Staff at activated sludge treatment plants with sedimentation need to know exactly how much capacity their secondary clarifiers have. Knowing the secondary clarifiers’ capacity can help staff avoid solids loss and associated permit violations during peak-flow events. It also helps utility managers plan better for improvements needed due to population growth, capacity shortfalls, or more-stringent regulations for peak wet weather treatment. Peak-flow events are statistically infrequent, so historical capacity records usually are not informative enough. Desktop evaluations of capacity — based on surface overflow rates or solids loading rates in textbooks, regulations, or manuals — do not account for variations in clarifier designs, variable mixed liquor solids settleability and compatibility, and widely varying flow conditions during peak-flow events. The best assessments come from process models supported by field measurements. Given the range of process models available, plant managers should know which one evaluates clarifier capacity most reliably and consistently. So, researchers compared two models commonly used to evaluate secondary clarifier treatment capacity: state point analysis (SPA) and 2Dc. They modeled secondary clarifiers at five municipal wastewater treatment plants to determine the advantages and disadvantages of both models.