How’s the hardness of rubber measured?

Shore hardness (durometer) is a measure of the resistance of a material to deformation by an applied force. Named after its inventor, Albert Ferdinand Shore, Shore hardness offers different scales for measuring the strength of different materials (Shore was born in 1876 and lived until 1936, creating the "Shore" scale in the 1920s).

In total, and according to ASTM D2240-00, there are 13 scales (A, B, C, D, DO, E, H, M, O, OO, OOO, OOO-S, and R), whose values range from 0 to 100 indicating the hardness of materials, such as soft rubbers, rigid plastics and super-soft gels. These hardness scales were created so that everyone can talk about these materials and have a common reference point for them.

Shore hardness, using the Shore A or Shore D scale, is the preferred method for rubbers and thermoplastic elastomers, and is also commonly used for "softer" plastics such as polyolefins, fluoropolymers and vinyls.

The Shore A scale is used for "softer" rubbers, while the Shore D scale is typically used for "harder" rubbers. For example: 30 Shore A is much softer than 80 Shore A. Once a material reaches Shore 95 A, it will be more like a plastic than a flexible material in feel. Shore D refers to rigid materials, while Shore A refers to flexible rubbers.