The performance of spatial joins is becoming increasingly important in many applications, particularly in the scientific domain. Several approaches have been proposed for joining spatial datasets on disk and few in main memory. Recent results show that in main memory, grids are more efficient than the traditional tree based methods primarily developed for disk. The question how to configure the grid, however, has so far not been discussed. In this paper we study how to configure a spatial grid for joining spatial data in main memory. We discuss the trade-offs involved, develop an analytical model predicting the performance of a configuration and finally validate the model with experiments.