This research aims to address the challenges of digital transformation in education by understanding the digital competence of teachers through a mixed-methods approach. The grounded theory is employed to develop the Teachers' Digital Competence Model (TDCM), which is structured around three facets: development, pedagogy, and ethics. Within these facets, six key dimensions were identified: digital ethics and safety, digital engagement, digital resources, digital teaching and learning management, digital assessment, and empowering students. The TDCM was operationalized by creating the Teachers' Digital Competence Self-assessment Instrument (TDCSI), utilizing a self-perceived scale to measure teachers' digital competence. The selection of TDCSI questionnaire items was carried out using item analysis and exploratory factor analysis, followed by confirmatory factor analysis to construct a structural equation model and assess the reliability and validity of the instrument. The TDCSI comprises 24 items with a Cronbach's a of 0.937 and factor loadings ranging from 0.535 to 0.834, demonstrating a reasonable structure. This study provides valuable insights into digital competence, offering a practical instrument for measuring teachers' digital proficiency. The findings are poised to inform future research on digital competence, as well as support endeavors to enhance teacher training and professional development programs in the context of educational digital transformation.