This document defines the principle and the methods for the determination of the degradability coefficient of rocky material. The degradability coefficient IDG distinguishes the behaviour of certain rocky material and is used to show the change in the geotechnical characteristics (particle size, clay content, plasticity, etc.) in relation to the characteristics seen immediately following excavation. Changes in the particle size occur due to the combined action of climatic or geohydrological elements (frost, soaking-drying cycles) and mechanical stress to which it is subjected. In the case of degradable rocky material, this leads to a fairly significant and continuous reduction in the mechanical and geometric characteristics of the works in which they are used. The two methods developed in this document for the determination of IDG are not equivalent, so any result obtained by this document can refer to the method used.