The Break Off Test is a recently developed nondestructive test for concrete. Although many experimental investigations have been carried out on this test, no in-depth theoretical evaluation has been done. In this study the behavior of the break off test specimen is investigated and the theoretical basis of the test explored. Based on linear elastic fracture mechanics, a model to predict the strength - manometer reading relationship of the test is proposed and compared with experimental results with good correlation. It was found that the ACI recommendation on the modulus of rupture (MOR) may be very conservative for certain members. The MOR of a rectangular beam is different from that observed from a circular cross section such as the break off test specimen. New MOR values are suggested for small rectangular beams and members with circular cross sections.