Trees in urban areas store carbon directly through photosynthesis, but they also provide the added benefit of reducing carbon emissions produced by fossil-fuel burning power plants, by means of energy conservation from strategically-planted trees near buildings, as well as by area-wide reductions in the urban heat island effect. Quantifying the role of urban forests is an important prerequisite to managing the vegetation to optimize benefits, and also serves to assign value to the important ecosystem services provided by urban trees. Decisions by policy-makers regarding the management and use of urban trees requires accurate and precise information about the state of the resource. This paper creates a methodology for conducting a carbon inventory in an urban forest in the Washington, DC area, one that requires a minimum of data gathering. The methodology could serve as a tool for other similar high-density urban areas to measure carbon resources in urban forests and to serve as the basis for further research. Carbon trading systems may provide opportunities for forest owners to sell carbon credits to entities that produce CO2 in excess of national or international limits; quantifying urban forest carbon would be necessary as a baseline for future carbon offset projects.