A Faecal Contamination Index for interpreting heterogeneous diarrhoea impacts of water, sanitation and hygiene interventions and overall, regional and country estimates of community sanitation coverage with a focus on low- and middle-income countries.

•An index for estimating faecal contamination in the community is proposed that combines a broad set of WASH indicators.•Faecal community contamination is an important determinant of a sanitation intervention's effectiveness to reduce diarrhoea.•Above a certain level of faecal contamination, interventions are less likely to lead to reductions of diarrhoea.•Less than half of the global population lives in communities in which coverage with basic sanitation services exceeds 75%.