A calibration apparatus includes a fixture, which is coupled to hold a distal end of a medical probe. A plurality of weights, which have respective masses and respective bottom surfaces that are oriented at respective angles with respect to the distal end of the probe, are coupled to rest on a distal tip of the probe so as to apply to the distal tip respective force vectors that cause a deformation of the distal tip relative to the distal end. A calibration processor is configured to receive from the probe measurements indicative of the deformation of the distal tip in response to the force vectors, and to compute, based on the measurements, the masses and the angles, calibration coefficients for assessing the force vectors as a function of the measurements.