X-ray reflectivity (XRR) is one of the primary measurement techniques for thickness calculation of thin films and multilayer period determination. This technique can also be used for the analysis of organic thin film multilayer structures. In this method, the accuracy of thickness calculation depends on precision of the determination of the local maxima of XRR curve. The analysis of the XRR curves is cumbersome because of the noise which is recorded while measurement. It can be improved using computer data analysis algorithms for noise reduction and determination of the local maxima on the XRR curve. One of such algorithms, widely used in the data spectroscopy analysis, is Savitzky–Golay (S–G) algorithm. In this paper, the application of S–G algorithm for thickness determination of self-assembled ion liquid nanolayer of dimethyldiallylammonium chloride (PDDA) is shown.