Abstract The hot air method proposed by Arya et al. to determine the relationship between diffusivity and soil moisture content uses a graphical procedure which is time-consuming and subjective. The results obtained by different observers vary greatly. In this paper, an algorithm is proposed by which the diffusivity values can be computed reproducibly. An empirical function with four parameters is used. Two of them are ascertained by the boundary conditions, the other two have been optimized by a standard procedure. The computation is rapid and unambiguous. The algorithm also enables an analytical expression for the evaporation that is generated by the drying of a sample by hot air and in this way it gives a reliable judgement of the appropriateness of the data obtained. The method has been tested on samples obtained from residual, silty soils from the Ardèche Basin, France.