The use of highly-enriched fuels is an effective method for reducing the number of spent fuel assemblies and improving fuel cycle economics, e.g., with >5 wt% 235U. However, from a criticality safety point of view, such high enrichment levels require a significant investment for the considerable modification of most facilities and equipment. Erbia-credit super-high-burnup fuel offers an effective solution that can solve the problem: Small amounts of erbia added to the entire amount of UO2 powder can reduce the reactivity level to less than that observed at a 5 wt% enrichment level, thus eliminating the need for the modifications mentioned above. A series of criticality safety analyses has been performed to determine the minimum and sufficient content of erbia that can guarantee a suitable erbia credit. As a noteworthy result, the erbia content required was determined for corresponding values of uranium enrichment in a range >5 wt%, as indicated in our ECOS (Erbia COntent for Sub-criticality judgment) diagram. This paper outlines a series of criticality safety analyses and explains how the minimum erbia content can be determined to ensure subcriticality for a >5 wt% enrichment fuel to ensure that the fuel obtained is equivalent to that whose enrichment is <5 wt%.