An analytical method is derived to describe the distribution of water quantity saved among customers within a water-use sector who adopt a water conservation action. Analytical results tend toward lognormal distributions with long tails, quantifying a smaller subset of customers that show potential to achieve large savings. Example effectiveness distributions are shown for seven long-term conservation actions potentially implemented by urban, domestic water users in Amman, Jordan. Monte Carlo simulations verify the analytical derivations. The probabilistic outputs contrast with common methods that estimate conservation action effectiveness as a product of typical (average) characteristics for disaggregated customer groups. Implications to size water conservation programs to meet conservation objectives and target customers to adopt conservation actions are discussed.