A simple analytical model has been developed to determine the time required for a room to fill with products of combustion from a small fire. The room is assumed to be closed except for small openings at either the floor or ceiling level and the assumption is made that the leak is large enough to allow the transient pressure term in the energy equation to be neglected. Products of combustion are assumed to occupy a layer next to the ceiling and the model predicts the growth of the thickness and the mean density of this layer as a function of time. The analysis shows that times required to fill a typical room are small. For example, a typical bedroom fills with products from a 20 kW fire in several minutes. The time required to fill a room and the mean density of ceiling layer are determined in terms of fire size, room geometry, leak position, fire elevation and geometry.