Downtime is arguably the single most significant contributor to system inefficiency in a multistage manufacturing system. Achieving near-zero downtime has been the ultimate goal of production operation management at the plant floor. Accurate estimation of the impact of each downtime incident is of great importance for deciding where to allocate limited resources among various manufacturing stages. In this paper, we focus on quantitative analysis of the impact of each individual downtime event in terms of permanent production loss and financial cost. We start from the transient analysis of a single downtime event and later extend to more generic scenarios where downtime incidents occur concurrently at different stages. Apart from the analytical study, a practical computation procedure using real-time production records can be readily derived and implemented at the plant floor. Case studies are conducted to demonstrate its potential in facilitating the decision making at plant floor on project identification, prioritization, and budget allocation in a multistage manufacturing system.