Various methods to measure fracture toughness using in‐dentation precracks were compared using soda‐lime glass as a test material. In situ measurements of crack size as a function of applied stress allow both the toughnessKcand the residual‐stress factor χ to be independently determined. Analysis of the data showed that stress intensity factors based on classical half‐penny crack shapes overestimate toughness values and produce an apparentR‐curve effect. This is due to a constraint on crack shape imposed by primary lateral cracks in soda—lime glass. Models based on elliptical cracks were developed to account for the crack