How to Read Flange Dimensions: Complete Guide with Class 150/300 Size Charts

Meta Title：How to Read Flange Dimensions | Class 150/300 Size Chart Meta Description：Learn to read flange dimensions: OD, bolt circle, bore and thickness. Includes free Class 150 & 300 size charts and measurement tips for engineers.

1. Introduction

Mismatched flange dimensions are one of the most common and costly headaches for piping engineers, procurement teams and site installers. A wrong NPS, incorrect bolt circle diameter or mixed-up pressure class will lead to wrong goods delivery, on-site rework, project delays, extra freight and fabrication cost. Even a small dimension error will make two mating flanges impossible to connect.

This full guide breaks down every key flange dimension term, teaches you how to check Class 150 / Class 300 ASME B16.5 flanges, how to measure flanges correctly on site, and helps you avoid costly specification mistakes. We also provide downloadable flange size chart PDF and custom quotation support for non-standard flanges.

2. Key Flange Dimension Terminology Explained

All standard ASME B16.5 flanges share these core parameters:

• NPS (Nominal Pipe Size)：Nominal pipe size, the matching pipe size of the flange. Not equal to the actual outer diameter of the pipe. Corresponding metric mark is DN.

• OD (Outside Diameter)：Overall outer diameter of the flange plate. Critical for installation space check.

• BCD / Bolt Circle Diameter：The diameter of the circle passing through the center of all bolt holes. This is the most important mating dimension; two matched flanges must have identical BCD.

• Bolt Hole Diameter：Diameter of each bolt hole, decides the stud bolt size you can use.

• Bore / Inside Diameter：Inner opening of flange, matching pipe ID for welding or threading.

• Flange Thickness：Thickness of flange body, directly related to pressure rating. Class 300 flanges are thicker than Class 150 under the same NPS.

• Raised Face Height：Height of the sealing raised face (RF). Affects gasket installation and sealing performance.

Important reminder: Pressure Class (Class150 / 300) is NOT equal to working pressure value. It is a rating series defined by ASME B16.5 for pressure-temperature performance. Same NPS, different Class = totally different OD, BCD, thickness and bolt layout.

3. Class 150 & Class 300 Standard Flange Size Reference Table (ASME B16.5, WN/RF for reference)

Note: Data for quick reference only, please refer to official ASME B16.5 table for formal procurement. | NPS | Class | OD (in) | BCD (in) | Bolt Qty | Bolt Hole Dia (in) | Min Thickness (in) | | ---- | ---- | ---- | ---- | ---- | ---- | ---- | | 2" | 150 | 6.00 | 4.75 | 4 | 0.75 | 0.69 | | 2" | 300 | 6.50 | 5.00 | 8 | 0.75 | 0.88 | | 3" | 150 | 7.50 | 6.00 | 4 | 0.75 | 0.75 | | 3" | 300 | 8.25 | 6.625 | 8 | 0.88 | 1.00 | | 4" | 150 | 9.00 | 7.50 | 8 | 0.75 | 0.81 | | 4" | 300 | 10.00 | 7.875 | 8 | 1.00 | 1.12 | | 6" | 150 | 11.00 | 9.50 | 8 | 0.88 | 0.94 | | 6" | 300 | 12.50 | 10.625 | 12 | 1.00 | 1.31 | | 8" | 150 | 13.50 | 11.75 | 8 | 0.88 | 1.06 | | 8" | 300 | 15.00 | 13.00 | 12 | 1.125 | 1.44 |

4. Correct Method to Measure Flanges On Site

When you have an existing flange on-site and no drawing, follow this sequence to avoid mistakes:

1. Confirm the flange type first: WN, SO, BL, Threaded, Socket Weld etc.

2. Measure Bolt Circle Diameter (BCD) correctly: Measure from center of one bolt hole to the center of the opposite bolt hole, NOT edge to edge of adjacent holes.

3. Measure flange overall OD, then flange thickness.

4. Count total bolt holes and measure single bolt hole diameter.

5. Measure bore inner diameter, check raised face size.

6. Cross-check all data against ASME B16.5 Class 150 / 300 table, confirm NPS and pressure class.

7. Record sealing face type: RF / RTJ / FF.

Common mistake: Only measure OD to judge size. Same OD may exist between different classes, BCD is the priority verification item.

5. Common Dimension Marking Mistakes & Notes

1. Confusing Class 150 and Class 300: Same NPS, different Class cannot mate, bolt holes will not align.

2. Mixing inch and mm units: Many rework cases happen due to unit conversion error.

3. Ignoring sealing face: Same dimension, wrong face type will cause leakage.

4. Neglecting flange type difference: WN, SO, Blind flanges have different hub and bore structure even under same NPS & Class.

5. Tolerance range: ASME B16.5 has standard machining tolerance, custom non-standard flanges cannot apply standard size charts.

6. Material difference does not change standard dimension: 304 / 316 / A105 standard flanges follow identical ASME B16.5 dimension tables.

6. Free Download: Full Pressure Class Flange Dimension Chart PDF

We prepared a complete ASME B16.5 flange dimension PDF covering Class150, 300, 600, 900, 1500, 2500. Fill in the form below to get the file, including WN, SO, BL, TH, SW flange size tables in both inch and mm. ✅ Suitable for piping engineer, procurement, QC and site inspection.

7. Non-Standard Custom Flange | Submit Drawings for Quotation

Beyond standard ASME B16.5 flanges, we support custom machining for non-standard flanges:

• Custom OD, BCD, thickness, special bolt layout

• Special sealing face, RTJ groove, large diameter flanges

• Material: 304, 316L, Monel400, Inconel600, duplex steel, carbon steel A105 etc.

• Support ASME, API, CCS and other third-party inspection certificates.

Send your drawing, DXF or dimension list to our team, we will provide formal quotation and production schedule quickly.

Conclusion

Reading flange dimensions correctly is the foundation of piping procurement and installation. Always confirm NPS + Pressure Class + BCD + OD + sealing face before ordering. Keep our Class150 / 300 chart handy and download the full PDF for daily reference. If you have any doubt on flange selection or dimension checking, our engineering team is ready to support you.