We present a model for predicting a representative width for the three-dimensional cells observed on detonation fronts in reactive gases. The physical premise is that the 3D unsteady cellular process for irregular cells is stochastic and produces the same burned mass per unit of time as the average planar steady ZND process. Graph theory defines an ideal cell whose grouping is equivalent to that of the real 3D cell front. Geometric probabilities determine the mean burned fraction that parameterizes the model. The ZND model closes the problem with the relation time-position of a fluid element in the ZND steady reaction zone. Comparison of measured and calculated widths shows agreement better than or within the accepted experimental uncertainties, depending on the reactive mixture. The quality of this estimate depends solely on that of the chemical kinetic scheme used for the ZND calculations, the modeling assumptions aside. The model is readily implementable as a post-process of ZND profiles and provides instantaneous estimates of the cell width, length, and reaction time, as well as the ZND reaction length and time.