Shadows severely affect the performance of solar photovoltaic (PV) systems. A proper description of this effect is useful for sizing and simulating PV systems when shadows cannot be avoided. Shading factors represent the basis for simulating the effect of shadows on solar modules. These factors can be used to estimate shading losses, calculate their I-V and P-V curves under shading conditions, or develop new maximum power point tracking (MPPT) techniques. Open-source libraries focused on solar energy have gained popularity in recent years. One of the currently most popular ones is the PV_LIB toolbox initially developed by Sandia Laboratories. PV_LIB significantly facilitates solar energy calculations. However, it currently lacks functions for taking into account shaded conditions. In this paper, a detailed Matlab-based method for calculating the shading factors is provided. The method has been used for elaborating a toolbox for shading calculations. The current work could help extend the functionalities of the PV_LIB toolbox. The results were compared against other currently popular computer programs, namely the System Advisor Model (SAM) and PVsyst. With this method, it is also possible to calculate shading factors with smaller time steps than possible with the mentioned programs. This work also shows the importance of using small time steps and how this can affect the accuracy of the calculated shading factors. The contribution of this work is providing a way of quantifying shadow losses in PV systems with Matlab, allowing for better accuracy, flexibility, and transparency during the calculation. The functions developed in this work can be accessed by contacting the authors.