Wasteback Machine is an open-source JavaScript library for analysing the historical size, composition, and environmental impact of websites using data from the Internet Archive’s Wayback Machine. Developed as part of doctoral research at the University of Edinburgh’s Institute for Design Informatics, the project investigates digital resource growth and sustainable web practices, providing a method to retrospectively measure the environmental impact of websites by repurposing archival data. Current assessment tools for measuring page size rely on contemporary data, capturing only the live state of a website. Without this temporal context, the cumulative environmental effects of a website’s growth remain invisible. This limitation reflects a broader challenge in reporting web-related emissions within Scope 3 (indirect value-chain emissions) of the Greenhouse Gas Protocol. To address this gap, Wasteback Machine introduces a novel methodology that repurposes archived webpages from the Internet Archive to enable retrospective estimation of environmental impacts. This approach reveals temporal trends and provides longitudinal insights that conventional measurement tools cannot capture. The underlying method retrieves archived webpages with high fidelity: removing artefacts, correcting modifications, and preserving temporal coherence. By delivering accurate historical data on page size (the volume of data transferred when a webpage is loaded) and composition, the tool enhances organisational accountability, supports empirical research on website emissions, and informs the integration of design-level interventions into broader decarbonisation strategies. The library’s design also supports integration into research and analytics workflows focused on web growth and sustainability. The project is now expanding its scope to incorporate additional internet archives, improving temporal coverage and accuracy. Future work includes the development of a web application to make these analyses more accessible to researchers, practitioners, and the wider public.