Vacuum system design which meet the customer's requirements best is the goal of any vacuum equipment manufacturer. To achieve this the reliable modeling of the adequate system of pumps and conductances is a necessary and decisive precondition. Being in use for nearly two decades, Leybold's purpose-built software tool CompuVac has been successfully revised and transferred to the Windows@super TM@ environment now. The program helps to predict essential parameters as the effective pumping speed, pump down behaviour and power consumption reliably. Problems specific to a given vacuum system can be detected and purposively solved.