Individuals want to know which organisations to donate to, and a variety of organisations have developed ranking systems to guide them. This paper explores charity ranking, with a particular focus on the increasing role of impact and 'cost-effectiveness'. Ranking systems are composed of a selection of metrics, which may miss important components and, as a result, create a set of unintended outcomes. We argue that an emphasis on cost-effectiveness and impact in ranking promotes simple, technocratic activities, negatively affects human rights-based interventions and de-prioritises inventions that work in remote, complex settings. The topic of charity ranking and its influence on private donors is limited in the literature, and this paper seeks to make a contribution to this important debate.