This paper develops a method to measure and compare social performance of microfinance investments at the level of microfinance investment vehicles. Drawing from measurement theory, it develops formal quality criteria that individual social performance indicators, the selection, and the aggregation of such indicators into a single metric need to satisfy. Social performance indicators are selected for both microfinance investment vehicles, and their underlying portfolio. The method presented here uses data of the microfinance investment universe to determine a rating framework for the underlying of microfinance institutions, in addition to a unique set of variables captured at MIV level. The paper demonstrates the approach in a sample calculation and serves as a guideline for a future empirical application among microfinance investment vehicles.