The local practice of planning urban open spaces is generally geared towards allocating open spaces in terms of quality. In a decade where the natural phenomenon is being intensified by effects of anthropogenic climate change. It is important to plan open spaces not solely in terms of land area coverage but essentially in terms of ecosystem function. Furthermore, as studies have correlated urbanization as a driver of anthropogenic climate change effects, it is strategic for urbanizing countries such as the Philippines, to plan urban open spaces as a climate change mitigation strategy, particularly, in terms of its function, as a carbon sink. This study proposes developing a tool to measure and monitor carbon storage for use in planning green spaces in urban areas. The development of the tool is an offshoot of a carbon storage assessment done in a study in 2014 (Yap et al. 2014) using estimation factors. Devised to have both urban green space carbon storage assessment and management procedures, the tool provides results that can be the basis of preserving and allotting open spaces during the conduct of actual local planning activities. different public urban open spaces in Antipolo City were used as case studies in developing the tool. These were the Town Plaza, the Antipolo Church Grounds, the City proper Public Streetscape, and the Rizal Provincial Capitol and Ynares Center Complex. The Hinulugang Taktak National Park was included as part of the study area per instructions from the City Planning and development Office. The concept of net primary productivity is the scientific basis of developing the carbon storage assessment aspect of the tool, as it is an indicator of carbon storage growth in an ecosystem within a period of time.