Durometer Scale Chart: Shore A vs Shore D vs Shore OO
What a durometer reading actually means, which Shore hardness scale to use, why the scales do not convert, and where common gasket materials fall — with an interactive reading guide and reference chart.
Built for: reading material data sheets, writing gasket specs, and picking the right hardness for flange conditions.
Durometer Scale Explorer
How it works — formula, worked example & reference
Formula: There is no formula — per ASTM D2240, each Shore scale uses its own indenter geometry and spring force, so scales do not cross-convert. This guide places a reading on its scale and describes what that firmness means in practice.
Worked example: A gasket spec reading Shore A 60 sits mid-scale: firm handshake-rubber feel, typical of general-purpose solid EPDM, neoprene, and silicone gasket grades.
| Feel / anchor (typical, approximate) | Shore OO | Shore A | Shore D |
|---|---|---|---|
| Gel insole, very soft foam | 20-60 | — | — |
| Soft silicone sponge, weatherstrip | 70-90 | 5-25 | — |
| Rubber band | — | ~25 | — |
| Pencil eraser | — | ~40 | — |
| Tire tread | — | 60-70 | — |
| Shoe heel, cart wheel | — | 80-95 | 30-45 |
| Hard hat, cutting board | — | — | 60-80 |
Anchors are typical published approximations for orientation, not conversions (ASTM D2240).
These results are engineering estimates for preliminary material selection; final validation is the responsibility of your engineering team.
Common questions
Can you convert Shore A to Shore D?
Not exactly. ASTM D2240 defines each scale with a different indenter and spring force, so there is no true mathematical conversion. Published cross-references are approximations for orientation only; when a spec calls a scale, measure on that scale.
Which durometer scale should I use?
Shore OO for very soft foams and gels, Shore A for most rubber and elastomer gaskets, Shore D for hard rubber and plastics. If readings fall above ~90 on a scale, move to the next harder scale.
What durometer should my gasket be?
Softer materials (Shore A 20–50) conform to rough or uneven flanges at low bolt load; firmer materials (Shore A 50–80) hold shape and resist extrusion. Compression set and CFD matter as much as hardness — hardness alone does not size a seal.
What is the tolerance on a durometer reading?
Material specs commonly allow ±5 points. Readings also vary with specimen thickness (ASTM D2240 requires 6 mm minimum) and dwell time.