The calibration factors for atomic force microscopy (AFM) friction force measurements in liquid media are shown to be different by 25-74% compared to measurements in air. Even though it is significantly more precise, the improved wedge calibration method using a universal calibration specimen suffers, as all other widely applied methods, from the drawback that friction force calibration factors acquired in air cannot be used for measurements in liquids for the most common liquid cell designs. The effect of laser light refraction and the dependence of the calibration factors on the refractive index of the imaging medium is captured quantitatively in a simple model that allows one to conveniently rescale the values of lateral photodiode sensitivity obtained in air. Hence a simple, yet precise calibration of lateral forces is now also feasible for AFM in liquids.