This paper provides an overview to Acumen Fund’s approach to measuring social returns on investments. Rather than seek an absolute standard for social return across an extremely diverse portfolio, Acumen Fund looks to quantify an investment’s social impact and compare it to the universe of existing charitable options for that explicit social issue. This methodology, called the BACO ratio (for "best available charitable option"), is a useful starting point for assessing the social impact and cost-effectiveness of each of Acumen Fund’s investments.