We present a model for the forces created by gas flow in a narrow gap between moving surfaces in a micromechanical structure. The partial differential equation describing the thin gas film behaviour is realized with a finite difference mesh, satisfying the boundary conditions for the gas flow. Thus different surface geometries and freedoms of motion can be modelled with different mesh topologies. The elements in the mesh are elementary electrical components, controlled sources and capacitances. The model presented is applied to capacitive silicon accelerometers having rectangular holes in the mass.