Anisotropic etching of V-grooves in a masked substrate provides the basis for two simple methods for in situ measurement of etch rates of Si(100). The width of the (100) facet defining the base of the groove and thus its surface area decreases at a rate which is determined by the etch rate in the (100) direction. By measuring voltammograms at regular intervals during etching we were able to monitor the change in geometry of the groove. In a complementary approach, in situ optical microscopy was used for determining etch rates in real time. An application of the method is described.