This paper presents a comparative analysis of two primary methodologies for calculating Scope 3 greenhouse gas (GHG) emissions: the activity-based approach and the spend-based approach. Scope 3 emissions, encompassing all indirect emissions within a company’s value chain, pose significant challenges in data acquisition and accuracy. The activity-based method provides a detailed calculation by directly measuring emissions from all activities in the value chain, thus offering granular insights into environmental impacts but requiring extensive data collection. In contrast, the spend-based method estimates emissions based on financial expenditures on goods and services, simplifying data collection but potentially sacrificing specificity for convenience. This study assesses the efficacy, challenges, and suitability of each approach depending on various industry needs, the complexity of supply chains, and regulatory compliance requirements, such as those from the Corporate Sustainability Reporting Directive (CSRD) and the Science Based Targets initiative (SBTi). The findings suggest a potential hybrid approach, leveraging the detailed accuracy of the activity-based method where feasible, supplemented by the broader overview provided by the spend-based method for initial assessments and less critical areas.