While both the Rashba–Edelstein effect and the inverse Rashba–Edelstein effect rely on a spin accumulation, the figure of merit of the processes is commonly computed by accounting for the spin current density related to the spin accumulation, instead of the spin accumulation itself, in analogy with the spin Hall angle for the spin Hall effect. Indeed, the efficiency of the Rashba–Edelstein effect and of the inverse Rashba–Edelstein effect can be estimated by means of the Rashba–Edelstein length, i.e., the ratio between the charge current density, flowing on the surface of the investigated material, (i.e., a surface charge current density) and the three-dimensional spin current density (since the spin accumulation can diffuse in the three-dimensional space).