In this paper, we present a method to assess social vulnerability through the creation of an Open Source Vulnerability Index (OS-VI). The OS-VI provides context to environmental hazards and allows NGOs and local agencies to better tailor services and provide targeted preemptive vulnerability reduction and resilience-building programmes. A deductive indicatorbased approach is utilised to incorporate a wide range of vulnerability indicators known to influence vulnerability. Unlike many vulnerability indices, the OS-VI incorporates flood risk as well as the loss of capabilities and the importance of key services (health facilities and food stores) through the measurement of accessibility when determining an area's level of social vulnerability. The index was developed using open-source mapping and analysis software and is composed completely of open-source data from national data sets. The OS-VI was designed at the national level, with data for all proxy indicators available across the entirety of England and Wales. For this paper, a case study is presented concerned with one English county, Norfolk. Highlights• We produce an open-source vulnerability index.• Accessibility to health care found to be severely affected by flooding.• High vulnerability areas found to be disproportionately impacted by flooding.• Urban extent of an area found to increase its level of vulnerability.• Flood affected areas more likely to be composed of elderly, sick and poor.