BackgroundThere are considerable differences among mental healthcare services, and especially in developed countries there are a substantial number of different services available. The intensity of mental healthcare has been an important variable in research studies (e.g. cohort studies or randomized controlled trials), yet it is difficult to measure or quantify, in part due to the fact that the intensity of mental healthcare results from a combination of several factors of a mental health service. In this article we describe the development of an instrument to measure the intensity of mental healthcare that is easy and fast to use in repeated measurements.MethodsThe Mental Healthcare Intensity Scale was developed in four stages. First, categories of care were formulated by using focus group interviews. Second, the fit among the categories was improved, and the results were discussed with a sample of the focus group participants. Third, the categories of care were ranked using the Segmented String Relative Rankings algorithm. Finally, the Mental Healthcare Intensity Scale was validated as a coherent classification instrument.Results15 categories of care were formulated and were ranked on each of 12 different intensities of care. The Mental Healthcare Intensity Scale is a versatile questionnaire that takes 2-to-3 min to complete and yields a single variable that can be used in statistical analysis.ConclusionsThe Mental Healthcare Intensity Scale is an instrument that can potentially be used in cohort studies and trials to measure the intensity of mental healthcare as a predictor of outcome. Further study into the psychometric characteristics of the Mental Healthcare Intensity Scale is needed.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12888-023-04752-6.