Diffraction effects from interstratified phyllosilicates have been studied extensively, and several computer programs such as NEWMOD© are available to facilitate interpretation of X-ray diffraction (XRD) profiles. However, accurate quantification of samples containing multiple interstratified phyllosilicate minerals is difficult due to the generally subjective nature of interpretations. More recent automated fitting interpretations require extensive computational effort, involving numerous calculations of profiles with different fitting-parameter values. Computational cost in time per calculation is the key factor influencing the overall efficiency of automated profile fitting. In general, the matrix methodology developed by Kakinoki and Komura is more efficient than the NEWMOD© architecture. A new matrix methodology was developed that reduces the number of matrix operations such as multiplication and addition, and these modifications improve calculation efficiency by up to a factor of three, with even greater improvement for small crystallite sizes (< 20 layers). A new computer program, ClayStrat, based on the modified matrix methodology, was developed to calculate one-dimensional XRD profiles from interstratified phyllosilicates along the Z direction.