The number of cavities, or the number of individual parts molded in one injection molding cycle, is determined primarily by the number of parts required. Most injection molding companies operate at least a 5 day 24 hour production facility. This enables the overhead cost of equipment to be low. Number of pieces per hour is calculated by the shots per hour (3600 seconds / cycle time) times the number of cavities. Normally a 90 percent efficiency is used for this calculation. Most injection molds are even multiples of cavities. One, two, four, eight and so forth. Balancing of the runner system for equal fill is important. The cavitation required for a given part should be no more than 80 percent of the total capacity of a 24 hours per day, 5 days per week, 50 weeks per year on a single tool. Time for preventive maintenance and repairs must be allowed. Another determining factor for cavitation is part size. Lower part cost can be obtained using more cavities, however tool cost will be more.