Flange Gaskets Calculator (pipeline piping)

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Gaskets calculates the dimensions and properties of flange gaskets such as; RTJ, BX, RX, CAF, etc.


Gaskets are required to contain fluid under pressure and, it is hoped, that gasket compression does not damage the flange mating surfaces. Gaskets are therefore manufactured from softer material than the flange (mating surfaces) as they are expected to deform and flow into and fill any gaps and undulations in the flanges and yet provide sufficient strength to resist the pressure contained within. After uncoupling the mating faces the gasket may be reused or discarded depending upon the level of permanent deformation and work hardening.

The material from which a gasket is made is therefore important. If it is too hard it will not deform sufficiently and may damage the flange, and if it is too soft the gasket material will simply flow out of the gap between the flanges or have insufficient strength to withhold internal pressure. It must also be able to withstand operating temperatures and coincident contamination from any chemicals with which it may come into contact.

Flange Gaskets

Typical flange gasket designs including RTJ and flat
Fig 1. Typical Designs

Flange gaskets are annular rings either of flat section (Spiral Wound or CAF) or heavy section steel rings (ring type joints) rated according to their pressure carrying capacity. Fig 1 shows the principal gasket types used in the piping, drilling and pipeline industries, each of which is targeted for a particular application:
(see Materials below)

R Oval

These gaskets are general purpose and very flexible (in terms of their usability range). They tend to be manufactured from soft iron but may also be manufactured from various stainless steels.

R Oval gaskets will fit into grooves with sloping sides (23°) and/or rounded bottoms and are also capable of sealing grooves deformed during previous use whether with octagonal or oval ring type joints.

Main applications are pipelines and drilling equipment.

R Octagonal

These gaskets are general purpose. They tend to be manufactured from various stainless steels although they can be manufactured from soft iron.

R Octagonal gaskets only fit in grooves with sloping sides (23°). They are capable of higher internal pressures and greater flange bending moments than R Oval gaskets but are normally discarded after uncoupling as they can tolerate less deformation.

Main applications are pipelines and drilling equipment.


These gaskets are for very high pressure applications. They are usually manufactured from various stainless steels. The hole in this gasket is used for pressure energised self-sealing.

BX gaskets only fit in grooves with sloping sides (23°). They are capable of greater flange bending moments than the RX gaskets but lower internal pressures and are normally discarded after uncoupling.

Main applications are pipelines.


These gaskets are for the highest pressure applications and are always manufactured from stainless steel. The holes in this gasket are used for pressure energised self-sealing which is biased to the outside diameter of the gasket. The sealing capacity of these gaskets increases proportionally with internal pressure.

RX gaskets only fit in grooves with sloping sides (23°). They are capable of greater internal pressure than the BX gaskets but lower flange bending moments. They are also longer than BX gaskets and the stud bolts will need to be lengthened to accommodate them. These gaskets are always discarded after uncoupling.

Main applications are pipelines.


CAF gaskets are flat and manufactured from pliable materials that readily deform against non-flat surfaces (such as serrations). They are normally used for relatively low pressure loading⁽¹⁾ applications such as piping systems and discarded after uncoupling.

However, copper, aluminium and soft iron 'CAF-type' gaskets are also used where bending moments are present in the flange joint, internal fluid pressure is high and serrations are less than 1/64th of an inch (0.4mm) or the flange faces are flat and smooth.

Main applications are relatively low pressure piping systems.

Spiral Wound

These gaskets are similar in design and application to fibrous CAF gaskets but interleaved with stainless steel, copper or aluminium. They are used in joints that require regular uncoupling and can be reused several times.

Design Codes

All internationally recognised design codes, standards and recommended practices recognise the ANSI pressure class numbering system. There are a couple of alternative pressure class numbering systems (API and ISO) the relationship between which is explained in our Pressure Classes web page.

The ANSI (B 16), API (6A), BS (3293), MSS (SP-44) and ISO (7005) design specifications provide standard flange and gasket sizes and materials suited to a specific pressure class rating. They do not address the use of non-standard dimensions or the application of external mechanical loads.

ASME VIII, Division 1, Appendix 2, along with Appendices P and S, provides an internationally recognised calculation method for standard and non-standard flanges based upon gasket seating and bending moments induced by the bolts. The gasket factors and seating stresses ASME use to define flange bolt loading requirements for various gaskets are based upon shape (m) and material seating stress (y) respectively, some of which are listed below for your flange calculations.

Flat CAF-type Gaskets

Material Gasket Factor 'm' Seating Stress 'y' (psi)
Elastomers < 75A (Shore) 0.5 0
Elastomers > 75A (Shore) 1 200
Elastomers with cotton filler 1.25 400
1-ply elastomers-asbestos filled 2.75 3,700
2-ply elastomers-asbestos filled 2.5 2,900
3-ply elastomers-asbestos filled 2.25 2,200
Vegetable fibre 1.75 1,100
1/8" asbestos (with binder) 2 1,600
1/16" asbestos (with binder) 2.75 3,700
1/32" asbestos (with binder) 3.5 6,500
Aluminium jacket-asbestos filled 3.25 5,500
Copper or brass jacket-asbestos filled 3.5 6,500
Iron jacket-asbestos filled 3.75 7,600
Monel jacket-asbestos filled 3.5 8,000
4% to 6% chrome jacket-asbestos filled 3.75 9,000
Stainless jacket-asbestos filled 3.75 9,000
Nickel alloy jacket-asbestos filled 3.75 9,000

Flat Spiral Wound (asbestos filled) Gaskets

Material Gasket Factor 'm' Seating Stress 'y' (psi)
Carbon 2.5 10,000
Metals 3 10,000

Corrugated Gaskets

Material Gasket Factor 'm' Seating Stress 'y' (psi)
Aluminium jacket-asbestos filled 2.5 2,900
Copper or brass jacket-asbestos filled 2.75 3,700
Iron jacket-asbestos filled 3 4,500
Monel jacket-asbestos filled 3.25 5,500
4% to 6% chrome jacket-asbestos filled 3.25 5,500
Stainless jacket-asbestos filled 3.5 6,500
Nickel alloy jacket-asbestos filled 3.5 6,500
Aluminium 2.75 3,700
Copper or brass 3 4,500
Iron 3.25 5,500
Monel 3.5 6,500
4% to 6% chrome 3.5 6,500
Stainless 3.75 7,600
Nickel alloy 3.75 7,600

Grooved Gaskets

Material Gasket Factor 'm' Seating Stress 'y' (psi)
Aluminium 3.25 5,500
Copper or brass 3.5 6,500
Iron 3.75 7,600
Monel 3.75 9,000
4% to 6% chrome 3.75 9,000
Stainless 4.25 10,100
Nickel alloy 4.25 10,100

RTJ Gaskets

Material Gasket Factor 'm' Seating Stress 'y' (psi)
Iron 5.5 18,000
Monel 6 21,800
4% to 6% chrome 6 21,800
Stainless 6.5 26,000
Nickel alloy 6.5 26,000

Note: The above values are provided here for information only as they are required for input data in CalQlata's flanges calculator


The flange gaskets calculator is a dimensional database and calculator, it does not include the calculation factors or material properties ('m' & 'y') that must be considered when designing/selecting the flange that will use them. The above data must be entered manually in the flanges calculator.

Soft Iron

Soft iron includes very low carbon steel that has been stress relieved to allow significant grain growth. These materials provide excellent pressure sealing capacity due to their ability to deform but they offer poor chemical resistance. Whilst they can be (and often are) reused, this is only practical if work hardening from prior use is not significant as subsequent stress relieving can distort the gasket making it dimensionally unstable.

Gaskets manufactured from soft iron are frequently used for hydrostatic testing due to their low cost and low damage (to the flange) potential.

Stainless Steels

Stainless steels include chemically resistant alloys such as AISI 316 and 304, neither of which suffer from significant work hardening under deformation and therefore R Oval gaskets made from stainless steel may be reused if deformation during prior use is acceptable. These materials provide excellent high-pressure sealing capacity and good chemical resistance.

More expensive materials such as Duplex, Inconel, Monel, etc. are also used where chemical resistance is important, moreover these materials do not work harden with use. They could be re-useable many times as R Oval or R Octagonal gaskets with little detrimental effect, but they can also be very expensive.

Fabrics and Polymers

Non-metallic materials are generally preferred for piping flange gaskets due to their high flexibility and low relative cost. Asbestos provides satisfactory temperature and chemical resistance but its less attractive properties tend to ensure that alternatives are sought where possible. Vegetable fibre is a preferred alternative fabric to asbestos for low temperatures applications and copper or aluminium for high temperatures.

Preferred polymers tend to be fluoropolymers, which offer excellent temperature and chemical resistance but because of their tendency to flow under stress they need to be contained in a tongue and groove type flange facing.

Flange Gaskets Calculator - Technical Help

The flange gaskets calculator contains a database of the principal dimensions for 1000 standard circular pipeline and piping flange gaskets according to pressure rating and specification. On selecting a particular gasket from the database, its dimensions are inserted into the input/output data and used to calculate ASME's basic gasket seating width ('bₒ'), on which the effective gasket seating width (b) in flange calculations is based. Together with 'm', 'y' and database value 'Øᴳ', these two values ('b' & 'bₒ') define the flange bolt loading requirements for gasket seating and therefore the maximum stresses in the flange for this loading condition.

If you alter any database input value, the flange gaskets calculator will recalculate using your modified value(s). In this way, you may determine a basic gasket seating width for non-standard gaskets. However, when you select an alternative gasket the input data will automatically revert to the database value(s) on your return.

Calculation Options

The 'BX', 'RX', 'R Oval' and 'R Octagonal' calculation options are all ring-type joint gaskets. They all calculate 'bₒ' using Facing Sketch⁽²⁾ (6). You cannot pass from any of the ring-type joints to the 'Facing Sketch' calculation option.

The 'Spiral Wound' and 'CAF' calculation options are for flat gaskets for which you need to select a 'Facing Sketch' design option.

The 'Facing Sketch' calculation option provides standard flange face designs for flat gaskets. When you select this option from either 'Spiral Wound' or 'CAF' the selected 'Facing Sketch' will remain active whenever you return to either of the flat gasket options.

Input Data

'Øᴳ' is the pitch (nominal) diameter of the gasket
'N' is the width across the face of the gasket (Fig 1) and is used to calculate 'Øₒ', 'Øᵢ' and 'bₒ'
'w' is the width of a flange nubbin. This value is not included in the gasket database and must be entered manually if present. If there is no nubbin, you should set this value equal to 'N'. If you have selected a Facing Sketch that uses 'w' in its calculation and you have set 'w'>'N' a warning will be displayed as the calculation for 'bₒ' will be based upon a nubbin width greater than the gasket width (N), which is a fictional condition for the ASME formulas.

Output Data

'bₒ' is the basic seating width for the gasket and used to calculate the effective seating width (b) in the ASME flange calculation
'Øₒ' and 'Øᵢ' are the outside and inside diameters of the gasket respectively and provided for information only. These values are not used in the flange calculations.
't' is the gasket thickness and included in the database (Fig 1). This information is not used in the calculation but is provided for flange bolting length determination

Data Export

Flanges imports 'Øᴳ', 'N', 'w' and 'bₒ' from this (flange gaskets) calculator. You must, however, remember to save⁽³⁾ the gasket calculation prior to importing the output data into Flanges.


The flange gaskets calculator applies only to circular piping and pipeline flange gaskets of any design or size.


The flange gaskets calculator is as accurate as the design code ASME VIII, Division 1, Section 2.


  1. For example, piping with a ½" wall thickness: 25,000psi internal pressure in a ½" diameter pipe will result in an end cap force of 491lbf (2189N), a longitudinal stress of 1250psi (8.6MPa) and a hoop stress of 1250psi (8.6MPa), whereas just 250psi in a 36" pipe will produce an end cap force of 254,469lbf (1,134,700N), a longitudinal stress of 8937psi (61.6MPa) and a hoop stress of 9000psi (62MPa). When CalQlata refers to low or high 'pressure loading' it is referring to the condition that produces the lowest or greatest end cap forces and stresses as a result of fluid pressure. In the aforementioned cases, the highest pressure loading occurs in the pipe with the lowest pressure.
  2. 'Facing Sketch' is the term used in the ASME VIII design code (Table 2-5.2) to describe various gasket sealing profile options that define the basic gasket seating width ('bₒ')
  3. You save a calculation either by closing the calculator or selecting menu item 'File'>'Save Data'

Further Reading

You will find further reading on this subject in reference publications(47, 48 & 49)