Most theoretical models related to thermal performance and the evaluation of pressure drop on compact heat exchangers, consider that the mass flow rate is constant and equal in all core channels. However, this hypothesis in some situations is erroneous, the effect of fluid maldistribution affects heat exchangers performance. Therefore, in the present work, an analysis of the phenomenon of fluid maldistribution in headers was performed. Experimental tests were carried out on polymeric headers, using Polyamide 2200, in order to evaluate the fluid nonuniformity. The tests were carried out with compressed air at room temperature in a wide range of mass flow rate totaling 300 experimental data. A theoretical model was used to predict the fluid maldistribution level inside the header. Based on the coefficient of variation (CoV), the presence of fluid maldistribution for low flows was verified. Besides, a new theoretical model, the standard deviation of fluid nonuniformity (), based on the shape factor (SF), was presented to measure maldistribution level without experimental tests. The proposed model proved to be efficient to predict the fluid nonuniformity inside the header, presenting an average error of 18% in comparison to the CoV.