The maximum stress intensity factor of a surface crack has been inferred by measuring the reflection coefficient of a Rayleigh wave incident to the crack. An acoustic surface wave wedge transducer was used to excite the incident wave and to measure the reflected‐wave amplitude. The fracture stress of Pyrex glass specimens containing the acoustically measured cracks was determined in biaxial flexure. The values of the fracture stress predicted from acoustic data were found to be in excellent agreement with the measured values, with less than a 15% error.