This paper presents a method for calculating the overall equipment effectiveness (OEE), which is one of the most effective key performance indicator (KPI) to measure the productivity and quality levels of an equip- ment-centric manufacturing company. OEE is the most representative time-related KPI. For ease of measurement, OEE is calculated on the basis of the quantity of the processing materials and theoretical cycle time instead of using time-related sub-indicators. Hence, setting up theoretical cycle times is the most important aspect of the OEE measurement. Although processing materials and methods are very important in setting up a cycle time, they have not been extensively considered for setting up theoretical cycle times in previous research. Through an analysis of actual cycle times from real data, this paper shows that the relationship between the processing materials and methods and the theoretical cycle times is significant. Accordingly, this paper proposes that the theoretical cycle times can be determined on the basis of manufacturing item groups, which are classified as processing materials and methods. This paper also shows that there is a marked difference between the results of the OEE measured by the existing method and the proposed one. The proposed method will be helpful in measuring the performance of real manufacturing fields clearly and will be widely applicable to various industries.