Pipe Size: IPS, NPS, DN, NB and Pipe Schedules Explained in Detail

Complete Guide to Pipe Size: IPS, NPS, DN, NB and Pipe Schedules Explained in Detail

Understanding pipe size terminology is one of the most essential requirements for anyone working in piping engineering, mechanical design, plant construction, or maintenance. Despite its importance, pipe sizing is often misunderstood due to the use of nominal designations that do not directly represent actual physical dimensions. This article provides a comprehensive and practical explanation of the major pipe sizing systems used worldwide — IPS, NPS, DN, NB — along with the role of pipe schedules and their impact on pipe geometry and engineering performance.

This guide is intended for students, fresh engineers, and experienced professionals who require a clear and reliable reference for everyday engineering applications.

1. Fundamentals of Pipe Geometry

Every pipe is defined by three fundamental geometric properties:

• Outside Diameter (OD) – the external diameter of the pipe
• Inside Diameter (ID) – the internal flow passage of the pipe
• Wall Thickness (t) – the thickness of the pipe wall

These three parameters are linked by the basic geometric relationship:

ID = OD − 2 × t

This formula governs flow capacity, pressure resistance, weight, flexibility, and mechanical strength of the piping system. All further discussions on pipe sizing are built upon this fundamental relationship.

2. IPS – Iron Pipe Size System

The Iron Pipe Size (IPS) system is the earliest form of standardized pipe sizing. In the IPS system, pipes were originally classified according to their approximate inside diameter in inches. As industrial pressures increased and applications diversified, wall thicknesses varied while the nominal designation remained unchanged.

However, this approach created compatibility problems with fittings and flanges. To solve this issue, the industry shifted toward fixing the outside diameter for each nominal size while allowing wall thickness to vary according to strength requirements. This evolution laid the foundation for modern pipe sizing systems.

Although the IPS terminology is largely historical today, its influence remains embedded in modern standards.

3. NPS – Nominal Pipe Size System

The Nominal Pipe Size (NPS) system is the dominant pipe sizing system used internationally. NPS is a dimensionless designation expressed in inches, but the NPS number does not directly represent the actual diameter of the pipe.

The fundamental rule of the NPS system is:

For a given NPS, the outside diameter remains constant, while wall thickness changes with the schedule.

Example: NPS 2

• Nominal size: 2 inches
• Actual outside diameter: 2.375 inches
• Schedule 40 wall thickness: approximately 0.154 inches
• Schedule 80 wall thickness: approximately 0.218 inches

As wall thickness increases, the inside diameter decreases, while the outside diameter remains unchanged.

4. Pipe Schedule and Wall Thickness

Pipe schedule is the classification system that defines wall thickness. Common schedules include SCH 10, SCH 20, SCH 40, SCH 80, SCH 160, and XXS.

• Higher schedule number indicates thicker wall
• Thicker wall results in greater pressure resistance
• Thicker wall reduces inside diameter
• Outside diameter remains constant for the same NPS

Example: NPS 6

The outside diameter of NPS 6 pipe is approximately 6.625 inches for all schedules. Only the wall thickness and inside diameter vary between Schedule 40 and Schedule 80.

5. Special Behavior of Large Pipes: NPS 14 and Above

For pipe sizes between NPS 1/8 and NPS 12, the NPS number and the actual outside diameter are different. Beginning at NPS 14, the nominal size numerically matches the outside diameter in inches.

This transition simplifies identification of large-diameter pipes while maintaining compatibility with established sizing standards.

6. DN – Nominal Diameter (Metric System)

DN (Diamètre Nominal) is the metric equivalent of NPS and is standardized under ISO systems. DN values are expressed in millimeters and represent nominal, not actual, diameters.

For example, DN 400 is approximately equivalent to NPS 16. However, actual outside and inside diameters must always be obtained from applicable standards or manufacturer data.

7. NB – Nominal Bore

NB (Nominal Bore) is a commonly used term in piping and valve specifications. It is essentially equivalent to DN and represents a nominal flow size rather than a precise measurement.

Valve internal flow areas vary by manufacturer and design; therefore, NB should always be verified using technical documentation.

8. Engineering Implications of Pipe Sizing

8.1 Flow Capacity

Inside diameter directly affects flow velocity, friction losses, and pump sizing. Increasing schedule thickness reduces inside diameter and increases velocity for the same flow rate, potentially increasing erosion and pressure drop.

8.2 Pressure Rating

Thicker walls provide higher pressure containment capability. Pipe schedule selection is therefore governed by internal pressure, temperature, corrosion allowance, and code requirements.

8.3 Structural Strength and Weight

Higher schedules increase pipe weight, affecting support spacing, stress analysis, and installation methods.

9. Complete Pipe Specification Format

Every pipe specification must include:

• Nominal size (NPS or DN)
• Schedule or wall thickness
• Material grade

Example: 6” NPS Schedule 40 ASTM A106 Grade B.

10. Common Errors in Practice

• Assuming nominal size equals actual diameter
• Changing schedule without considering ID reduction
• Ignoring standard dimension charts
• Using NB alone without verifying valve geometry

11. Industry Standards Governing Pipe Dimensions

• ASME B36.10 – Carbon Steel Pipes
• ASME B36.19 – Stainless Steel Pipes
• ISO 4200 – Metric Pipe Dimensions
• EN 10220 – European Pipe Dimensions

12. Conclusion

Pipe sizing is a controlled and standardized engineering system that ensures compatibility, safety, and reliability across industries. NPS, DN, and NB are nominal designations, while actual pipe dimensions are determined by standardized tables and schedules. A thorough understanding of these principles is essential for proper piping design, stress analysis, procurement, fabrication, and installation.

Accurate pipe sizing remains the foundation of safe and efficient piping systems.