Wind-farm blockage is investigated by means of an analytical model based on the linearized Reynolds-averaged Navier-Stokes equation. Despite the simplifications, useful insight is obtained about the induction region upstream of a single wind turbine and of a cluster of turbines. Since the model is linearized, superposition methods are allowed and the farm blockage is obtained as a linear superposition of all the induction zones of each turbine present in the farm, including the mirrored ones due to the presence of the ground. The model is validated against data from wind tunnel experiments, and it is later used to assess blockage in velocity and power for wind farms with different layouts and from several wind directions.