A direct, accurate and convenient procedure for calibrating the spring constants of probes used for force microscopy/spectroscopy is described. It amounts to deflecting an 'unknown' cantilever with a 'standard' lever, where the standard lever has been precalibrated. The absolute and relative accuracies of the procedure are ±20-30% and ±10-20%, respectively; the former is limited by uncertainties in the determination of the spring constant for the 'standard' lever, as well as that for the 'unknown'. The method differs from others of the static deflection variety by its exploitation of the routine features of current instruments. The technique is compared with other static and dynamic methods currently being used.