A durometer determines a sample’s ability to resist permanent indentation, one of many ways to assess hardness. These devices are typically used to assess plastic and rubber.

The fundamental test applies pressure in a consistent fashion and without shock, to measure the depth of the indentation. The depth of an indentation is reliant upon the hardness of the material, its viscoelastic qualities, the form of the presser foot, and the period of time of the test.

There are 12 durometer scales, denoted by the letters of the alphabet: A, B, C, D, DO, M, O, OO, OOO, OOO-S, and CF. The most typical scales are A and D – with A including softer rubbers, like car tires, the D scale being for hard rubbers like bowling balls.