We have developed a simple and accurate method for calibrating the amplitude of vibration of quartz tuning fork sensors commonly used in at. force- and near field optical-microscopy. Unlike interferometric methods, which require a complex optical setup, the method we present requires only a simple measurement of the electro-mech. properties of the tuning-fork oscillator and can be performed in a matter of minutes without disturbing the exptl. setup. Comparison with interferometric methods shows that an accuracy of better than few percent can be routinely achieved.