An a posteriori error estimate is derived for the finite element formulation of the nonlinear, integro-differential equations that govern the steady-state behaviour of particulate systems (more commonly known as population balance equations). Since this estimate is derived in terms of calculable parameters it may be used as a means of quantitatively assessing the quality of an obtained numerical solution and additionally it has the potential to be incorporated into adaptive mesh refinement algorithms. Several numerical case studies are investigated and it is demonstrated that the estimate reliably predicts an upper bound of the error in numerically obtained solutions.