This thesis explores the critical intersection between early-stage entrepreneurship and sustainability by addressing the lack of accessible tools for impact assessment in startups. While sustainability has become a growing priority across industries, startups often lack the time, resources, and methodological guidance to evaluate their environmental and social contributions effectively. Existing frameworks such as GRI, SROI, and IRIS+ provide valuable foundations but are often misaligned with the operational constraints and agility of startups. To bridge this gap, the research develops a structured, questionnaire-based assessment tool designed to support startups in measuring and managing their ESG (Environmental, Social, and Governance) performance. The tool combines universal indicators with sector-specific materiality filters, offering both comparability and relevance.