Flange Size Chart: How to Read and Use ASME Flange Dimension Tables
A flange size chart isn’t complicated, but if you’ve never worked with one before, the number of columns and footnotes can make things confusing when you’re just looking for a quick reference to one item. Pipe size, outside diameter, bolt circle, number of bolts, bolt diameter, raised face diameter, thickness… it’s a lot to consider, but it is all necessary to call out the right part.
We’ve been helping engineers, procurement managers, and contractors source the right flanges since 1986. In that time, we’ve answered a lot of questions about how to read flange dimension tables, so here’s the straightforward breakdown.
What Is a Flange Size Chart?
A flange size chart, more formally called a flange dimension table, is a reference document that lists the standardized physical dimensions for flanges across different pipe sizes and pressure classes. These dimensions are governed by ASME and ANSI standards, primarily ASME B16.5 for pipe sizes ½” through 24″, and ASME B16.47 Series A and B for larger diameter flanges from 26″ through 60″ and beyond.
The purpose of these standardized dimensions is interchangeability. When you order a 6″ class 150# weld neck flange from any compliant manufacturer anywhere in the world, the bolt circle, bolt hole diameter, outside diameter, and raised face dimensions should be identical. That’s what makes ASME-standard flanges so practical for industrial piping systems.
Key Dimensions on a Flange Size Chart
Before you can read a flange dimension table, you need to understand what each column represents. Here are the critical dimensions:
Nominal Pipe Size (NPS)
NPS is the standardized designation for pipe size. It’s not an actual measurement; it’s a nominal reference number. Though it would make sense to believe so if you’re a first timer, a 2″ NPS pipe does not have a 2″ outside diameter. The OD of a 2″ pipe is actually 2.375″. Once you understand that NPS is a reference number rather than a true dimension, a lot of the initial confusion clears up.
Outside Diameter (OD) of the Flange
This is the total outer diameter of the flange itself, not the pipe. This OD dimension is often confused with the NPS, and people will often call out a flange by describing the OD first. Flange OD increases with both pipe size and pressure class. A higher pressure class flange has a larger OD (and more material mass) than a lower class flange at the same NPS.
Bolt Circle Diameter (BCD)
The bolt circle is the diameter of the imaginary circle that passes through the center of all the bolt holes. This is a critical dimension for mating flanges, both flanges in a pair must have the same bolt circle for the bolts to line up. Mismatched bolt circles mean the flanges won’t connect, no matter how close the ODs are, so it’s important that these line up.
Number of Bolts and Bolt Hole Diameter
The number of bolt holes is always a multiple of four, 4, 8, 12, 16, 20, because flanges need to be assembled in 90° orientations and still line up. Bolt hole diameter is slightly larger than the bolt to allow for installation. Both the bolt count and hole size increase with pressure class and flange size.
Flange Thickness
Thickness is usually the measurement of the base of the flange where the bolt pattern is located, and this increases with pressure class. A Class 600# flange is significantly thicker than a Class 150# flange at the same NPS because it needs to withstand higher internal pressure. Thickness is also relevant when calculating clearance in tight installations. Sometimes thickness can also be used to describe the measurement of the flange from the top of the hub to the bottom of the face, in which case you will prefer to use the terms “overall length” or “full length through the hub” to describe these flanges.
Raised Face (RF) Diameter
Most ASME B16.5 flanges have a raised face, a slightly elevated circular seating surface where the gasket sits. The RF diameter is smaller than the flange OD and is the surface that actually contacts and compresses the gasket. Knowing the RF diameter is important for gasket selection, and knowing the pressure class determines the thickness of said raised face.
Sample ASME B16.5 Class 150# Flange Dimensions
Here’s a condensed reference table for ASME B16.5 Class 150# raised face flanges across common pipe sizes:
| NPS | Flange OD | Thickness | # Bolts | Bolt Dia. | RF thickness |
|---|---|---|---|---|---|
| ½” | 3.50″ | 0.38″ | 4 | 0.62″ | 1/16″ |
| 1″ | 4.25″ | 0.50″ | 4 | 0.62″ | 1/16″ |
| 2″ | 6.00″ | 0.69″ | 4 | 0.75″ | 1/16″ |
| 4″ | 9.00″ | 0.88″ | 8 | 0.75″ | 1/16″ |
| 6″ | 11.00″ | 0.94″ | 8 | 0.88″ | 1/16″ |
| 8″ | 13.50″ | 1.06″ | 8 | 0.88″ | 1/16″ |
| 10″ | 16.00″ | 1.12″ | 12 | 1.00″ | 1/16″ |
| 12″ | 19.00″ | 1.19″ | 12 | 1.00″ | 1/16″ |
All dimensions per ASME B16.5. Dimensions in inches. Bolt circle diameter (BCD) varies, refer to full ASME B16.5 tables for complete data.
For a complete flange bolt chart including all pressure classes and pipe sizes, see our flange bolt chart reference guide.
How Pressure Class Affects Flange Dimensions
This is where engineers and procurement teams sometimes get tripped up. Each pressure class, 150#, 300#, 600#, 900#, 1500#, 2500#, has its own dimension table. Flanges at the same NPS but different pressure classes are NOT interchangeable, so check all values.
A 4″ Class 300# flange has a larger OD, a larger bolt circle, more bolt holes, and greater thickness than a 4″ Class 150# flange. They will not bolt together, and even if you could force them together, the mating surfaces wouldn’t align correctly.
Always verify pressure class before ordering. In the field, this is one of the most common sources of mismatched flanges – someone orders a replacement flange by size alone without confirming the pressure class.
ASME B16.5 vs. ASME B16.47: Which Standard Applies?
The standard you’re working from depends on the pipe size:
- ASME B16.5 covers flanges from ½” NPS through 24″ NPS in pressure classes 150# through 2500#
- ASME B16.47 covers large diameter flanges from 26″ NPS through 60″ NPS, in two series: Series A (MSS SP-44) and Series B (API 605)
For the vast majority of piping applications, ASME B16.5 is the governing standard. ASME B16.47 comes into play for large-diameter pipeline and vessel applications, think major transmission pipelines, large storage vessels, and similar infrastructure.
Practical Tips for Using Flange Dimension Tables
- Always confirm NPS, pressure class, and flange type before pulling dimensions. A weld neck and a slip-on flange at the same NPS and class share the same bolt pattern but have different hub dimensions.
- Use the bolt circle diameter to verify mating flanges match – this is the most field-critical dimension to check first, along with the OD stamping.
- When ordering replacement flanges, measure the existing flange OD and bolt circle, then cross-reference to the dimension tables to confirm NPS and class. Don’t rely on markings alone as they can be worn or misread, but check both anyway
- For custom or non-standard sizes, dimensions won’t be in the standard tables. That’s where working with a supplier who can provide custom fabrication matters, as we would offer to your provided prints.
Need Flanges in the Right Size and Class?
Texas Flange carries ASME B16.5 and B16.47 flanges in sizes from ½” through 60″+ across all standard pressure classes, in over 50 material grades. Whether you’re working from a dimension table or need help confirming the right spec for your application, we can help.
Contact us for a quote and we’ll get you the right flanges, the right size, the right class, ready to ship and install for your project.
