Risk matrices are used to rate and rank risks of hazardous events for processes. They provide for the lookup of the risk level for an event using its severity and likelihood levels which are estimated subjectively. Risk levels are associated with requirements for risk reduction to achieve tolerable risk. Often, risk matrices are defined using numerical values for event severity and likelihood levels. Therefore, the resulting risk levels denote numerical values of risk. Consequently, such risk matrices must be calibrated with reference to appropriate numerical risk tolerance criteria, or process safety target levels, to define appropriate risk reduction requirements. Calibration poses several pitfalls for the unwary. Many practitioners are unaware of these pitfalls and use risk matrices that are calibrated improperly producing incorrect risk reduction requirements. This paper describes how these pitfalls can be avoided and provides calibration procedures. Use of these procedures will help to avoid incorrect decisions on risk reduction for processes. (C) 2015 Elsevier Ltd. All rights reserved.