This paper outlines a procedure for the calibration of the lateral force applied to a triangular cantilever tip by means of a lateral force microscope. The force is directed perpendicular to the cantilever major axis. The lateral force results in a torsion of the cantilever. The torsion signal is monitored as a voltage on a four-sector photodiode. The conversion from a torsion signal to a lateral force depends on the optical geometry of the instrument and the cantilever shape, dimensions and material properties. The calibration procedure used requires a calculation of the elasticity of the cantilever and experimental data. In case tripping forces are ignored, the lateral force can be taken as the friction force.