASME, ANSI, and ASTM?

The core concepts of NPS (Nominal Pipe Size) and Schedule were established by ASME (American Society of Mechanical Engineers) in its standards. They were subsequently approved by ANSI (American National Standards Institute) as U.S. national standards, thereby gaining authoritative status worldwide.

1.Developing Organization: ASME

ASME B36.10M

Its full name is “Welded and Seamless Steel Pipes” This standard formally defines the NPS size series for carbon steel and alloy steel tubes along with their corresponding outside diameters, and establishes a correspondence table between Schedule Numbers (such as SCH 40, SCH 80, STD, XS, XXS) and specific wall thickness values.

ASME B36.19M

This is a supplemental standard for stainless steel pipes. It defines the nominal pipe size (NPS) dimensions and wall thicknesses (Schedule) for stainless steel pipes. Their outside diameters are identical to those of carbon steel pipes of the same NPS in B36.10M to ensure interchangeability, though wall thicknesses differ slightly for certain NPS sizes.

2.Approval and Certification Body: ANSI

• After ASME develops a draft standard, it is submitted to ANSI.
• ANSI conducts open reviews to ensure its development process is transparent, fair, and consensus-based.

After review, the standard was published with the ANSI prefix, becoming an American National Standard.
Consequently, you will see the complete standard numbers as: ANSI/ASME B36.10M and ANSI/ASME B36.19M. This “ANSI/” prefix significantly enhances the standard’s legal standing and recognition in international trade and large-scale projects.

3.Implementation and reference of standards: ASTM

ASTM is the American Society for Testing and Materials, which does not participate in defining nominal pipe size (NPS), schedule, or other specifications. However, when ASTM develops specific material product standards (such as ASTM A106 seamless pipe or ASTM A53 welded pipe), it directly references and adopts the definitions of NPS, outside diameter, and schedule as specified in ANSI/ASME B36.10M.

For example: In the ASTM A106 standard, it explicitly states that “the dimensions and wall thickness of steel pipes shall comply with the requirements of ASME B36.10M.” Detailed dimension and tolerance tables are then provided, with the data in these tables sourced directly from ASME B36.10M.

Schedule Chart

Stainless Steel Pipe

Carbon Steel Pipe