This document specifies carbon usage effectiveness (CUE) as a key performance indicator (KPI) for quantifying the CO2 emissions of a data centre during the use phase of the data centre life cycle. CUE is a simple method for reporting the CO2 intensity of the data centre operating. By reporting CO2 emissions, it is possible to present the data centre's contribution to climate change (enhanced greenhouse effect). This document: a) defines the CUE of a data centre; b) introduces CUE measurement categories; c) describes the relationship of this KPI to a data centre’s infrastructure, information technology equipment and information technology operations; d) defines the measurement, the calculation and the reporting of the parameter; and e) provides information on the correct interpretation of the CUE.