Experimental results and numerical simulations have demonstrated that a protective barrier can effectively reduce blast load and, therefore, protect structures from an external explosion. However, there are no formulae in the open literature that can be used to estimate the blast loads on a structure behind a barrier. In this paper, pseudo-analytical formulae based on numerical results are derived to estimate the reflected pressure-time history on a rigid wall behind a protective barrier. Numerical simulations of blast wave propagation are carried out to estimate the peak reflected pressure and the impulse on a rigid wall behind a blast barrier. The shock wave front arrival time and positive phase duration are extracted from the numerical results. Pseudo-analytical formulae, which are derived from the best-fitted curves of the numerical results, are suggested. These formulae can be used with those given in TM5-1300 or other methods for blast pressure estimation in the no-barrier case, to estimate pressure-time histories at various building locations behind a protective barrier.