Abstract Current debate on the sustainability of using cleaner fish to control parasitic lice in salmon farming suffers from extremely variable scientific evidence on the efficacy of this practice. This paper presents novel experimental results on evacuation rates of salmon lice through the digestive tract of ballan wrasse. These results are combined with quantitative field data on contents of salmon lice in ballan wrasse, to derive a method to study the efficacy of ballan wrasse in cleaning salmon of salmon lice. From a fitted binomial regression model on the probability of finding lice in the digestive tract after ingestion, we found a median evacuation time of 11.0 h. The mean evacuation time was 12.2 h. Furthermore, by integration, we found that if a wrasse on average consumes one louse per day, then the expected number of observable lice in the digestive tract is 0.472. This gave an estimated daily consumption of salmon lice per wrasse expressed as the number of salmon lice in the digestive tract divided by 0.472. As an example, analyses of lice contents in the digestive tract of 6406 ballan wrasse used as cleaner fish in salmon farming revealed that salmon lice were found in 2.9% of the wrasses, with a mean number of 0.15 lice per fish. This translates to an estimate of 0.32 lice consumed per day per ballan wrasse. The present way of estimating the efficacy of wrasse as cleaner fish may contribute to a more robust evaluation of louse control effects of ballan wrasse.