In an era of urbanization there is an increasing need to benchmark the performance of cities and do this against a robust set of measures that can be repeated and replicated over time. Open city data initiatives are on the increase and with the release of IS037120 city indicators it is timely to critically appraise the synergies between open data and city indicators. Hence, in this paper we present a novel measure for assessing the completeness of open city data in the context of measuring city indicators. We apply this measure known as the “City Indicator Data Openness Measure” (CIDOM) in the context of two global cities, Toronto and Melbourne. Specifically we do this in the context of the ISO 37120 educational indicator 6.4: “Primary Education Student/Teacher Ratio. This exercise has identified that there exists fundamental barriers in realizing the vision of open data if it is to support the reporting and analysis of city indicators. It raises further issues about sourcing data and also provides another argument towards developing a semantic approach for city indicators.