The objective of this study is to propose measurements for the complexities of rule based systems so as to indicate development efforts. The measurements of rule based systems include those for facts, rules, and inference relationships. Fact counts are used to measure the amounts of facts. Measurements for rules consist of volume, conditions and actions, and number of variables and wildcards. Inference measurements reflect inference relationships among rules, including distribution of rule relationships, inference ratio, and inference count. Empirical analyses are performed on Expert System Consultant (an expert system for selecting ESBTs) to illustrate the usage of these measurements. From the analyses, it is demonstrated that the measurements possess desired characteristics such as expressiveness and extendibility, and can be used for managing the development of rule based systems.