In industrial piping, a flanged connection is only as good as its fit. “Flange-to-flange dimensions” don’t refer to one measurement, but to the set of standardized dimensions that ensure two separate flanges can bolt together properly to form a sealed, pressure-tight joint.
Getting these dimensions wrong means bolts won’t align, gaskets won’t seal, and the connection will fail. This guide explains the critical dimensions, the standards that govern them, and how to ensure compatibility for your project.
The Core Dimensions for Connection
For two flanges to mate, several key dimensions must match exactly. These are defined by published standards like ASME B16.5 for common sizes (NPS 1/2″ to 24″) and ASME B16.47 for larger diameters.
Here are the non-negotiable dimensions for a proper flange-to-flange connection:
| Dimension | What It Is | Why It Must Match |
| Nominal Pipe Size (NPS) | The nominal size of the pipe the flange is designed for (e.g., NPS 4″, NPS 12″). | Ensures the flanges are designed for the same pipeline size. |
| Pressure Class | The pressure-temperature rating (150, 300, 600, etc.). | Determines the flange’s thickness, bolt size, and all other critical dimensions. A Class 150 flange cannot bolt to a Class 300 flange of the same NPS. |
| Bolt Circle Diameter (BCD) | The diameter of the circle that passes through the center of all bolt holes. | The single most important dimension for fit-up. If the BCDs differ, the bolt holes will not align. |
| Number of Bolt Holes | The total count of bolt holes in the flange. | Must be identical for all flanges in the connection. |
| Bolt Hole Diameter | The diameter of the holes for the bolts/studs. | Must accommodate the correct bolt size with a standard clearance (typically 1/8 inch larger than the bolt diameter). |
How Dimensions Change: Pressure Class and Size
Flange dimensions are not universal for a given pipe size. They increase with the Pressure Class to handle higher forces. For example, a 6-inch flange has very different physical sizes depending on whether it’s Class 150 or Class 600.
The table below illustrates this, showing how the Outside Diameter (OD) and Bolt Circle Diameter (BCD) grow for a common pipe size (NPS 6″) as the pressure rating increases.
| Pressure Class | Flange Outside Diameter (OD) | Bolt Circle Diameter (BCD) | Number of Bolts |
| Class 150 | 11.00 inches | 9.50 inches | 8 |
| Class 300 | 12.50 inches | 10.62 inches | 12 |
| Class 600 | 14.00 inches | 12.50 inches | 12 |
The Standards Landscape: ANSI/ASME vs. Others
The most common standard in the United States and many international projects is ASME B16.5 (often historically called ANSI B16.5). Its dimensions are imperial (inches).
It’s crucial to know that flanges made to other international standards have completely different dimensions and are generally not interchangeable. For instance, a PN16 flange (ISO standard) will not bolt to an ANSI Class 150 flange, even if they are for the same nominal pipe size.
When sourcing flanges, you must specify the complete standard (e.g., ASME B16.5), the NPS, and the Pressure Class to ensure you get parts with compatible flange-to-flange dimensions.
Frequently Asked Questions
Q: How do I measure an existing flange to identify it?
A. You need to measure three things: 1) the Outside Diameter (OD), 2) the Bolt Circle Diameter (BCD) (measure from the center of one bolt hole to the center of the hole directly across from it), and 3) the number and size of the bolt holes. Compare these measurements to published ASME B16.5 dimension tables to find the matching NPS and Pressure Class.
Q: Are the dimensions the same for all flange types (Weld Neck, Slip-On, Blind, etc.)?
A. Yes, for the connecting dimensions. The OD, BCD, bolt hole count, and bolt hole diameter are standardized within a given NPS and Pressure Class for all flange types covered by ASME B16.5 (like Weld Neck, Slip-On, Blind, etc.). This ensures a Blind flange can seal against a Weld Neck flange of the same specification.
Q: What is the most common mistake when ordering flanges?
A. The most common error is mixing Pressure Classes. Assuming a “6-inch flange” is enough information leads to failure. You must always specify both the Nominal Pipe Size (NPS) and the Pressure Class (e.g., “NPS 6, Class 300”).
Q: What if my old flange doesn’t match any standard table?
A. This happens with custom, specialty, or very old equipment. In these cases, you must take all critical dimensions (OD, BCD, thickness, bore) and have a flange custom-manufactured to match. This is a common service provided by specialized manufacturers.
Need help identifying a flange or sourcing components with guaranteed compatible dimensions? Accurate flange-to-flange dimensions are the foundation of a leak-free, safe piping system. Our team provides expert guidance and supplies flanges manufactured to exact ASME B16.5 specifications.
Submit your flange specifications or request a dimensional chart for your project: http://texasflange.com/lp12/