In the discussion on further measures to enhance the energy efficiency of ships, four options for specific metrics for calculating operational efficiency have been proposed to MEPC, viz. the Annual Efficiency Ratio (AER, formerly Annual EEOI), the Individual Ship Performance Indicator (ISPI), the Fuel Oil Reduction Strategy (FORS) and an unnamed United States’ proposal. The respective Working Group of MEPC 66 agreed that further work was needed on all the metrics and that no metric should be excluded at that point in time (MEPC 66/WG.9). The Committee encouraged interested delegations to voluntarily submit data from monitoring programmes and metric testing to the Committee (MEPC 66/21, Paragraph 18.104.22.168). This study was commissioned by Germany as a contribution to the discussion on possible metrics and appropriate indicators for operational energy efficiency.