In some cases, the accuracy and efficiency of a particle tracking model may be greatly enhanced by determining the time for a particle to travel a specified distance rather than the distance traveled during some time interval. For instance, it may be desirable to know when a particle reaches a boundary or interface or a location where the system properties change. In this work, we derive an analytical expression in the form of a probability function describing the distribution of random times necessary for a particle to diffusively travel a specified distance. Upon substitution of the specified distance and a random number for the probability, the expression may be solved for the necessary time. Comparison of the results from this equation with an empirical expression determined by James and Chrysikopoulos (Chem. Eng. Sci. 56(23)(2001)6535) shows excellent agreement. Recommendations for implementing the analytical solution into particle tracking algorithms with or without a deterministic velocity component are included. This general equation is amenable to use in modeling both fractured and porous media systems. Published by Elsevier Science Ltd.