The toughness of unreinforced thermoplastics is often high, but limited by a transition to brittle behaviour with increasing impact speed and decreasing temperature. Izod or Charpy impact strength data are widely regarded as a primary performance index for these materials, but cannot be used for quantitative design. Although fracture mechanics analysis offers a substantial advance, the impact fracture resistance for some polymers varies substantially with configuration (the shape and size of the body and the impact speed) rather than being a material constant as expected. A model of brittle impact fracture as a process of local fusion within a rapidly-loaded crack-tip craze explains this variation quantitatively. The model leads to a method of correlating impact fracture in different configurations, which is demonstrated here using a case study.