The process of privatising services historically provided by the state has blurred the boundaries between what is considered to be 'private' and 'public'. However, few efforts have been made in the educational arena to develop tools to measure this process. Most of the previous research has relied on narrow definitions about what is private and what is public. This work proposes a tool to measure the degree of publicness-privateness of school systems, avoiding binary separation of the concepts. We develop an index and test this tool in two different landscapes: London and Santiago. In these cases, it serves to illustrate major changes in the levels of public-private participation in both school systems, reflecting differences between the systems and over time. We conclude that the index has potential for development and use in the analysis of public and private dimensions in education in broader international contexts.