This report introduces the Digital Wellbeing Index (DWI), a five-dimension framework for evaluating the impact of digital environments on human health, identity, and personal agency. Arguing that screen time alone is an inadequate metric for assessing digital health, the report proposes five measurable dimensions: Screen Quality, Algorithmic Dependency, Identity Coherence, Social Comparison Load, and Digital Boundaries. Each dimension is grounded in peer-reviewed research and targets a distinct mechanism through which digital platforms influence psychological health, self-perception, and behavioral patterns. The report draws on evidence from JAMA Pediatrics, JAMA Psychiatry, Pew Research Center, and the U.S. Surgeon General's 2023 advisory on youth mental health. It builds on two prior independent reports by the author, Digital Identity Drift (2026) and The Algorithmic Cage (2026), forming the third installment of an independent research series on digital literacy and human agency. The Digital Wellbeing Index is intended as a practical resource for researchers, educators, mental health professionals, platform designers, and individuals seeking a more accurate understanding of their relationship with digital technology.