A flange dimensions chart provides the measurements needed to identify, select, compare, and install pipe flanges correctly. Flanges are widely used in plumbing, water treatment, chemical processing, oil and gas, HVAC, power plants, manufacturing, and industrial piping systems.
Unlike a simple pipe size chart, a flange dimensions chart contains several measurements, including nominal pipe size, flange outside diameter, bolt circle diameter, bolt-hole diameter, number of bolt holes, flange thickness, bore diameter, and raised-face dimensions.
Flange dimensions can vary significantly according to the applicable standard, pressure class, nominal pipe size, flange type, material, and facing. Therefore, a 4-inch flange, for example, does not have one universal set of dimensions.
This guide covers common ASME flange dimensions, ANSI flange dimensions, Class 150 flange dimensions, Class 300 flange dimensions, metric flange dimensions, bolt patterns, flange thickness, bore sizes, and the most important terminology used when reading a flange dimensions chart.
What Are Flange Dimensions?
Flange dimensions are the physical measurements used to define the size and configuration of a flange.
The most important flange dimensions include:
- Nominal pipe size
- Outside diameter
- Inside bore
- Bolt circle diameter
- Bolt-hole diameter
- Number of bolt holes
- Flange thickness
- Raised-face diameter
- Raised-face height
- Hub dimensions
- Neck dimensions where applicable
These measurements ensure that a flange can be properly connected to another flange, pipe, valve, pump, or piece of equipment.
A flange should never be selected based only on its nominal pipe size. The pressure class and standard must also be considered.
Flange Dimensions Chart
The following is a general reference chart for common ASME B16.5 Class 150 steel flanges. Dimensions are representative and should be verified against the applicable standard before fabrication or pressure-service use.
| NPS | Flange OD | Bolt Circle Diameter | Bolt Hole Diameter | Bolt Holes |
|---|---|---|---|---|
| 1/2 in | 3.50 in | 2.38 in | 0.62 in | 4 |
| 3/4 in | 3.88 in | 2.75 in | 0.62 in | 4 |
| 1 in | 4.25 in | 3.13 in | 0.62 in | 4 |
| 1-1/4 in | 4.63 in | 3.50 in | 0.62 in | 4 |
| 1-1/2 in | 5.00 in | 3.88 in | 0.62 in | 4 |
| 2 in | 6.00 in | 4.75 in | 0.75 in | 4 |
| 2-1/2 in | 7.00 in | 5.50 in | 0.75 in | 4 |
| 3 in | 7.50 in | 6.00 in | 0.75 in | 4 |
| 4 in | 9.00 in | 7.50 in | 0.75 in | 8 |
| 5 in | 10.00 in | 8.50 in | 0.88 in | 8 |
| 6 in | 11.00 in | 9.50 in | 0.88 in | 8 |
| 8 in | 13.50 in | 11.75 in | 0.88 in | 8 |
| 10 in | 16.00 in | 14.25 in | 1.00 in | 12 |
| 12 in | 19.00 in | 17.00 in | 1.00 in | 12 |
| 14 in | 21.00 in | 18.75 in | 1.13 in | 12 |
| 16 in | 23.50 in | 21.25 in | 1.13 in | 16 |
| 18 in | 25.00 in | 22.75 in | 1.25 in | 16 |
| 20 in | 27.50 in | 25.00 in | 1.25 in | 20 |
| 24 in | 32.00 in | 29.50 in | 1.25 in | 20 |
These values are useful for general reference, but exact dimensions depend on the flange type and applicable standard.
Understanding Flange Outside Diameter
The flange outside diameter, often abbreviated as OD, is the maximum diameter across the flange.
It is one of the easiest flange dimensions to measure and can be useful when identifying an unknown flange.
For example:
- 2-inch Class 150 flange → approximately 6 inches OD
- 3-inch Class 150 flange → approximately 7.5 inches OD
- 4-inch Class 150 flange → approximately 9 inches OD
- 6-inch Class 150 flange → approximately 11 inches OD
- 8-inch Class 150 flange → approximately 13.5 inches OD
- 12-inch Class 150 flange → approximately 19 inches OD
The flange OD is much larger than the pipe’s nominal diameter because the flange needs sufficient surface area for bolting and gasket sealing.
Flange Bolt Circle Diameter
The bolt circle diameter (BCD) is another important measurement shown on a flange dimensions chart.
The bolt circle is an imaginary circle passing through the centers of all the bolt holes.
For example, a common 4-inch Class 150 flange has a bolt circle diameter of approximately 7.5 inches.
Bolt circle dimensions are critical because the mating flange must have a compatible bolt pattern.
When replacing a flange, matching the nominal pipe size alone is not enough. The bolt circle, bolt-hole size, and number of bolt holes should also be checked.
Flange Bolt Hole Diameter
The bolt-hole diameter is the diameter of the holes through which the flange bolts pass.
The hole is normally slightly larger than the nominal bolt diameter to provide assembly clearance.
Common bolt-hole dimensions vary depending on:
- Flange size
- Pressure class
- Flange standard
- Bolt size
- Flange type
For this reason, bolt-hole diameter should always be taken from the appropriate dimensional table rather than estimated.
Number of Flange Bolt Holes
The number of bolt holes is another important part of a flange dimensions chart.
Small flanges commonly use four bolt holes, while larger flanges may use eight, twelve, sixteen, twenty, or more.
For example, common Class 150 configurations include:
| NPS | Typical Bolt Holes |
|---|---|
| 1/2 in | 4 |
| 3/4 in | 4 |
| 1 in | 4 |
| 1-1/4 in | 4 |
| 1-1/2 in | 4 |
| 2 in | 4 |
| 2-1/2 in | 4 |
| 3 in | 4 |
| 4 in | 8 |
| 5 in | 8 |
| 6 in | 8 |
| 8 in | 8 |
| 10 in | 12 |
| 12 in | 12 |
| 14 in | 12 |
| 16 in | 16 |
| 18 in | 16 |
| 20 in | 20 |
| 24 in | 20 |
The exact bolt arrangement should always be confirmed against the applicable flange standard.
Flange Thickness Chart
Flange thickness is the thickness of the main flange body.
It is affected by:
- Nominal size
- Pressure class
- Material
- Flange type
- Standard
- Design requirements
A higher pressure class generally uses a more substantial flange design than a lower class.
For this reason, there is no single universal flange thickness for a particular nominal pipe size.
For example, a 4-inch Class 150 flange and a 4-inch Class 600 flange are not expected to have the same thickness.
What Is Flange Bore Diameter?
The bore is the opening through the center of the flange.
The bore allows the pipe, fluid, gas, or process material to pass through the flange.
Bore dimensions depend on the flange type and pipe configuration.
Weld neck flanges are especially important when considering bore because the flange bore is designed to align with the pipe and welding configuration.
A flange with an incorrect bore may create:
- Installation problems
- Flow restrictions
- Welding difficulties
- Misalignment
- Unwanted internal projections
Therefore, bore diameter should be checked when selecting a replacement flange.
Flange Dimensions in Millimeters
Many international engineering projects use metric measurements.
The following table converts common Class 150 flange outside diameters to millimeters.
| NPS | Flange OD | Approx. OD in mm |
|---|---|---|
| 1/2 in | 3.50 in | 88.9 mm |
| 3/4 in | 3.88 in | 98.6 mm |
| 1 in | 4.25 in | 108.0 mm |
| 1-1/4 in | 4.63 in | 117.6 mm |
| 1-1/2 in | 5.00 in | 127.0 mm |
| 2 in | 6.00 in | 152.4 mm |
| 2-1/2 in | 7.00 in | 177.8 mm |
| 3 in | 7.50 in | 190.5 mm |
| 4 in | 9.00 in | 228.6 mm |
| 5 in | 10.00 in | 254.0 mm |
| 6 in | 11.00 in | 279.4 mm |
| 8 in | 13.50 in | 342.9 mm |
| 10 in | 16.00 in | 406.4 mm |
| 12 in | 19.00 in | 482.6 mm |
| 14 in | 21.00 in | 533.4 mm |
| 16 in | 23.50 in | 596.9 mm |
| 18 in | 25.00 in | 635.0 mm |
| 20 in | 27.50 in | 698.5 mm |
| 24 in | 32.00 in | 812.8 mm |
Class 150 Flange Dimensions
Class 150 is one of the most widely encountered flange classes in general piping.
Common Class 150 flange sizes range from small instrument connections to large industrial piping.
Some representative dimensions are:
| NPS | OD | Bolt Circle | Bolt Holes |
|---|---|---|---|
| 1 in | 4.25 in | 3.13 in | 4 |
| 2 in | 6.00 in | 4.75 in | 4 |
| 3 in | 7.50 in | 6.00 in | 4 |
| 4 in | 9.00 in | 7.50 in | 8 |
| 6 in | 11.00 in | 9.50 in | 8 |
| 8 in | 13.50 in | 11.75 in | 8 |
| 10 in | 16.00 in | 14.25 in | 12 |
| 12 in | 19.00 in | 17.00 in | 12 |
| 16 in | 23.50 in | 21.25 in | 16 |
| 20 in | 27.50 in | 25.00 in | 20 |
| 24 in | 32.00 in | 29.50 in | 20 |
Class 150 dimensions should not be used for other pressure classes.
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Class 300 Flange Dimensions
Class 300 flanges are generally larger and heavier than Class 150 flanges for the same nominal pipe size.
This is because they are designed for higher pressure-temperature service when used with suitable materials and conditions.
Compared with Class 150, a Class 300 flange may have:
- Greater outside diameter
- Larger bolt circle
- Different bolt-hole dimensions
- More or larger bolts
- Greater flange thickness
- Different facing dimensions
For this reason, Class 150 and Class 300 flanges should not be assumed to be interchangeable.
Class 600 Flange Dimensions
Class 600 flanges are commonly used in more demanding industrial applications.
Typical applications include:
- Oil and gas pipelines
- Petrochemical plants
- Refineries
- Power generation
- High-pressure process systems
- Industrial steam systems
Class 600 flange dimensions are different from Class 150 and Class 300 dimensions.
The complete flange specification should include the nominal size, class, material, flange type, and facing.
Flange Dimensions by Pressure Class
Common ASME flange pressure classes include:
- Class 150
- Class 300
- Class 400
- Class 600
- Class 900
- Class 1500
- Class 2500
The class number should not be interpreted as a universal pressure rating in psi.
Actual pressure-temperature ratings depend on:
- Material group
- Operating temperature
- Flange standard
- Design conditions
- Applicable pressure-temperature tables
Therefore, flange selection should always use the correct rating table.
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ANSI Flange Dimensions
Many people search for ANSI flange dimensions, but modern piping specifications commonly reference ASME standards.
The phrase “ANSI flange” is still widely used when referring to flange dimensions associated with common ASME pressure classes.
Typical search terms include:
- ANSI 150 flange dimensions
- ANSI 300 flange dimensions
- ANSI 600 flange dimensions
- ANSI flange bolt pattern
- ANSI flange bolt dimensions
- ANSI flange OD chart
- ANSI flange dimensions chart
When ordering a flange, it is better to specify the actual applicable ASME standard, pressure class, size, type, and facing rather than relying only on the term “ANSI flange.”
Metric Flange Dimensions
Metric piping systems often use DN designations.
Common sizes include:
| DN | Approximate NPS |
|---|---|
| DN15 | 1/2 in |
| DN20 | 3/4 in |
| DN25 | 1 in |
| DN32 | 1-1/4 in |
| DN40 | 1-1/2 in |
| DN50 | 2 in |
| DN65 | 2-1/2 in |
| DN80 | 3 in |
| DN100 | 4 in |
| DN125 | 5 in |
| DN150 | 6 in |
| DN200 | 8 in |
| DN250 | 10 in |
| DN300 | 12 in |
| DN350 | 14 in |
| DN400 | 16 in |
| DN450 | 18 in |
| DN500 | 20 in |
| DN600 | 24 in |
DN is a nominal designation and should not be treated as an exact outside or inside diameter.
PN Flange Dimensions
European flange systems commonly use PN pressure designations.
Examples include:
- PN6
- PN10
- PN16
- PN25
- PN40
- PN63
- PN100
- PN160
- PN250
- PN400
The dimensions of PN flanges can differ from ASME Class flanges, even when the nominal pipe size appears similar.
Therefore, a DN100 PN16 flange should not automatically be assumed to match a 4-inch ASME Class 150 flange.
The bolt pattern, outside diameter, thickness, and other dimensions can differ.
Also Read: Plywood Thickness Chart: Standard Size Guide & Dimensions.
Flange Facing Dimensions
The flange face is the surface that contacts the gasket or mating flange.
Common flange facing configurations include:
Raised Face
The sealing surface is raised above the main flange face.
Flat Face
The gasket surface remains flat across the flange.
Ring-Type Joint
A specially machined groove accommodates a metallic ring gasket.
The flange facing must be compatible with the mating flange and gasket.
Weld Neck Flange Dimensions
A weld neck flange has a long tapered hub designed to be butt welded to pipe.
Important weld neck dimensions include:
- Flange outside diameter
- Bolt circle
- Bolt-hole diameter
- Flange thickness
- Bore
- Hub diameter
- Hub length
- Neck dimensions
- Welding end dimensions
Weld neck flanges are frequently selected for demanding piping applications because their design provides a gradual transition between the pipe and flange.
Slip-On Flange Dimensions
A slip-on flange is designed to fit over the pipe.
Its dimensions include:
- Outside diameter
- Bolt circle
- Bolt-hole diameter
- Number of holes
- Bore
- Thickness
Slip-on flanges are commonly used in general industrial piping.
The pipe is positioned inside the flange and welded according to the applicable installation procedure.
Blind Flange Dimensions
A blind flange has no central bore and is used to close a piping connection.
Important blind flange dimensions include:
- Outside diameter
- Bolt circle
- Number of bolt holes
- Bolt-hole diameter
- Thickness
- Facing dimensions
Blind flanges are commonly used at:
- Pipe ends
- Valve outlets
- Equipment nozzles
- Maintenance isolation points
- Temporary closures
Because blind flanges withstand pressure over their entire face, their design differs from that of ordinary pipe-opening flanges.
Socket Weld Flange Dimensions
Socket weld flanges are commonly used for smaller pipe sizes.
Important dimensions include:
- Flange OD
- Bolt circle
- Bolt holes
- Bore
- Socket diameter
- Socket depth
- Thickness
The socket dimensions must correspond to the intended pipe size and applicable standard.
Threaded Flange Dimensions
Threaded flanges have internal pipe threads.
They can be useful in installations where welding is not practical or desirable.
Important dimensions include:
- Flange OD
- Bolt circle
- Bolt-hole diameter
- Number of holes
- Thread size
- Thread pitch
- Bore
- Thickness
Thread compatibility is essential when selecting threaded flanges.
Lap Joint Flange Dimensions
A lap joint flange is generally used together with a stub end.
The flange can rotate around the stub end, making bolt-hole alignment easier during installation.
This design can be useful for:
- Frequently dismantled piping
- Corrosion-resistant piping systems
- Systems using expensive pipe materials
- Applications requiring easier alignment
The stub-end dimensions must be compatible with the lap joint flange.
How to Measure Flange Dimensions
If you need to identify an unknown flange, follow a systematic measurement process.
Measure the Outside Diameter
Measure straight across the widest part of the flange.
Measure the Bolt Circle
Determine the distance through the centers of the bolt holes.
Count the Bolt Holes
Record the total number of holes.
Measure the Bolt Holes
Measure the diameter of an individual bolt hole.
Measure the Bore
Measure the central opening.
Measure Thickness
Measure the thickness of the flange body.
Identify the Face
Determine whether it has:
- Flat face
- Raised face
- Ring-type joint
- Another specialized face
Check Markings
Look for markings showing:
- Size
- Pressure class
- Material
- Standard
- Manufacturer
- Heat number
These markings can make flange identification much easier.
How to Read a Flange Dimensions Chart
A typical flange dimensions chart may use abbreviations such as:
NPS — Nominal Pipe Size
OD — Outside Diameter
ID — Inside Diameter
BCD — Bolt Circle Diameter
PCD — Pitch Circle Diameter
BH — Bolt Hole
No. of Holes — Number of Bolt Holes
T — Flange Thickness
RF — Raised Face
Bore — Internal flange opening
Understanding these abbreviations makes it easier to compare flange specifications.
Flange Dimensions vs Pipe Dimensions
Pipe and flange dimensions are related but are not the same.
For example, a common 4-inch NPS steel pipe has an outside diameter of approximately 4.5 inches.
A 4-inch Class 150 flange has an outside diameter of approximately 9 inches.
Therefore:
Pipe OD ≠ Flange OD
The flange must be larger to provide enough material for the gasket sealing surface and bolting system.
Why Flange Dimensions Matter
Correct flange dimensions are essential for creating a safe and functional piping connection.
Incorrect dimensions can cause:
- Bolt-hole misalignment
- Gasket problems
- Pipe misalignment
- Leakage
- Installation delays
- Equipment connection problems
- Flow restrictions
- Maintenance difficulties
This is why flange selection should be based on complete dimensional information rather than nominal pipe size alone.
Common Flange Dimension Mistakes
Using Nominal Size Alone
A flange’s nominal size does not define its complete dimensions.
Ignoring Pressure Class
Class 150, Class 300, and Class 600 flanges have different dimensions.
Ignoring the Standard
ASME, EN, DIN, JIS, and other flange systems can use different bolt patterns.
Measuring Only the Outside Diameter
Several flange sizes and configurations may have similar outside dimensions. More measurements are needed for reliable identification.
Forgetting the Facing Type
A raised-face flange is not identical to a flat-face or RTJ flange.
Using the Wrong Bolt Pattern
Bolt circle and bolt-hole dimensions must match the mating flange.
Frequently Asked Questions About Flange Dimensions
What dimensions are included in a flange dimensions chart?
A typical flange dimensions chart includes nominal size, outside diameter, bolt circle diameter, bolt-hole diameter, number of bolt holes, bore diameter, thickness, and sometimes hub and facing dimensions.
What is the OD of a 4-inch flange?
A common 4-inch Class 150 flange has an outside diameter of approximately 9 inches (228.6 mm).
What is the bolt circle of a 4-inch Class 150 flange?
A common 4-inch Class 150 flange has a bolt circle diameter of approximately 7.5 inches (190.5 mm).
How many bolt holes are on a 4-inch Class 150 flange?
A typical 4-inch Class 150 flange has 8 bolt holes.
Are all 4-inch flanges the same dimensions?
No. Dimensions vary according to pressure class, standard, flange type, facing, and other specifications.
What is flange BCD?
BCD means Bolt Circle Diameter. It represents the diameter of the imaginary circle passing through the centers of the bolt holes.
What is PCD on a flange?
PCD means Pitch Circle Diameter and is commonly used interchangeably with bolt circle diameter in flange terminology.
What is the difference between flange OD and pipe OD?
The pipe OD is the actual outside diameter of the pipe, while flange OD is the maximum outside diameter of the flange. The flange is normally much larger.
What is the difference between Class 150 and Class 300 flanges?
They have different pressure-temperature ratings and commonly different dimensions, including thickness, bolt patterns, and outside diameter.
Can two flanges with the same pipe size be connected together?
Not necessarily. Their pressure class, standard, bolt pattern, facing, and other dimensions must be compatible.
Final Flange Dimensions Reference
A flange dimensions chart is an essential reference for anyone working with piping systems, mechanical equipment, plumbing, industrial installations, and pipe fabrication. The most important measurements are flange outside diameter, bolt circle diameter, bolt-hole diameter, number of bolt holes, bore diameter, and flange thickness.
However, these dimensions cannot be considered independently. The complete flange specification should include the nominal pipe size, pressure class, standard, flange type, facing type, and material.
For example, a 4-inch Class 150 flange is substantially different from a 4-inch Class 300 or Class 600 flange. Likewise, an ASME flange may not have the same bolt pattern as a similarly sized flange manufactured to an EN, DIN, or JIS standard.
When identifying or purchasing a flange, check:
- Nominal pipe size
- NPS or DN
- Pressure class
- Flange standard
- Flange type
- Outside diameter
- Bolt circle diameter
- Number of bolt holes
- Bolt-hole diameter
- Bore diameter
- Flange thickness
- Facing type
- Material
- Gasket requirements
- Bolt requirements
For general identification and educational purposes, this flange dimensions chart provides a useful starting point. For pressure-containing, fabrication, or safety-critical applications, exact dimensions and pressure-temperature ratings should always be verified against the applicable current standard and manufacturer documentation.