This position paper puts forward a proposal by the World Business Council for Sustainable Development (WBCSD) for the development of a harmonized approach for businesses to measure and value their interactions with society: A Social Capital Protocol. While there are many tools and approaches for measuring social impact available, companies are increasingly calling for a single harmonized and broadly accepted approach which will: Clarify best practice for business by providing a focal point for the development of best practice approaches, tools and standards for social performance measurement and management, Scale up the positive social impacts of business by informing appropriate action throughout a company’s operations and supply chain, Improve business credibility by providing a solid foundation for companies to report or disclose their impacts and dependencies externally, building on existing frameworks such as the International Integrated Reporting Framework. This paper’s objective is to put forward the WBCSD’s proposal for a joint solution – the development of a Social Capital Protocol. It outlines the need for action and proposes the development of a Social Capital Protocol as a solution. It outline the components of a Social Capital Protocol and what would be needed to deliver it, and features a number of member companies who are already leading the way in the space of measurement and valuation, including Alliance Trust, ArcelorMittal, BT, DSM, Holcim, J.P. Morgan, KPMG, Lafarge, Masisa, Nestlé, PWC,SABMiller, SCA, Schneider Electric, Sompo Japan Nipponkoa, and Veolia. The paper also profiles initial partners and experts, and how they will contribute to the development of the Social Capital Protocol including The B Team, CIRAIG, The Global Impact Investing Network, The GrameenFoundation, IFC, the Initiative for Global Development, the Roundtable for Product Social Metrics, The Social Return on Investment Network, and the William Davidson Institute. This proposal is already the result of valuable contributions from WBCSD members, partners, and peer reviewers, and we aim to provide a platform for further collaboration to make this proposal a reality.