Zone fire models are used by practising engineers every day in New Zealand, yet the models have limitations, and the uncertainty of these models has not been well documented. Comparisons with experimental data are simply comparison and do not analyse the uncertainty of the models, nor are they validation of the models. The object of this research has been to discuss the uncertainties in components of zone models and show how uncertainty within user supplied data affects the results obtained. The zone fire model selected for analysis is the second version of CFAST. A numerical uncertainty analysis is performed, utilising sensitivity factors as the basis of the analysis. In the analysis, no assumptions are made as to the independency of the input variables. A large amount of information is appended, with a discussion of pertinent results. Several input variables were identified to resulted in discernible uncertainty in the output. Consisting of the heat release rate, radiative fraction, ambient temperature, ambient pressure, and ceiling height.