In a case of computer simulation used for the verification of pneumatic system performance, one of the main problems is that various parameters can be used to describe flow characteristics of the system components. Although the Standard ISO 6358 offers two parameters: the sonic conductance C and the critical static pressure ratio b, these cannot be directly utilised in the selection of elements comprising a pneumatic system. In this paper, we present two algorithms for calculating the volumetric flow rate Q$_N$ and the flow coefficient K$_V$ as a function of sonic conductance C and critical pressure ratio b (recommended by the standard) toward the improved selection of pneumatic control valves.