An original mathematical approach to estimate the quantity of impurity particles in jet fuel from a vector of ISO numbers is proposed. The main goal of this study is to design a model that relates the proportion of free water (in parts per million by volume, ppmv) and the content of solid contaminant (mg/L) to the corresponding ISO numbers, traditionally used in jet fuel industry. This model is also applied to obtain the combination of ISO numbers corresponding to the alarm values standardized in ppmv and mg/L units. The new method provides estimations for the free water and solid impurities (simulated by the addition of Arizona dust in laboratory) separately, using sequences of ISO numbers corresponding to the sizes of 4, 6, 14, and 30 pm. It is based on the assumption that the granulometric distribution of water (and that of the solid contaminant) is always fixed, i.e., adding more or less percentage of water (or solid contaminant), the number of particles of a given size will vary proportionally. Using this assumption and starting with an ISO code vector (or a set of them obtained by subsequent measurements), a new procedure is proposed to split it into two vectors, corresponding to the ISO codes of water and Arizona dust, respectively. Finally, the contents of free water and solid contaminant are calculated from those two ISO code vectors and validated using design of experiment techniques. This procedure allows us to translate the standardized alarms measured using free water (ppmv) and solid contaminants (mg/L) into ISO numbers.