A finite element a posteriori method to compute rigorous inexpensive upper and lower bounds for engineering outputs of interest, in the field of large strain elasticity, is presented. Based on a general Lagrangian formulation, the bound computation requires global calculations on a finite element coarse mesh and the solution of local decoupled Neumann problems over each element of previous mesh. A new procedure to solve local problems is introduced, whereby bounds are computed locally over the continua for the exact solution in a completely analytical manner. The resulting method shows a considerable gain in computational performance and, besides being robust, can easily be incorporated into existing finite element codes without major modifications.