A simple formulation is developed that permits a quick and accurate estimate of the maximum increase in reactivity that an unmoderated fissile system can experience if subjected to high compaction forces. The keff estimates from the proposed model are compared with Monte Carlo and transport theory values and indicate that for low-density fissile systems and realistic compressions, there is excellent agreement. Good agreement is achieved for denser systems up to ∼20% compression. Some practical areas of application and extension of the model are indicated in which large uniform external pressures may exist.