Although it is generally known that ray tracing is 'time consuming', yet rewarding with respect to image quality, there are few attempts to predict the rendering time for a given model in advance. This paper focusses on the develop-ment of such a technique. The cost of ray tracing using adaptive spatial subdivisions has been studied by analysing the probability that a ray intersects an object. Per spatial subdivision cell the surface area relative to the cell size provides a measure for this proba-bility. This cost function is refined by taking into account possible overlap when multiple objects inhabit the same cell. A further refinement is applied by comput-ing the average tree depth of the spatial subdivision and by assuming that each ray will on average traverse the spatial subdivision at this depth. To evaluate and validate our method we applied it to some complex models and compared the results with the actual rendering cost.