Lorentz Force Velocimetry (LFV) is a noncontact method for flow measurement in electrically conducting fluid, especially in high temperature, opaque and aggressive molten metal. The principle is based on exposing the flow to a magnetic system and measuring the drag force acting upon it . The aim of present paper is to study the application of LFV for open channel liquid metal flows, to numerically obtain the relationship between the measured Lorentz force and the flow rate. This provides the calibrating criterion of LFV. To this end, we firstly investigate a metal bar with different cross-section shapes passing through the magnetic system; Secondly, we study the relationship by a multiphysics numerical model fully coupling Navier-Stokes equations and Maxwell equations.