Relations previously have been shown between the size of the fracture mirror and the nominal breaking stress. In the present paper, this relation is derived analytically involving one unknown quantity which must be determined experimentally. This quantity is δm√ρ where δm is the intrinsic strength of the glass and the effective radius of the crack tip. Tests on glass rods indicated that the breaking stress could be determined from the radius of the fracture mirror. For 80% of the specimens the values obtained were within 8% of those obtained from the test loads.