Recently derived improved boundary conditions for the reflection of weak shock waves from an open end of a duct can be well approximated by a centered expansion wave if the center of the later is located on the leading characteristic of the reflected wave and some distanceeoutside the duct. A relation is derived betweene/D(Dis the duct diameter), the shock pressure ratiop1/p0, and &ggr; (the ratio of specific heats). This relationship is represented with sufficient accuracy by the empirical formulaeD=0.90+0.677(p1/p0)−1(&ggr;+1.08)−3.8.This result makes it possible to apply the improved boundary conditions without losing the convenience of of the customarily used wave diagram procedure.