The rapid advancement and deployment of Web applications call for a precise yet simple model for capacity planning and analysis purposes. The most widely deployed Web application architecture is the 3- tiered system, which is composed of a front-end Web server, an application server and a backend database server. In this paper, we present an analytical model of the 3-tiered Web application architecture. We show by using queueing network theory, we can model the 3- tiered Web application architecture accurately. A testbed is built to measure model parameters based on industry standard server components and TPC-W benchmark. Validation results show that the proposed model predicts performance measures such as response time and throughput accurately.