The effect of cracks on the overall elastic and conductive properties is controlled by the crack density parameter; it is a scalar in the case of random crack orientations and a tensor in anisotropic cases of non-random orientations. It also applies to thin platelets in matrix composites. In the present work, this 3D tensor parameter is determined analytically from the 2D densities and orientations of crack traces in cross-sections or specimen boundaries, in anisotropic cases of one or several families of parallel cracks. The theoretical results on the 3D-2D connections are compared with numerical simulations. This work supplements a substantial literature on 3D-2D-1D connections for materials with inhomogeneities other than cracks (pores, various foreign particles, etc.)