The proposed method of forecasting a single-room fire growth based on inverse modelling relies upon the assimilation of temperature measurements at the doorway level from floor to soffit height. These measurements are processed in the context of a two-zone model to obtain transient profiles of neutral plane height, X-N, and upper layer temperature, T-u. An additional correlation is used to deduce the smoke layer height, h, within the fire room. The obtained profiles of T-u, X-N and h over a given period of time (the assimilation window') are used to estimate the fire growth factor, , the time delay, t(0), and the heat loss factor, (c), with a two-zone inverse modelling procedure. Instantaneous displays of the future fire development are then delivered. Experimental data of four furniture fires in an ISO-room are analysed to illustrate the proposed forecasting methodology, which provides positive lead times between 20 and 235 s, depending on the fire growth rate.