Determining key performance indicators (KPI) is a first step in achieving environmental sustainability of manufacturing operations. KPI selection is a multi-criteria decision making problem, because of various criteria that must be considered. Intuitively, one can rank candidate KPIs by specifying a numerical value indicating the effectiveness of a KPI in satisfying each criterion. However, linking selection criteria to KPI objectives, ranking how well a KPI satisfying a given criterion, and assigning a value to each rank lead to better KPI rankings. Values for each score are crucial. This paper shows steps to capture values to derive a criterion value function that is used to rank candidate KPIs. Selected KPIs can be used for assessing and monitoring sustainability performance, which must be considered together with including traditional (e.g., throughput) measures. A machine shop is used to show how an objective of reducing emissions from energy use in manufacturing can be pursued by monitoring the energy used to produce a unit product.