Polymer durometry is the measurement of the hardness of plastic and rubber materials using an indentor. Many different hardness scales and indentor geometries have been standardized for durometry. Indentation into a solid is governed by linear elastic mechanics, and this paper relates the Young’s modulus to standardized hardness scale reading. We nondimensionalized these indentation mechanics, to uncover a normalized Young’s modulus, Υ, and the dimensionless hardness, H, and then, a new dimensionless indentor stiffness that we call mechanical indentability, Mi. We use Mi for classifying standardized hardness scales. Our results allow practitioners to determine the Young’s modulus from measured hardness. Further, our results can be used to convert between hardness scales, not only between hardness scales of the same indentor geometry, but also between those of differing geometry.