This paper continues the development of the use of personal computers to design vacuum systems by determining the gas flow through the system as a function of the system pressures. Previous papers have presented a compatible set of short‐tube gas‐flow equations for circular tubes for all pressures from atmosphere to high vacuum1and a computer methodology for combining components in a series arrangement to obtained a complete system design.2Some of the problems of parallel pumped systems are discussed in the present paper and a general outline is presented for their computer solution. The additional data on transmission probabilities through short rectangular tubes which was presented at the 1990 National AVS meeting as a part of this paper has been removed and is to be published separately.