One approach to measuring and managing the complexity of software, as it evolves over time, is to exploit software metrics. Metrics have been used to estimate the complexity of the maintenance effort, to facilitate change impact analysis, and as an indicator for automatic detection of a transformation that can improve the quality of a system. However, there has been little effort directed at applying software metrics to the maintenance of grammar-based software applications, such as compilers, editors, program comprehension tools and embedded systems. In this paper, we adapt the software metrics that are commonly used to measure program complexity and apply them to the measurement of the complexity of grammar-based software applications. Since the behaviour of a grammar-based application is typically choreographed by the grammar rules, the measure of complexity that our metrics provide can guide maintainers in locating problematic areas in grammar-based applications.