ABSFAC is a FORTRAN program that rapidly and efficiently calculates the absorption factor T in multifaceted convex crystals and in other materials whose shape can be modeled by a convex polyhedron. The program also calculates derivatives of T with respect to small outward movements of the polyhedron faces and the mean path length for the scattered rays. Although written for X-ray scattering, the programme is relevant to other scattering provided the underlying assumptions are valid. The program uses a mathematically exact method for calculating quantities from given sample shape data. The size of any numerical error (typically of the order of 1 in 1014) that arises during the calculation is estimated by the program and is much less than the uncertainty in specifying the scattering geometry. Excellent accuracy is obtained for a very wide range of absorption coefficients. An option within the program permits storing of the calculated geometrical factors for any one orientation of the sample and scattering vector, so that the absorption can be rapidly recalculated for different values of the absorption constant. This is particularly relevant for energy dispersive scattering. The program can also calculate the absorption in samples that are only partly illuminated by the incident beam.

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