Exchanging structured business documents is inevitable for successful collaboration in electronic commerce. A prerequisite, for fostering the interoperability between business partners utilizing different business document standards, is a mapping between different standards. However, the effort involved in creating those mappings is hard to estimate. For example, the complexity of standardized formats is one crucial aspect affecting the effort of the mapping process. Therefore, a notion of complexity is desirable for both, manual as well as automatic mapping processes. For this reason we develop an initial set of metrics, based on well established metrics for XML Schema, allowing to analyze the complexity of business document standards. Having such metrics at hand allows estimating the complexity and hence the mapping effort of a business document standard, prior to the actual mapping process. We demonstrate the complexity metrics on three different business document standards from the electronic commerce domain.