The fracture toughness of A533 steel at room temperature, 0 deg and −70 deg F, was evaluated with 1 and 2-in-thick contoured double cantilever beam (DCB) specimens. To establish the effect of rate on thickness requirements, and to assess the strain rate sensitivity of KIc both initiation and arrest toughnesses were measured in tests covering a wide range of loading rates. The effect of thickness on the measured toughness, Kc, is discussed with reference to the parameter βc = 1/B (Kc/σY)2 where B is specimen thickness and σY is the yield stress at the temperature and strain rate of the test. Measurements at each temperature defined a single level of Kc for initiation, independent of loading rate and thickness, when the criterion βc ≤ 1.0 was satisfied. The values were 85–90 ksi in. at room temperature and 55–60 ksi in. at 0 deg and −70 deg F. As an alternative to identifying these numbers as KIc, an extrapolation function which associates KIc with an infinite plate thickness was applied to the data. The extrapolation defined KIc = 70 ksi in. at room temperature and KIc = 55 ksi in. at the 0 deg and −70 deg which values are consistent with the measured lower bound of arrest toughness. There was essentially no strain rate sensitivity of KIc. The difference between initiation and arrest toughness was small, and the dependence of Kc on loading rate could be attributed to the rate dependence of σY.