Ray-optical propagation models are often used for the prediction of the field strength (and delay spread) in wireless mobile communication networks. However, these models suffer from long computation times. For large areas in urban or indoor scenarios, the computation times are in the range of hours which is too long for the planning of mobile radio networks. In this paper, we present a fast algorithm based on a ray-tracing technique with scalable approximation accuracy for field strength prediction in cellular mobile network planning together with a preprocessing of the geographical database. Automatic network design methods perform a huge number of field strength estimations and, therefore, require a fast and accurate approximation. However, common prediction techniques either give only rough estimations or are too complex for fast evaluation. The proposed new ray-tracing technique obtains its speedup by taking advantage of the topological information inherent in the used triangulation data structure of the investigated terrain. The applicability of the fast ray-tracing technique is demonstrated for a single transmitter scenario.