This communication reports a geometrical factor that is necessary in the diffusion boundary condition across surface steps. Specifically, this factor relates adatom concentration to its spatial gradient at a surface step, and it describes the fraction of jump attempts that cross the step. In this communication, the authors show that the factor is 1/pi using theoretical formulation and further verify the formulation using numerical simulations for triangular, square, and hexagonal surface lattices. (C) 2012 Elsevier B.V. All rights reserved.