In general terms materials with low hardness (softer materials) are more flexible and thus seal more readily on rougher surfaces or have better conformity, although they are more susceptible to wear, abrasion and extrusion.

A decrease in hardness can also be expected to reduce breakout friction with dynamic seals but running friction is reduced with increasing hardness (provided the unit surface load is reduced by reducing the squeeze). Basically, a higher hardness figure should give lower rubbing friction in any dynamic seal, although this depends specifically on adjusting the squeeze to compensate. Retaining the same squeeze and increasing the hardness of the elastomer can increase both the breakout friction and the running friction.

Standard measurement of elastomer hardness are: IRHD (International Rubber Hardness Degrees), BS(British Standard) degrees and Shore ADurometer. The first two are identical, however, Shore A Hardness may be measured as an instantaneous reading or as a 30 second reading. In the former case the value obtained is about 5 degrees higher than IRHD.

The hardness of any elastomer can be modified by compounding. Hardness is thus a control factor in the formulation of an elastomer for specific applications.

Hardness values used for seal materials may range from as soft as 40 to 50 °ShA up to a very hard compound of 95 °ShA hardness.

Hardness (and therefore friction) is also affected by swelling. Swelling tends to reduce hardness and at the same time will increase the squeeze. Similarly, hardness is affected by temperature, decreasing with increasing temperature, and vice versa. At the same time the squeeze is modified by the expansion or contraction of the seal.