We describe a method for the determination of spring constants for cantilevers used in atomic force microscopes that utilizes the hydrodynamic drag. This simple method is applicable when a spherical colloidal particle has been placed on the tip of the cantilever and requires no extra equipment. The method is applicable to all types of cantilevers and requires that the radius of the colloidal particle and the viscosity of the fluid be known. The deflection of the cantilever under the action of the hydrodynamic drag force in a viscous fluid is measured. As the drag force is known theoretically, the spring constant is determined by application of Hooke's law.