This paper introduces the Spatial Analysis of surface Runoff in ArcGis (SARA v1.0) interface to estimate curve number and runoff volume for hydrologic evaluations. This version runs in a vector platform as with other extensions of ArcGIS, introducing changes to the original Natural Resources Conservation Services-Curve Number method, and allowing adaptation to local conditions. The programming syntax was developed in Visual Basic 6.0 as an ActiveX Dynamic Link Library (DLL) to interact with ArcMap, and it is released as Free and Open Source Software (FOSS) that can be modified or upgraded to other programming languages.