This paper describes a simplified calculation method for pipeline decompression wave speed based on a rigorous equation of state for pure CO2 as well as mixtures with significant impurities. Calculations are performed assuming homogeneous equilibrium for the estimation of the speed of sound in the two-phase region and calculations are performed along an isentropic decompression path. These calculations are important for the design of pipelines and can be used to estimate the required wall thickness and/or material toughness when combined with e.g. the Battelle two curve method, thereby ensuring that a potential running ductile fracture is arrested. The calculations are validated against available literature data and is offered as an open source tool. For pure CO2 at supercritical conditions the model results match experimental results very well, whereas for the dense liquid phase the pressure plateau in the decompression wave speed curve is over-predicted. For CO2 with impurities the model calculations generally match experimental data, except for the experiment with a significant fraction of hydrogen and for the experiment with the highest amount of impurities of approx. 6%. In these two cases the pressure plateau is under-predicted.