A novel hybrid field/zone fire model, coupling the SMARTFIRE CFD fire model to both the CFAST zone model and a custom zone model is presented. The intention of the hybrid model is to reduce the computational overheads incurred in using fire field models to simulate large geometries such as large buildings or large passenger ships, while maintaining the accuracy of the fire field model. In using the hybrid model, only the most important parts of the geometry are fully modeled using the field model. Other less important parts of the geometry are modeled using the zone model. From the field model’s perspective, the zone model is used to represent parts of the geometry as an accurate boundary condition. By using this approach, many computational cells are replaced by a simple zone model, saving computational costs. Two tests cases demonstrating the technique are presented. It is shown that the hybrid approach is capable of producing reasonably accurate predictions of fire development while substantially reducing computational costs. It is shown that by removing some 56% of the CFD solution domain, the hybrid case can achieve a saving of 48% in the run time.