Across decades of contemporary discussion on sustainable development, a core concern has been the balance between economic, social, and environmental dimensions. A critical strand of the debate focuses on economic growth versus economic degrowth and, more specifically, on whether economic growth can be sustainable in environmental terms and whether degrowth can be sustainable in social terms. This conceptual and theoretical article used the Sustainability Window, or “SuWi”’ method, to theoretically determine the sustainable window of economies. The window is defined as the upper and lower bounds of future change in GDP that could be deemed in line with achieving both environmental and social sustainability. The conceptual analysis considers all theoretically possible scenario paths for development by combining the outcome paths of economic, environmental, and social dimensions with the environmental and social productivities of GDP. Through SuWi analysis, it is found that only four of the logically possible scenario paths could be considered theoretically “sustainable”—two cases involving economic growth and two of degrowth. In the cases of each of the four paths, sustainability only emerges where they adhere to strict conditions in terms of environmental and social outcomes, as well as related productivities. The SuWi approach and its applied analytical formulas have many potential uses in 21st-century policymaking for sustainability, including supporting the United Nations Sustainable Development Goals. It provides a unique and comprehensive theoretical and analytical framework that enables the categorisation of the complex challenges of sustainability and quantitative analysis of policy choices. Such foresight analysis could greatly assist in providing an evidence base for future development planning and policy formulation, ex ante of locking in a pathway. Further implementation in applied studies that explore a comprehensive indicator set, robust and consistent across all relevant dimensions, offers a promising opportunity to advance empirical analysis of key questions in sustainable development globally at a critical juncture in human history.