In a Hall plate with finite size and contacts the Hall output voltage is given by the product of sheet resistance, input current, Hall mobility, magnetic flux density, and Hall geometry factor G(H). G(H) is an element of [0,1] accounts for the loss in signal due to the contacts. At weak magnetic field G(H) -> G(H0) is a function of geometrical parameters only, which makes it the crucial point for layout optimization. We show how to express G(H0) alternatively as a function of electrical parameters only, namely of input and output resistances over sheet resistance. This allows for an analytical optimization of signal-to-noise-ratio (SNR) without getting lost in the multitude of geometrical representations of equivalent Hall plates. In the course of this investigation we notice a hidden symmetry property of G(H), which we prove rigorously in the limit of small magnetic fields. The physical meaning of this symmetry in the case of Hall plates with equal input and output resistances is also explained. (C) 2017 Elsevier Ltd. All rights reserved.