This report aims to evaluate the reliability of the formulae commonly used to determine the severity of a compartment fire. It briefly explains the concept behind fire resistance rating for structural elements, describing how the severity of a 'real' compartment fire is equated to that of the 'standard' fire used in laboratories for fire resistance testing. This is followed by a discussion of the computer method used in this report in dissecting those formulae and the development of some computer techniques for calculating fire severity. Apart from that, various physical parameters of a compartment such as the opening sizes are looked at to determine the significance of their influence on the fire severity. This report also goes beyond the works carried out in the past and examines the validity of the formulae for scenarios that have not previously been considered and explores the validity of the calculation methods intended for steel member for reinforced concrete structure. Finally some discussions and conclusions are made from the findings.