A precise estimation of the edge strength becomes an important issue when designing structural glass beams or façade mullions. However, in literature, the glass edge strength is poorly documented compared to the surface strength. In particular, the experimental validation of an edge strength model is lacking. Therefore, in this project, a ratio is assessed between the flaw depth and the mirror zone depth to determine more easily the geometry factor used in the edge strength model. Glass specimens with arrised and ground edges were tested at a high stress rate (50 MPa/s) with an in-plane four-point bending test setup. After failure, the critical flaw and the corresponding mirror zone was examined by means of an optical microscope. As the ratio of these values could be determined rather accurately, in future the geometry factor can be determined only by measuring the mirror zone depth instead of the flaw depth.