Settings The Faculty of Medicine, Alexandria University, Egypt. Purpose The aim of this work was to study the contribution of posterior corneal astigmatism to total corneal astigmatism. Patients and methods A descriptive prospective observational study that included 100 eyes of 100 patients aged 18 years and above. Scheimpflug camera imaging was performed using a Pentacam. The following data were recorded: anterior corneal radii of curvature, posterior corneal radii of curvature and astigmatism, simulated keratometry (sim K) and astigmatism, the true net power and astigmatism, and central corneal pachymetry. All included participants signed a written informed consent. Results The mean age was 38.11 ± 12.69 years. The orientation of the steep axis of the anterior corneal surface was vertical in 70% of the cases, resulting in with-the-rule astigmatism. For the posterior corneal surface, the steep axis orientation was vertical in 94% of the cases, resulting in against-the-rule (ATR) astigmatism. The mean sim K astigmatism was higher than that of the true net K astigmatism by 0.12 ± 0.18 D (range -0.3-0.4 D) at 4 ± 5° (range 0-21°). Conclusion Anterior corneal with-the-rule astigmatism tends to change to ATR astigmatism with age. The posterior corneal surface showed ATR astigmatism regardless of the age. Using data from the anterior corneal surface only resulted in a higher astigmatism of 0.12 D compared with using data from both the anterior and the posterior surfaces. Also, data from the anterior corneal surface alone resulted in a difference in the axis of more than 10° in 11% of the cases.