The importance of XML (eXtensible Markup Language) can't be understated; their usefulness may range from data sharing to data transport in software systems. Schema languages describe the structure of an XML document and the common schemas languages are Document Definition Type (DTD), W3C XML Schema and RelaxNG. Applications depend heavily on XML documents to be free of error and this makes it imperative to determine the quality of such schema document. Schema metrics is used to achieve this, and several of them have been proposed in recent years. In this paper we present the existing schema metrics and to make comparative studies on all schema metrics, figuring out the features, advantages and limitations of each metrics.