The PECUNIA Resource Use Measurement Instrument (PECUNIA RUM) is an internationally standardised, harmonised and validated, generic, self-reported questionnaire that measures resource use for costing from a societal perspective. It is structured in sections covering the impacts of health or wellbeing on different services including health and social care use, medication, informal help, education, work, justice, and private expenses. The PECUNIA RUM is consistent with the harmonised PECUNIA Costing Concept and other PECUNIA Costing Tools (PECUNIA RUC Templates, PECUNIA RUC Compendium) to achieve cross-country and cross-sectoral comparability. and can be used complementary to the PROM-MH Compendium, a repository of standardised mental health outcome measures. Development Version 1 was initially developed as a modular, pen-and-paper questionnaire in English by the scientific partners of the PECUNIA Consortium between 2019 and 2021 alongside the following steps: a) comprehensive scoping review to identify existing RUM recommendations, b) feedback from health economist focus group on the first draft, c) initial wording review, d) professional English language editing, e) piloting with former mental health care users and carers, and f) formal translatability assessment. More information about the initial development process can be found here. To improve usability, transferability and translatability, the PECUNIA RUM underwent further adaptations and language simplifications during 2024, leading to the creation of a more concise, updated and flexible Version 2, which has matching proxy-completion and flexible recall period versions. Translations are available in multiple languages such as Dutch, German, Macedonian, Russian, Ukrainian, Rumanian. A comprehensive User Manual is also available to provide guidance on effective use. Researchers considering digital implementation should review adaptation plans as part of licensing to maintain the PECUNIA RUM’s measurement integrity.