The Chicxulub impact crater, which is the principal source of impact debris in Cretaceous-Tertiary boundary sediments, is currently buried by a ~1 km thick sequence of carbonate sediments. Because the crater is not exposed for direct scrutiny, its size has remained uncertain, and in particular, estimates ranging from ~100 to ~300 km have been made on the basis of gravity and magnetic field data. In this study, the physical properties of the K/T boundary impact ejecta and Chicxulub impact melt sheet have been used to test the geophysical estimates of the impact crater's size. On the basis of the measured thickness of impact ejecta in Haiti and North America, the calculated volume of the impact melt sheet, and the chemical and isotopic composition of the melt sheet, the Chicxulub impact crater is inferred to be ~180 km in diameter and to contain a ~3 to 7 km thick melt sheet and breccia sequence within a centrally located ~100 km diameter region that corresponds to the limits of the transient cavity. ¿ American Geophysical Union 1995