ANSI Flange Dimensions Chart

Contents

1 1. What Is an ANSI Flange?
2 2. ANSI Flange Standards and ASME B16.5
3 3. Key ANSI Flange Dimensions and Terminology
4 4. ANSI Flange Pressure Classes: 150, 300, 400, 600, 900, 1500, and 2500
5 5. Complete ANSI Flange Dimensions Chart
6 6. ANSI Class 150 Flange Dimensions Chart
- 6.1 Class 150 dimensions in millimetres
7 7. ANSI Class 300 and Class 600 Flange Dimensions Charts
8 8. ANSI Class 900, 1500, and 2500 Flange Dimensions Charts
9 9. How to Read and Measure ANSI Flange Dimensions
10 10. How to Select the Correct ANSI Flange Size and Pressure Class
11 Conclusion

ANSI flanges are widely used to connect pipes, valves, pumps, pressure vessels, and other equipment in industrial piping systems. Their standardized dimensions allow components from different manufacturers to be assembled while maintaining proper alignment, pressure containment, and sealing performance.

An ANSI flange dimensions chart provides essential measurements such as outside diameter, flange thickness, bolt circle diameter, number of bolt holes, bolt-hole diameter, and raised-face dimensions. These values vary according to the nominal pipe size, flange type, and pressure class.

Although the term “ANSI flange” remains common in the piping industry, most dimensional requirements are now published by the American Society of Mechanical Engineers (ASME). For example, ASME B16.5 covers pipe flanges and flanged fittings from NPS ½ through NPS 24, while ASME B16.47 covers larger flanges from NPS 26 through NPS 60.

This guide explains ANSI/ASME flange dimensions, pressure classes, terminology, and measurement methods. It also provides dimensional charts to help engineers, fabricators, purchasers, and maintenance personnel select compatible flanges for their piping systems.

1. What Is an ANSI Flange?

What Is an ANSI Flange?

An ANSI flange is a standardized mechanical component used to create a removable connection between sections of pipe or between a pipe and another piece of equipment. Two flanges are normally joined using bolts or studs, with a gasket installed between their sealing faces to prevent fluid leakage.

The expression “ANSI flange” refers to flanges manufactured according to dimensional and pressure-temperature standards originally developed or approved by the American National Standards Institute. However, ANSI does not directly design or manufacture flanges. Today, the relevant standards are primarily maintained by ASME, so the technically correct term is usually “ASME flange” or “ANSI/ASME flange.”

Common standards include:

ASME B16.5: Pipe flanges and flanged fittings from NPS ½ to NPS 24.
ASME B16.47: Large-diameter steel flanges from NPS 26 to NPS 60.
ASME B16.36: Orifice flanges used for differential-pressure flow measurement.
ASME B16.48: Line blanks, including spectacle blinds, spades, and spacers.

ANSI/ASME flanges are available in several configurations, including weld neck, slip-on, socket weld, threaded, lap joint, and blind flanges. Each design provides a different method of connecting to the pipe and is suited to particular pressure, temperature, maintenance, and installation requirements.

Flanges are also assigned pressure classes such as Class 150, 300, 400, 600, 900, 1500, and 2500. The class number is not the flange’s maximum working pressure in psi. Its actual pressure rating depends on the flange material and operating temperature, as specified in the applicable ASME pressure-temperature tables.

For two ANSI flanges to connect correctly, several characteristics must be compatible:

Nominal pipe size
Pressure class
Flange standard
Flange facing type
Bolt-hole quantity and diameter
Bolt-circle diameter
Gasket dimensions
Material and pressure-temperature rating

Standardizing these features makes ANSI/ASME flanges interchangeable within the limits of the applicable standard, simplifying piping design, component replacement, fabrication, and maintenance.

2. ANSI Flange Standards and ASME B16.5

ANSI Flange Standards and ASME B16.5

The term “ANSI flange” is still commonly used in the piping industry, but most modern flange dimensions and pressure-temperature ratings are established by ASME standards. ANSI approves the standards, while ASME develops and maintains the technical requirements.

The principal standards associated with ANSI/ASME flanges include:

Standard	Scope
ASME B16.5	Pipe flanges and flanged fittings from NPS ½ to NPS 24
ASME B16.47	Large-diameter steel flanges from NPS 26 to NPS 60
ASME B16.36	Orifice flanges used in flow-measurement systems
ASME B16.48	Spectacle blinds, spades, and ring spacers
ASME B16.20	Metallic gaskets for pipe flanges
ASME B16.21	Nonmetallic flat gaskets for pipe flanges
ASME B16.25	Butt-welding end preparation
ASME B16.1	Gray iron pipe flanges and flanged fittings
ASME B16.42	Ductile iron pipe flanges and flanged fittings

ASME B16.5 flange size range

ASME B16.5 is the primary standard used for steel pipe flanges. It covers nominal pipe sizes from NPS ½ through NPS 24 and includes the following pressure classes:

Class 150
Class 300
Class 400
Class 600
Class 900
Class 1500
Class 2500

The standard specifies critical dimensional and technical requirements, including:

Flange outside diameter
Flange thickness
Bolt-circle diameter
Number and diameter of bolt holes
Flange facing dimensions
Hub dimensions
Bore dimensions
Dimensional tolerances
Materials and markings
Pressure-temperature ratings
Testing and inspection requirements

ASME B16.5 covers weld neck, slip-on, socket-weld, threaded, lap-joint, and blind flanges. However, not every flange type is available in every nominal size and pressure class.

ASME B16.47 for large-diameter flanges

Flanges larger than NPS 24 are generally covered by ASME B16.47. This standard applies to steel flanges from NPS 26 through NPS 60 in Classes 75, 150, 300, 400, 600, and 900.

ASME B16.47 includes two flange series:

Series A: Generally heavier and designed with larger bolt diameters and fewer bolt holes.
Series B: Generally lighter, with smaller bolt diameters and a greater number of bolt holes.

Series A and Series B flanges of the same nominal size and pressure class are not normally interchangeable because their outside diameters, bolt circles, bolt quantities, and bolt-hole dimensions may differ.

ANSI class and actual working pressure

A common mistake is to treat the flange class number as its allowable working pressure in psi. For example, a Class 300 flange is not automatically rated for 300 psi under all operating conditions.

The actual allowable pressure depends on:

Flange material group
Operating temperature
Pressure class
Applicable ASME standard
Gasket and bolting limitations

The pressure-temperature tables in ASME B16.5 or ASME B16.47 must therefore be consulted when determining the allowable working pressure.

3. Key ANSI Flange Dimensions and Terminology

Key ANSI Flange Dimensions and Terminology

Understanding flange terminology is essential when reading an ANSI flange dimensions chart. The chart measurements define whether a flange will fit the pipe, align with the mating flange, and provide a reliable sealed connection.

Nominal Pipe Size

Nominal Pipe Size, abbreviated as NPS, identifies the approximate pipe size connected to the flange. It is a nominal designation rather than a direct measurement of the pipe’s inside or outside diameter.

For NPS ¹⁄₈ through NPS 12, the NPS value does not equal the actual pipe outside diameter. From NPS 14 upward, the nominal size generally matches the pipe outside diameter in inches.

A flange must be selected according to the pipe’s NPS and, where applicable, its wall thickness or schedule.

Flange outside diameter

The flange outside diameter, usually represented by O or D, is the maximum diameter measured across the flange body. It affects the amount of installation space required around the flange.

The outside diameter generally increases with:

Nominal pipe size
Pressure class
Flange design
Applicable dimensional standard

Higher-pressure flanges are usually larger and thicker because they require stronger bolting and greater structural capacity.

Flange thickness

Flange thickness, commonly represented by T, is measured from the back of the flange to the basic flange face. The raised face is normally excluded unless the chart or drawing specifically states otherwise.

A thicker flange provides greater resistance to bending caused by bolt preload and internal pressure. Flange thickness generally increases as the pressure class rises.

Bolt-circle diameter

Bolt-circle diameter, also called BCD, PCD, or diameter of bolt circle, is the diameter of the imaginary circle passing through the centers of all bolt holes.

The bolt-circle diameter must match between mating flanges. Even if two flanges have the same NPS and outside diameter, they cannot be connected if their bolt-circle dimensions differ.

Number of bolt holes

The number of bolt holes varies according to flange size and pressure class. The holes are equally spaced around the bolt circle to distribute gasket-compression force uniformly.

A standard flange joint should use every specified bolt hole. Operating a flange with missing bolts can produce uneven gasket loading, flange rotation, and leakage.

Bolt-hole diameter

Bolt-hole diameter is slightly larger than the nominal bolt or stud diameter. This clearance helps accommodate manufacturing tolerances and simplifies flange alignment during installation.

For example, a flange designed for ¾-inch bolts may have bolt holes larger than ¾ inch. Therefore, bolt size should not be determined solely by measuring the bolt hole.

Flange bore

The flange bore is the central opening through which the process fluid flows. Its dimensions depend on the flange type and the pipe wall thickness.

For weld neck and socket-weld flanges, the bore may be specified according to the pipe schedule. Matching the flange bore with the pipe’s inside diameter helps maintain a smooth flow path and reduces turbulence.

Raised-face diameter and height

A raised-face flange has a circular sealing surface elevated above the main flange face. Its diameter provides the seating area for the gasket.

Typical raised-face heights are:

Class 150 and 300: approximately 1.6 mm or 1⁄16 inch
Class 400 and above: approximately 6.4 mm or ¼ inch

Raised-face height is generally additional to the listed minimum flange thickness, although the applicable standard and dimensional notes should always be checked.

Hub diameter and hub length

Weld neck, slip-on, socket-weld, and threaded flanges may include a hub. Important hub dimensions include:

Diameter at the base of the hub
Diameter at the welding end
Hub length
Transition profile

The tapered hub of a weld neck flange transfers stress gradually from the flange into the pipe, making it suitable for high-pressure, high-temperature, cyclic, and severe-service applications.

Flange facing

The flange facing is the surface that contacts the gasket. Common facing types include:

Raised Face (RF)
Flat Face (FF)
Ring-Type Joint (RTJ)
Tongue-and-Groove
Male-and-Female

Mating flanges must have compatible facing arrangements. The gasket must also be selected for the facing type, flange class, process fluid, pressure, and temperature.

4. ANSI Flange Pressure Classes: 150, 300, 400, 600, 900, 1500, and 2500

ANSI Flange Pressure Classes: 150, 300, 400, 600, 900, 1500, and 2500

ANSI/ASME pressure classes categorize flanges according to their pressure-temperature capability and dimensional design. ASME B16.5 includes Classes 150, 300, 400, 600, 900, 1500, and 2500.

As the pressure class increases, a flange generally becomes thicker and may have a larger outside diameter, larger bolts, or more bolt holes. However, the exact dimensional changes depend on the nominal pipe size and flange type.

Class 150

ANSI Flange Pressure Classes: 150

Class 150 is the most common low-pressure flange class. It is widely used in:

Low-pressure water piping
Cooling-water systems
Utility air systems
Low-pressure steam service
General chemical-processing systems
Fire-water piping

Despite its designation, a Class 150 flange is not limited to 150 psi. At ambient temperature, some materials may have an allowable pressure higher than 150 psi. The rating decreases as operating temperature increases.

Class 300

Class 300 flanges are heavier and stronger than Class 150 flanges. They typically have greater flange thickness, larger bolts, and different bolt-circle dimensions.

Common applications include:

Medium-pressure steam
Oil and gas processing
Chemical plants
Refinery piping
Compressed-gas systems
High-pressure utility services

A Class 300 flange cannot normally be directly bolted to a Class 150 flange because their dimensions and drilling patterns differ.

Class 400

Class 400 is less frequently used than Classes 300 and 600. It is available for applications requiring pressure capacity between these two common classes.

Because Class 400 components may be less readily available, designers often select Class 600 when greater standardization, inventory availability, or future replacement is important.

Class 600

Class 600 flanges are commonly used for high-pressure and high-temperature process services. Typical applications include:

Refinery process lines
Petrochemical systems
High-pressure steam
Gas transmission
Offshore facilities
Power-generation systems

Class 600 flanges have heavier bodies and stronger bolting arrangements than lower-class flanges. Correct bolt tightening and gasket selection become increasingly important because of the higher joint loads.

Class 900

Class 900 flanges are designed for severe pressure and temperature conditions. They are often found in:

High-pressure oil and gas production
Compressor discharge systems
High-pressure steam systems
Process injection lines
Offshore and subsea facilities

Ring-Type Joint facings are frequently selected for Class 900 service, particularly where high pressure or cyclic loading requires a robust metal-to-metal sealing arrangement.

Class 1500

Class 1500 flanges are used for very-high-pressure services where joint integrity is critical. Their heavy construction requires careful consideration of piping loads, equipment nozzle capacity, installation clearance, and lifting requirements.

Typical applications include high-pressure gas processing, chemical injection, wellhead-related systems, and high-pressure process piping.

Class 2500

Class 2500 is the highest pressure class included in ASME B16.5. These flanges are exceptionally thick and require large-diameter, high-strength bolting.

Typical applications include:

Very-high-pressure gas systems
High-pressure injection service
Critical refinery processes
Specialized chemical-processing systems
Severe high-pressure equipment connections

Class 2500 flanges are available only in a limited size range compared with lower pressure classes.

Comparison of ANSI flange pressure classes

Pressure class	General service level	Typical applications
Class 150	Low pressure	Water, air and low-pressure utilities
Class 300	Medium pressure	Steam, chemical and refinery piping
Class 400	Medium to high pressure	Specialized industrial systems
Class 600	High pressure	Refinery, petrochemical and power systems
Class 900	Severe high pressure	Oil and gas, high-pressure steam
Class 1500	Very high pressure	Gas processing and injection systems
Class 2500	Extremely high pressure	Critical and specialized process service

Pressure class selection must be based on the pressure-temperature rating for the exact flange material rather than a general class description. The flange, gasket, bolting, valves, and connected piping components must all be suitable for the system’s design pressure, design temperature, fluid, and service conditions.

5. Complete ANSI Flange Dimensions Chart

An ANSI flange dimensions chart identifies the measurements needed to confirm whether two flanges can be connected. The most important dimensions are the flange outside diameter, minimum thickness, bolt-circle diameter, number of bolt holes, and bolt-hole diameter.

The following symbols are commonly used in flange dimensional tables:

Symbol	Dimension	Description
NPS	Nominal Pipe Size	Nominal size of the connected pipe
OD	Outside Diameter	Maximum diameter across the flange
T	Flange Thickness	Minimum thickness of the flange body
BCD	Bolt-Circle Diameter	Diameter through the centers of the bolt holes
n	Number of Holes	Total number of equally spaced bolt holes
d	Bolt-Hole Diameter	Diameter of each drilled bolt hole
RF	Raised Face	Raised gasket-contact surface
Bore	Internal Opening	Central opening through the flange
X	Hub Diameter	Diameter at the base of the flange hub
Y	Flange Length	Overall length through the hub
A	Weld-End Diameter	Outside diameter at the welding end

Dimensions controlled by pressure class

For a given nominal pipe size, several flange dimensions change when the pressure class changes:

Outside diameter
Flange thickness
Bolt-circle diameter
Number of bolt holes
Bolt-hole diameter
Required bolt or stud size
Raised-face diameter
Ring-joint dimensions, when applicable

Therefore, pipe size alone is not enough to determine flange compatibility. For example, an NPS 4 Class 150 flange has a different drilling pattern from an NPS 4 Class 300 flange.

Dimensions controlled by flange type

Some dimensions also vary according to flange type:

Weld neck flange: bore, hub diameter, weld-end diameter and overall length
Slip-on flange: bore, hub diameter and overall length
Socket-weld flange: socket bore, socket depth and pipe bore
Threaded flange: threaded bore and thread dimensions
Lap-joint flange: bore, hub and corner radius
Blind flange: flange thickness and internal profile

The outside diameter, bolt circle, number of holes and bolt-hole diameter are generally common to different flange types within the same NPS and pressure class. However, thickness and hub-related dimensions may vary by flange type.

Metric and inch dimensions

ANSI/ASME flange charts may present dimensions in either inches or millimetres. The basic conversion is:

Millimetres = inches × 25.4

Converted metric values may contain decimals because the original ASME dimensions are normally based on inch units. When inspecting a flange, the standard values and applicable dimensional tolerances should be used instead of rounding measurements excessively.

Important chart limitations

The tables below show the principal dimensions used for flange identification and bolt-pattern matching. Detailed manufacturing dimensions—such as bores, hubs, welding ends, facing finishes and tolerances—must be selected according to:

Flange type
Pipe schedule
Material
Facing type
Pressure class
Applicable standard edition

The charts should not be used alone to establish the allowable working pressure. Pressure-temperature ratings must be checked separately using the flange material group and operating temperature.

6. ANSI Class 150 Flange Dimensions Chart

Class 150 flanges are widely used for water, air, oil, low-pressure steam and general industrial piping. The table below presents the principal ASME B16.5 Class 150 drilling dimensions in inches.

NPS	Flange OD (in.)	Min. Thickness (in.)	Bolt Circle (in.)	No. of Holes	Hole Diameter (in.)
½	3.50	0.44	2.38	4	0.62
¾	3.88	0.50	2.75	4	0.62
1	4.25	0.56	3.12	4	0.62
1¼	4.62	0.62	3.50	4	0.62
1½	5.00	0.69	3.88	4	0.62
2	6.00	0.75	4.75	4	0.75
2½	7.00	0.88	5.50	4	0.75
3	7.50	0.94	6.00	4	0.75
3½	8.50	0.94	7.00	8	0.75
4	9.00	0.94	7.50	8	0.75
5	10.00	0.94	8.50	8	0.88
6	11.00	1.00	9.50	8	0.88
8	13.50	1.12	11.75	8	0.88
10	16.00	1.19	14.25	12	1.00
12	19.00	1.25	17.00	12	1.00
14	21.00	1.38	18.75	12	1.12
16	23.50	1.44	21.25	16	1.12
18	25.00	1.56	22.75	16	1.25
20	27.50	1.69	25.00	20	1.25
22	29.50	1.81	27.25	20	1.38
24	32.00	1.88	29.50	20	1.38

Class 150 dimensions in millimetres

NPS	Flange OD (mm)	Min. Thickness (mm)	Bolt Circle (mm)	No. of Holes	Hole Diameter (mm)
½	88.9	11.2	60.3	4	15.7
¾	98.6	12.7	69.9	4	15.7
1	108.0	14.2	79.2	4	15.7
1¼	117.3	15.7	88.9	4	15.7
1½	127.0	17.5	98.6	4	15.7
2	152.4	19.1	120.7	4	19.1
2½	177.8	22.4	139.7	4	19.1
3	190.5	23.9	152.4	4	19.1
3½	215.9	23.9	177.8	8	19.1
4	228.6	23.9	190.5	8	19.1
5	254.0	23.9	215.9	8	22.4
6	279.4	25.4	241.3	8	22.4
8	342.9	28.4	298.5	8	22.4
10	406.4	30.2	362.0	12	25.4
12	482.6	31.8	431.8	12	25.4
14	533.4	35.1	476.3	12	28.4
16	596.9	36.6	539.8	16	28.4
18	635.0	39.6	577.9	16	31.8
20	698.5	42.9	635.0	20	31.8
22	749.3	46.0	692.2	20	35.1
24	812.8	47.8	749.3	20	35.1

The thickness values shown are basic minimum flange-body dimensions. Raised-face height, manufacturing tolerances and corrosion allowances may affect the flange’s measured overall thickness.

7. ANSI Class 300 and Class 600 Flange Dimensions Charts

Class 300 and Class 600 flanges are used where higher pressure-temperature capability is required. Compared with Class 150 flanges, they generally have thicker bodies, stronger bolting and different drilling patterns.

ANSI Class 300 flange dimensions

NPS	Flange OD (in.)	Min. Thickness (in.)	Bolt Circle (in.)	No. of Holes	Hole Diameter (in.)
½	3.75	0.56	2.62	4	0.62
¾	4.62	0.62	3.25	4	0.75
1	4.88	0.69	3.50	4	0.75
1¼	5.25	0.75	3.88	4	0.75
1½	6.12	0.81	4.50	4	0.88
2	6.50	0.88	5.00	8	0.75
2½	7.50	1.00	5.88	8	0.88
3	8.25	1.12	6.62	8	0.88
3½	9.00	1.19	7.25	8	0.88
4	10.00	1.25	7.88	8	0.88
5	11.00	1.38	9.25	8	0.88
6	12.50	1.44	10.62	12	0.88
8	15.00	1.62	13.00	12	1.00
10	17.50	1.88	15.25	16	1.12
12	20.50	2.00	17.75	16	1.25
14	23.00	2.12	20.25	20	1.25
16	25.50	2.25	22.50	20	1.38
18	28.00	2.38	24.75	24	1.38
20	30.50	2.50	27.00	24	1.38
22	33.00	2.62	29.25	24	1.62
24	36.00	2.75	32.00	24	1.62

An important example is the NPS 2 flange. A Class 150 flange uses four bolt holes on a 4.75-inch bolt circle, while a Class 300 flange uses eight bolt holes on a 5-inch bolt circle. The two classes are therefore not interchangeable.

ANSI Class 600 flange dimensions

NPS	Flange OD (in.)	Min. Thickness (in.)	Bolt Circle (in.)	No. of Holes	Hole Diameter (in.)
½	3.75	0.56	2.62	4	0.62
¾	4.62	0.62	3.25	4	0.75
1	4.88	0.69	3.50	4	0.75
1¼	5.25	0.81	3.88	4	0.75
1½	6.12	0.88	4.50	4	0.88
2	6.50	1.00	5.00	8	0.75
2½	7.50	1.12	5.88	8	0.88
3	8.25	1.25	6.62	8	0.88
3½	9.00	1.38	7.25	8	1.00
4	10.75	1.50	8.50	8	1.00
5	13.00	1.75	10.50	8	1.12
6	14.00	1.88	11.50	12	1.12
8	16.50	2.19	13.75	12	1.25
10	20.00	2.50	17.00	16	1.38
12	22.00	2.62	19.25	20	1.38
14	23.75	2.75	20.75	20	1.50
16	27.00	3.00	23.75	20	1.62
18	29.25	3.25	25.75	20	1.75
20	32.00	3.50	28.50	24	1.75
22	34.25	3.75	30.63	24	1.88
24	37.00	4.00	33.00	24	2.00

Class 300 versus Class 600

Some smaller Class 300 and Class 600 flanges share the same outside diameter and drilling pattern. This does not mean they have identical pressure ratings or that components may be substituted without engineering review.

Differences may exist in:

Minimum flange thickness
Hub dimensions
Weld-end dimensions
Bore dimensions
Facing details
Pressure-temperature ratings
Required materials and bolting

Flange compatibility must therefore be verified using the complete designation rather than bolt alignment alone. At a minimum, the NPS, pressure class, standard, flange type, facing, material and pipe schedule should be confirmed before installation.

8. ANSI Class 900, 1500, and 2500 Flange Dimensions Charts

Class 900, 1500, and 2500 flanges are designed for severe high-pressure and high-temperature services. These flanges have substantially thicker bodies, stronger hubs, larger bolts, and more robust sealing arrangements than lower-pressure flanges.

High-pressure flange joints commonly use Raised Face (RF) or Ring-Type Joint (RTJ) facings. RTJ flanges are particularly common in refinery, offshore, gas-processing, and high-pressure injection systems.

ANSI Class 900 flange dimensions

The following table provides principal ASME B16.5 Class 900 drilling dimensions in inches.

NPS	Flange OD (in.)	Min. Thickness (in.)	Bolt Circle (in.)	No. of Holes	Hole Diameter (in.)
½	4.75	0.88	3.25	4	0.88
¾	5.12	1.00	3.50	4	0.88
1	5.88	1.12	4.00	4	1.00
1¼	6.25	1.12	4.38	4	1.00
1½	7.00	1.25	4.88	4	1.12
2	8.50	1.50	6.50	8	1.00
2½	9.62	1.62	7.50	8	1.12
3	9.50	1.50	7.50	8	1.12
4	11.50	1.75	9.25	8	1.25
5	13.75	2.00	11.00	8	1.50
6	15.00	2.19	12.50	12	1.38
8	18.50	2.50	15.50	12	1.62
10	21.50	2.75	18.50	16	1.62
12	24.00	3.12	21.00	20	1.50
14	25.25	3.38	22.00	20	1.62
16	27.75	3.50	24.25	20	1.75
18	31.00	4.00	27.00	20	2.00
20	33.75	4.25	29.50	20	2.12
24	41.00	5.50	35.50	20	2.62

Class 900 dimensions do not always increase uniformly from one nominal size to the next. The applicable ASME table should therefore be consulted rather than estimating dimensions from adjacent pipe sizes.

ANSI Class 1500 flange dimensions

NPS	Flange OD (in.)	Min. Thickness (in.)	Bolt Circle (in.)	No. of Holes	Hole Diameter (in.)
½	4.75	0.88	3.25	4	0.88
¾	5.12	1.00	3.50	4	0.88
1	5.88	1.12	4.00	4	1.00
1¼	6.25	1.12	4.38	4	1.00
1½	7.00	1.25	4.88	4	1.12
2	8.50	1.50	6.50	8	1.00
2½	9.62	1.62	7.50	8	1.12
3	10.50	1.88	8.00	8	1.25
4	12.25	2.12	9.50	8	1.50
5	14.75	2.88	11.50	8	1.75
6	15.50	3.25	12.50	8	1.88
8	19.00	3.62	15.50	12	2.00
10	23.00	4.25	19.00	12	2.50
12	26.50	4.88	22.50	16	2.38
14	29.50	5.62	25.00	16	2.62
16	32.50	5.75	27.75	16	2.88
18	36.00	7.00	30.50	16	3.25
20	38.75	7.50	32.75	16	3.50
24	46.00	9.00	39.00	16	4.00

Some Class 900 and Class 1500 flanges in smaller sizes share common drilling dimensions. However, this does not establish full interchangeability. Their thicknesses, pressure ratings, hubs, bores, facing details, and required bolting may be different.

ANSI Class 2500 flange dimensions

Class 2500 flanges are used for extremely high-pressure service and are available over a more limited nominal-size range than lower classes.

NPS	Flange OD (in.)	Min. Thickness (in.)	Bolt Circle (in.)	No. of Holes	Hole Diameter (in.)
½	5.25	1.19	3.50	4	1.00
¾	5.50	1.25	3.75	4	1.00
1	6.25	1.50	4.25	4	1.12
1¼	7.25	1.75	5.12	4	1.25
1½	8.00	2.00	5.75	4	1.38
2	9.25	2.50	6.75	8	1.25
2½	10.50	2.75	7.75	8	1.38
3	12.00	3.00	9.00	8	1.62
4	14.00	3.50	10.75	8	1.88
5	16.50	4.25	12.75	8	2.25
6	19.00	4.88	14.50	8	2.62
8	21.75	5.75	17.25	12	2.38
10	26.50	7.25	21.25	12	3.00
12	30.00	8.50	24.38	12	3.38

Because Class 2500 joints develop extremely high bolt loads, flange alignment, gasket selection, bolting material, lubrication, tightening method, and piping support require careful engineering control.

The charts in this section provide general dimensional reference information. Final manufacturing, purchasing, fabrication, and inspection decisions should be verified against the applicable edition of ASME B16.5.

9. How to Read and Measure ANSI Flange Dimensions

Correctly identifying an existing flange requires more than measuring its outside diameter. Different pressure classes or flange standards may have similar overall dimensions, particularly in smaller sizes.

Before taking measurements, isolate and depressurize the piping system where necessary. Clean the flange sufficiently to expose its markings, bolt holes, facing, and hub profile.

Check the flange markings

Begin by examining the markings stamped or forged around the flange rim. Depending on the manufacturer and material specification, the marking may include:

Manufacturer’s name or trademark
Nominal pipe size
Pressure class
Material grade
Heat or batch number
ASME designation
Flange type
Bore or pipe schedule
Ring number for an RTJ flange

A typical marking might appear as:

4 – 600 – WN – RF – ASTM A105 – SCH 80

This identifies an NPS 4, Class 600 weld neck flange with a raised face, manufactured from ASTM A105 material and bored for Schedule 80 pipe.

Measure the flange outside diameter

Measure across the widest part of the flange using a tape, calliper, or outside-diameter measuring tool. The measurement should pass through the centre of the flange.

Compare the measured value with the OD column in the appropriate chart. Do not identify the flange using OD alone because multiple standards or pressure classes may use similar outside diameters.

Measure the bolt-circle diameter

For a flange with an even number of holes, measure from the centre of one bolt hole to the centre of the directly opposite hole.

If measuring between hole edges, the bolt-circle diameter can be calculated as:

BCD = outside-edge distance − bolt-hole diameter

or:

BCD = inside-edge distance + bolt-hole diameter

For flanges without directly opposite holes, the BCD should be calculated from the hole spacing and total number of holes or measured using a suitable flange template.

Count and measure the bolt holes

Count the total number of bolt holes and measure the diameter of one hole. Bolt holes are normally slightly larger than the bolts or studs installed through them.

Do not assume that the bolt diameter equals the measured hole diameter. The proper bolt size must be obtained from the relevant bolting table.

The combination of the following measurements usually provides a reliable pressure-class identification:

Flange OD
Bolt-circle diameter
Number of bolt holes
Bolt-hole diameter

Measure the flange thickness

Measure the flange body from its back surface to the basic flange face. Do not include the raised-face height unless the dimensional chart specifically includes it.

Corrosion, coating, machining, and manufacturing tolerances can affect the measured thickness. A heavily corroded flange should not be accepted merely because its remaining thickness is close to a nominal chart value.

Identify the flange type

Inspect the flange profile and its connection to the pipe:

A weld neck flange has a long tapered hub and a butt-weld connection.
A slip-on flange slides over the pipe and normally has fillet welds inside and outside.
A socket-weld flange contains a recessed socket into which the pipe is inserted.
A threaded flange has an internal pipe thread.
A lap-joint flange rotates around a separate stub end.
A blind flange has no central bore.

The flange type affects the applicable bore, hub, thickness, and installation requirements.

Identify the facing type

Inspect the gasket-contact surface to determine whether the flange has:

Raised Face
Flat Face
Ring-Type Joint
Tongue-and-Groove
Male-and-Female facing

For an RTJ flange, measure and identify the ring groove carefully. The mating flanges must use compatible groove dimensions and the correct gasket ring number.

Check the pipe schedule

Pipe schedule is particularly important for weld neck and socket-weld flanges. Two weld neck flanges may have the same size, class, and drilling pattern but different bores because they are intended for different pipe wall thicknesses.

The flange bore should match the pipe inside diameter as closely as practical to provide proper weld preparation and avoid a significant internal step.

Compare all dimensions

A flange should be identified by comparing all available information rather than relying on one measurement. A practical sequence is:

Read the flange markings.
Determine the nominal pipe size.
Measure the outside diameter.
Measure the bolt-circle diameter.
Count the bolt holes.
Measure the bolt-hole diameter.
Measure the flange thickness.
Identify the flange type and facing.
Confirm the material and pipe schedule.
Compare the results with the applicable ASME standard.

10. How to Select the Correct ANSI Flange Size and Pressure Class

Selecting an ANSI flange requires consideration of the entire operating system. A flange that physically fits the pipe may still be unsuitable because of its pressure rating, temperature capability, material, facing, or connection method.

Determine the nominal pipe size

Select a flange with the same NPS as the connected pipe or equipment nozzle. Confirm the pipe outside diameter and wall thickness because the NPS designation alone does not define the bore.

For a weld neck flange, specify the correct pipe schedule so that the flange bore corresponds with the pipe’s internal diameter.

Establish the design pressure and temperature

Flange class must be selected using both the design pressure and design temperature. The system’s normal operating conditions should not be used as substitutes when higher design conditions are specified.

The required class is established by checking the ASME pressure-temperature rating table for the selected material group.

A safe selection process is:

Identify the design pressure.
Identify the design temperature.
Select the flange material.
Find the material group in ASME B16.5.
Compare the available class ratings.
Select a class whose allowable pressure is at least equal to the design pressure.

Additional project, piping-code, corrosion, cyclic-service, or safety requirements may require a higher class.

Select a suitable flange material

The flange material must be compatible with the process fluid, piping material, design temperature, corrosion environment, and applicable piping code.

Common flange materials include:

ASTM A105 forged carbon steel
ASTM A350 LF2 low-temperature carbon steel
ASTM A182 F304 or F304L stainless steel
ASTM A182 F316 or F316L stainless steel
ASTM A182 alloy-steel grades
Nickel-alloy materials
Duplex and super-duplex stainless steels

Material compatibility includes more than corrosion resistance. Thermal expansion, galvanic corrosion, weldability, toughness, and pressure-temperature strength must also be considered.

Choose the flange type

The appropriate flange type depends on pressure, temperature, piping loads, maintenance requirements, and fabrication method.

Flange type	Typical selection considerations
Weld neck	High pressure, high temperature, cyclic or severe service
Slip-on	General low- to moderate-pressure piping
Socket weld	Small-bore, high-pressure piping
Threaded	Systems where welding is undesirable or impractical
Lap joint	Systems requiring frequent dismantling or flange alignment
Blind	Closing a pipe end, valve outlet or equipment nozzle

Weld neck flanges are often preferred for critical service because their tapered hubs distribute mechanical stress more gradually into the pipe.

Select the flange facing

The facing must suit the pressure class, gasket type, process service, and mating component.

Raised Face: Common in general refinery and process piping
Flat Face: Frequently used with cast-iron or nonmetallic equipment
Ring-Type Joint: Used for high-pressure, high-temperature, and critical service
Tongue-and-Groove: Provides controlled gasket positioning
Male-and-Female: Helps confine and locate the gasket

A raised-face flange should not normally be tightened directly against a flat-face cast-iron flange unless the joint is specifically designed for that arrangement. Excessive bending can damage the weaker flange.

Verify mating-flange compatibility

Before purchasing or installing a flange, confirm that both sides of the joint have compatible:

Nominal pipe size
Pressure class
Dimensional standard
Facing type
Bolt-circle diameter
Number and size of bolt holes
Gasket dimensions
Material requirements

A flange adapter may be required when connecting components manufactured to different standards, such as ASME, EN, JIS, or API.

Select the gasket and bolting

The flange joint should be treated as a system consisting of the flange, gasket, bolts or studs, nuts, and installation method.

The gasket must be suitable for:

Flange facing
Pressure class
Process fluid
Design pressure
Design temperature
Required emissions performance

Bolting must have the correct diameter, length, material grade, thread form, and mechanical strength. Bolts that fit through the holes are not necessarily suitable for the design conditions.

Consider external piping loads

Flanges may be exposed to forces and moments caused by:

Pipe weight
Thermal expansion
Equipment movement
Vibration
Misalignment
Wind or seismic loads
Unsupported valves and fittings

High external loads can cause flange rotation, uneven gasket compression, leakage, or fatigue failure. Proper piping support and flexibility analysis may therefore be required.

Final flange selection checklist

Before finalizing a flange specification, verify:

Correct NPS
Correct pressure class
Applicable ASME standard
Suitable flange type
Suitable material grade
Correct pipe schedule or bore
Compatible facing
Suitable gasket
Correct bolting
Acceptable pressure-temperature rating
Corrosion allowance, where applicable
Project specification and piping-code compliance
Compatibility with the mating flange

A complete flange description should clearly state all essential information. For example:

NPS 4, Class 600, weld neck, raised face, ASTM A182 F316L, Schedule 80 bore, ASME B16.5.

This complete designation helps prevent dimensional mismatches, incorrect materials, unsuitable pressure ratings, and installation delays.

Conclusion

ANSI/ASME flanges provide standardized, removable connections between pipes, valves, pumps, pressure vessels, and other piping equipment. Correct standardization allows components from different manufacturers to be assembled with compatible dimensions, bolting patterns, facing arrangements, and pressure-temperature capabilities.

The term “ANSI flange” remains widely used, although most current dimensional requirements are established by ASME. ASME B16.5 covers flanges from NPS ½ through NPS 24, while ASME B16.47 covers large-diameter flanges from NPS 26 through NPS 60.

An ANSI flange dimensions chart typically includes:

Nominal pipe size
Flange outside diameter
Minimum flange thickness
Bolt-circle diameter
Number of bolt holes
Bolt-hole diameter
Raised-face dimensions
Bore and hub dimensions

Pressure classes—including Class 150, 300, 400, 600, 900, 1500, and 2500—do not represent fixed pressure ratings in psi. The actual allowable working pressure depends on the flange material and operating temperature. The appropriate ASME pressure-temperature table must therefore be consulted during flange selection.

Flanges should never be matched by outside diameter or bolt-hole alignment alone. The nominal size, pressure class, standard, flange type, facing, material, pipe schedule, gasket, and bolting must all be compatible.

The dimensional charts in this guide are useful for preliminary selection and field identification. However, final engineering, procurement, fabrication, and inspection decisions should always be verified against the applicable edition of ASME B16.5, ASME B16.47, the governing piping code, and the project specification.

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