In recent years, navigability has become the pivot of website designs. Existing works fall into two categories. The first is to evaluate and assess a website's navigability against a set of criteria or check list. The second is to analyse usage data of the website, such as the server log files. This paper investigates a metric approach to website navigability measurement. In comparison with existing assessment and analysis methods, navigability metrics have the advantages of objectiveness and the possibility of using automated tools to evaluate large-scale websites. This paper proposes a number of metrics for website navigability measurement based on measuring website structural complexity. We will validate these metrics against Weyuker's software complexity axioms, and report the results of empirical studies of the metrics.