In the event of a fire, atrium buildings often require the use of smoke control systems to provide conditions for safe means of escape. Hot smoky gases entering an atrium from a fire within an adjacent compartment will rise as a thermal spill plume into a buoyant layer of gases formed beneath the atrium ceiling. This spill plume will entrain large amounts of air, which must be calculated to determine the quantity of gases entering the smoke layer. There are a number of calculation methods available to designers of smoke control systems involving the thermal spill plume. These methods are important to the designer in order to calculate the required fan capacity or vent area for a smoke exhaust ventilation system. This work describes research examining various uncertainties and limitations in the available calculation methods. A combination of both physical scale modelling and computational fluid dynamics modelling has been used in the analysis. This work has demonstrated that the presence of a downstand at the spill edge of a compartment opening appears to have little effect on the entrainment of air into the subsequent spill plume. A simplified spill plume formula has been developed to predict the mass flow rate of gases produced by a free spill plume, which inherently includes entrainment of air into the ends of the plume. An empirical correlation has also been developed to predict the entrainment of air from a compartment opening to a higher projecting balcony, and hence, the subsequent mass flow rate of gases at the spill edge. This work has addressed various uncertainties in spill plume calculations, providing robust and relevant simplified design formulae. This work has generally improved the guidance available to fire safety engineers for the spill plume in smoke control design.