In most cases, it is impossible to analyse drinking water for iron content at the sampling point. The water needs to be transported to a specialized laboratory, which can take several hours. This also applies to tap water. It was previously established that iron in water quickly passes from soluble to insoluble form. As a result, its concentration within 90 minutes decreases by 1.5-2 times compared to the initial value. The aim of the research is to develop a new mathematical model for the prediction of the concentration of iron in drinking water depending on the duration of time from the moment of sampling to the moment of analysis. The model is based on empirical data obtained from measurements of iron concentration in tap water of the water supply system of the city of Pokrovsk, Donetsk region, Ukraine. Based on the analysis of four developed and analysed polynomial models for two samples of tap water, second-degree polynomial equations are recommended. A correlation analysis confirmed the presence a strong correlation of the iron content in tap water and the time interval between sampling and analysis. It was experimentally established that from the 90th minute the physical process of a parabolic decrease in the concentration of iron in tap water practically attenuate. The recommended time range for using proposed mathematical model is from 0 to 120 minutes from the moment of tap water sampling.