A proper selection of material is a key factor in fire safety. The material can be selected on the basis some factors,which include heat release rate, mass loss rate, smoke produced. There are standard tests which measure these. One such equipment is a cone calorimeter, which measures heat release rate, mass loss rate, ignition time, smoke produced when a material is burned. This equipment helps to classify the materials based on these results. The tests are conducted in cone calorimeter under fixed external radiant heat flux. So a clear understanding regarding the heat flux distribution over the specimen surface is essential. Present work experimentally determines the heat flux distribution over the specimen at different heights below for both standard and large format (approximately double the dimension of standard) conical heater. Tests are carried out for fuel sample size of 10 cm x 10 cm placed horizontally under both heaters and compared. A numerical pyrolysis model is developed to predict the pyrolysis of fuel sample (black PMMA). The numerical model is used to predict the two tests and additional computations are made for 15 cm x 15 cm fuel slabs of same material for standard and large format heater. © Asia-Pacific Conference on Combustion, ASPACC 2019.All right reserved.