An automatic razor blade sharpener simultaneously sharpens multiple edges of a spring-loaded disposable razor blade cartridge by gently contacting the blade edges with a moving rubber or polymeric compliant belt that is motor driven. The disposable razor blade handle is set in a cradle aperture, which is spring loaded against the moving belt. A timer terminates the blade sharpening operation, indicated by a green LED. A liquid is dispersed on the moving belt forming a thin liquid layer on the belt surface, illuminated by transversely located light illumination and the reflected light is received by a transversely located sensor. When the blade contacts the thin liquid layer, deep grooves in the blade cutting edge reduce this reflection indicating that the blade is too worn to be sharpened and a red LED light is indicated. Absence of reflection indicates that the thin liquid film has evaporated or spilled out.