Calculations of atmospheric transmittance and radiance require the knowledge of the integrated amounts of the absorbing gases along the path. This report describes the calculation of the integrated amounts ('air mass' or 'column density') for various infrared absorbing gases for an arbitrary slant path through the atmosphere, including the effects of both curvature and refraction, and presents a Fortran program. FSCATM, to perform the calculation. Among the features of FSCATM are: 1. It calculates the layer-by-layer integrated absorber amounts and density-weighted pressure and temperature for an arbitrary slant path through the atmosphere. 2. It assumes a spherically symmetric atmospheric with exponential profiles of density and refractivity between layer boundaries. 3. It allows a variety of options for specifying the slant path. 4. It includes six representative atmospheric profiles of pressure and temperature, and of density for the gases H2O, CO2, O3, N2O, CO, CH3, and O2 and has provision for user-supplied profiles of up to 20 gases. 5. The output layering may either be generated internally or supplied by the user. 6. It portable to 32 bit word computers in single precision and compatible with both ANSI Standard FORTRAN 66 and 77. 7. It is modular and easily modified to suit the users' particular needs. A discussion of atmospheric profile data and a survey of the literature are included in appendices.