How to Convert Flange NPS to OD: The…

In the world of piping and flange systems, understanding Nominal Pipe Size (NPS) and Outer Diameter (OD) is crucial for engineers, technicians, and procurement professionals. But confusion often arises when trying to convert between NPS and OD—especially when dealing with international standards, drawings, or flange selection.

This comprehensive article explains how to convert flange NPS to OD, why it matters, and how to avoid mistakes during flange sourcing and installation. Throughout the article, we’ll also show how SASA FLANGE helps customers worldwide with accurate flange dimensioning, custom manufacturing, and technical consultation.

1. What Is NPS and How Is It Different from OD?

1.1 What is NPS?

NPS stands for Nominal Pipe Size, a standard used in North America to define the diameter of pipes and their corresponding flanges. The term “nominal” means the size is not the exact measurement—it’s a standardized reference.

NPS is expressed in inches (e.g., NPS 2, NPS 6, NPS 10)
It does not always match the actual OD, especially for smaller sizes

1.2 What is OD?

OD is Outer Diameter, the actual external diameter of a pipe or flange. OD is critical for correct fitment, alignment, and sealing in piping systems.

Example:

NPS 6 ≠ 6 inches OD
NPS 6 actually has an OD of 6.625 inches

That’s where confusion starts—and why proper conversion is important.

2. Why Convert Flange NPS to OD?

Knowing the OD from a given NPS is essential for:

Ensuring correct pipe-flange fitment
Interpreting engineering drawings
Ordering flanges to match existing pipes
Avoiding mismatched connections
Meeting ASME B16.5, B16.47, DIN, or JIS standards

Many engineers and buyers mistakenly assume NPS equals OD, which can lead to procurement errors. SASA FLANGE helps customers verify all dimensional parameters before finalizing orders.

3. General Rule: How to Convert NPS to OD

Use this simple rule:

For NPS ≤ 12:
OD is greater than NPS
(Based on ASME B36.10 or B36.19 standards)

For NPS ≥ 14:
OD ≈ NPS (Nominal equals actual)

Let’s look at examples of some common sizes:

NPS 1 = 1.315″ OD
NPS 2 = 2.375″ OD
NPS 4 = 4.5″ OD
NPS 6 = 6.625″ OD
NPS 8 = 8.625″ OD
NPS 10 = 10.75″ OD
NPS 12 = 12.75″ OD
NPS 14 = 14.00″ OD

From NPS 14 and up, the OD equals the nominal size.

4. How Does This Apply to Flanges?

Flanges are matched to pipe size using NPS, but their outer diameters (OD of the flange) vary depending on:

NPS size
Pressure class (150, 300, 600, etc.)
Standard used (ASME, DIN, JIS, etc.)

For example, a NPS 6 flange class 150 has:

Pipe OD = 6.625″
Flange OD = 10.62″ (as per ASME B16.5)

So converting NPS to flange OD must consider the flange type and pressure class.

5. Practical Steps to Convert Flange NPS to OD

Follow these steps:

Step 1: Know the NPS size

Start by identifying the nominal pipe size from documentation or pipe markings.

Step 2: Refer to flange standard

Determine if the flange follows:

ASME B16.5 (up to 24″)
ASME B16.47 (above 24″)
DIN EN 1092-1
JIS B2220
GOST or BS standards

Step 3: Look up pipe OD

Use ASME B36.10/36.19 charts or online references to get the actual pipe OD for the given NPS.

Step 4: Check flange OD based on class

Use a flange dimension chart to match NPS and pressure class to the flange OD.

Tip from SASA FLANGE: We always provide complete dimensional drawings with each quotation to help customers avoid confusion and mismatched selections.

6. Factors That Influence Flange OD

When converting flange NPS to OD, consider these:

Pressure Class – Higher pressure = larger flange OD
Flange Type – Weld neck vs blind vs slip-on affects dimensions
Standard – ASME vs DIN flanges have different OD values
Material – Some materials may require design adjustments (e.g., for stress or sealing)

7. Common Flange Size Conversion Examples

Here are a few real-world examples:

Example 1:

Flange: NPS 4″, Class 150, Weld Neck

Pipe OD = 4.5″
Flange OD = 9.00″

Example 2:

Flange: NPS 10″, Class 300, Blind

Pipe OD = 10.75″
Flange OD = 17.00″

Example 3:

Flange: NPS 24″, Class 600, Slip-On

Pipe OD = 24.00″
Flange OD = 38.75″

Without using a flange dimension chart, you can’t determine the OD just from NPS. That’s why SASA FLANGE provides ready-to-use charts and expert consultation.

8. Common Mistakes to Avoid

Assuming NPS = OD — Only true for NPS 14 and above
Ignoring pressure class — Changes OD significantly
Using wrong standard — ASME vs DIN can differ by several inches
Overlooking flange type — Each type has unique dimensions

Get it right the first time by double-checking with a trusted supplier like SASA FLANGE.

9. Tools for NPS to OD Conversion

ASME B16.5/16.47 flange charts
Digital caliper/tape measure for field measurement
Online calculators or flange dimension apps
3D CAD models or drawings
Technical support from SASA FLANGE

We recommend always verifying with official standards or the manufacturer’s technical sheet before finalizing orders.

10. When to Contact SASA FLANGE

If you need help converting NPS to OD or selecting the correct flange, our technical team at SASA FLANGE is ready to assist. We offer:

Full flange dimension charts in PDF or CAD
Custom flanges for special piping systems
ASME, DIN, EN, GOST standard support
Professional advice for international projects
Marked drawings and certificates with every order

Conclusion: NPS and OD—Know the Difference, Get the Right Flange

Understanding the difference between NPS and OD is essential for anyone involved in piping systems. While NPS serves as a nominal reference, OD is the actual measurement that determines fit, sealing, and system integrity.

By learning how to convert flange NPS to OD, you can:

Select the right flange every time
Avoid costly rework or installation issues
Improve procurement efficiency
Ensure system safety and compliance

Whenever you’re unsure, rely on experts like SASA FLANGE to help you make the right choice.