The compression index is estimated by several standard equations. The first is from Terzaghi 0.009(LL-10). The NRCS laboratories developed a good estimate from this equation: Cc = sqrt[0.0032 x LL x (e0-0.4)]. For instance, given a soil with a LL value of 42 and an intial void ratio of 0.76, you would estimate a compression index value of 0.23.