This paper deals with a unified approach to forward kinematics for both over-constrained and under-constrained cable robots. Moreover, little assumptions on the cable model are required. This makes the proposed method applicable to a variety of currently discussed cable models including the standard model, an elastic cable model, static cable sagging, or even finite elements model. Solutions to the forward kinematics problem are computed by minimizing potential energy in the cable robot. As shown in this paper, the method unifies forward kinematics of both over-constrained and under-constrained cable robots.