The Durometer is the International Standard Instrument used to measure the hardness of rubber or rubber-like materials. Durometers measure hardness by the penetration of an indentor into the rubber sample. The calibrated reading is expressed in a number value. The hardness values are always expressed in terms of plus or minus (±) five points (example: 60±5 Durometer).
It is important to note that when testing hardness of a rubber sample, the material should not be less than ¼" thick (.250").