For rubber sheet, gasket, and O-ring elastomers, the practical hardness window is 40 to 80 Shore A, and 70 Shore A is the de-facto default for industrial seals [S5][S8]. Readings are taken on a spring-loaded indentor per ASTM D2240 (ISO 7619-1): 0 means full penetration, 100 means no penetration, and the test takes under five seconds [S5].
The same datum drives three engineering decisions at once, namely bolt load needed to seat the gasket, ability to fill flange surface finish, and resistance to extrusion at system pressure. Most rubber compounds are manufactured around 65 Shore A, but usable span runs 20 to 90 Shore A across the elastomer families [S2][S3].
What the Shore A reading tells you on a flange
Higher numbers mean harder, less compliant material; lower numbers mean softer, more conformable material, and the convention applies across rubber, foam, and cellular elastomers [S4][S7]. A 70 Shore A reading is the running-shoe-sole reference point, the most common industrial gasket hardness, while 80 Shore A behaves like a leather belt, firm and limited in flex [S5].
The scale is not linear in sealing behaviour: dropping from 70 to 50 Shore A roughly halves the bolt torque required to achieve a given compression, but also roughly halves the pressure at which the gasket will cold-flow out of the joint [S5]. For soft materials (20 to 40 Shore A) the risk shifts to extrusion; for hard materials (80 Shore A and up) the risk shifts to flange damage, particularly on PVC, fibreglass, and glass-lined flanges [S5].
Hardness windows for the seven common elastomers
Nitrile (NBR) compounds are commonly supplied in the 40 to 90 Shore A range with stock thicknesses from 0.5 mm to 25 mm, and they pair that span with excellent resistance to petroleum oils, fuels, and greases [S1]. Neoprene (CR) is typically specified at 40 to 80 Shore A, with a -20 to +90°C operating window and BS 2752 commercial grades available, but it has poor low-temperature flexibility [S1].
Natural rubber spans 35 to 90 Shore A, gives very good mechanical properties and good alkali or acid resistance, but is attacked by oils, fats, fuels, and oxidising agents, and is limited to -30 to +80°C [S1]. Silicone (VMQ) covers 20 to 80 Shore A, retains flexibility down to -50°C, and resists +250°C, but has poor mechanical strength and only moderate oil or fuel resistance [S1].
EPDM, Neoprene, and Silicone each offer the common 40 to 80 Shore A span, while Butyl (IIR) is a narrow 60 to 65 Shore A material prized for gas impermeability and solvent resistance rather than compliance range [S1][S6]. PTFE is graded at 50 to 65 Shore A on the elastomer scale, but functions more as a plastic sealing face with -100 to +250°C capability and the lowest coefficient of friction of any common gasket material [S1].
Hardness as a four-criterion selection matrix
Read across the matrix below to pick a starting compound. The criteria are the four that actually move under bolt load: Shore A hardness, minimum service temperature, maximum service temperature, and oil/fuel resistance rating. [S5]
Family, Shore A span, Min temp, Max temp, Oil/fuel rating: NBR 40 to 90, -20°C, +110°C, Excellent; CR (Neoprene) 40 to 80, -20°C, +90°C, Good; EPDM 40 to 80, -40°C (typical), +120°C (typical), Poor; FKM (Viton) 60 to 90 (typical), -10°C (typical), +200°C (typical), Excellent; VMQ (Silicone) 20 to 80, -50°C, +250°C, Poor; NR (Natural) 35 to 90, -30°C, +80°C, Poor; IIR (Butyl) 60 to 65, -20°C, +90°C, Poor; PTFE 50 to 65, -100°C, +250°C, Excellent (chemical) [S1][S5].
Use this table to shortlist, not to finalise: FKM and PTFE both clear the chemical bar, but FKM is elastomeric (compresses under bolt load) while PTFE behaves as a plastic face that needs higher compressive stress to seat [S1][S5]. For diaphragm-style isolation between an elastomer body and a process fluid, a related comparison of EPDM and PTFE-faced diaphragms across chemistry, temperature, and cost is laid out in this diaphragm valve decision guide.
Cross-scale conversion: Shore A, Shore D, IRHD
For seals above 90 Shore A, the reading migrates to the Shore D scale, with the overlap fixed at approximately 90 Shore A equals 40 Shore D, and the IRHD (International Rubber Hardness Degrees) value tracking Shore A within roughly two points across the elastomer range [S3][S5]. Shore 00 is reserved for gels and very soft foams that would bottom out the A scale [S5].
For a given application the question is rarely "what is the absolute hardness" but "what hardness does my flange finish and my bolt load tolerate" [S4]. A soft 30 to 50 Shore A compound will fill a 3.2 to 6.3 µm Ra flange easily but may extrude above roughly 10 bar in a non-reinforced joint, while a 70 to 80 Shore A compound on the same flange will need roughly 1.5 to 2 times the seating stress and risks crushing on plastic or glass-lined flanges [S5].
Durometer is necessary, never sufficient
Hardness correlates with compression set, rebound, tear strength, and abrasion resistance, but it does not predict chemical compatibility, and it does not replace a full materials review for hydrocarbons, oxidising agents, or strong acids and alkalis [S1][S4]. A NBR at 70 Shore A still fails in chlorinated or strong oxidising service, regardless of how well it seats, while a 70 Shore A EPDM will fail in petroleum oil service within hours [S1].
For specification traceability on elastomers, the relevant standards are ASTM D2240 (indentation hardness), ISO 7619-1, ASTM D395 (compression set), and BS 2751 / BS 2752 for commercial NBR and CR grades respectively [S1][S5]. For a process engineer cross-checking gasket seating against the upstream valve spec, the Cv and bore selection logic for full-bore versus reduced-bore ball valves gives the same kind of trade-off envelope in a different component. A broader reference on the gasket material family and its operating envelope is also useful when the question is which elastomer class to enter the matrix with.
Failure modes tied directly to wrong hardness
Too soft, below roughly 40 Shore A for a pressurised joint, typically fails by extrusion and split-through, with the gap between flange faces acting as a die [S5]. Too hard, above roughly 80 Shore A on a non-metallic flange, typically fails by flange surface cracking, bolt-thread fatigue from excess torque, or by the gasket never reaching its seal-stress threshold [S5].
Hardness drift in service is itself a fault signal: a 70 Shore A NBR that measures 55 Shore A after a year in hot oil has plasticised, and the joint needs re-evaluation, not just re-torquing. Use a calibrated durometer and re-test at room temperature; do not infer hardness from finger pressure, since the working range of 40 to 80 Shore A is precisely where finger estimates are most unreliable.
Trackable signals for the next review cycle: watch for any update to ASTM D2240 micro-hardness revisions for thin-section gaskets under 1.5 mm, and any ISO 7619-1 amendment to IRHD normalising procedures for TPE and TPV gaskets, both of which sit on the 2026-2027 standards maintenance calendar at the time of writing. Specifying engineers should also re-check the advanced elastomer and chemical-resistant material reference when the service moves above 200°C or into strong oxidisers, since the rubber families covered here cap out well before that envelope.