This work aims to solve the problem of determining the funding structure for each pyme (small and mid-sized companies). Most of the research on this topic shows the carrying out of a descriptive analysis on the variables that influence the decision of mixing liabilities and equity in different contexts, while in this case we provide a tool that it is expected to improve the financial decision-making process. To introduce the established topic we propose a mathematical model for the financial manager of the pyme can establish the most convenient mix of capitals. For this, and previously, we identify the characteristics of the different sources of funding, the variables that influence the risk, and the association between the source and the destination of the funds. We develop the mathematical foundation of the model and then it is taken to an electronic calculus form so, upon providing the data for the different variables, the best combination between own funds and borrowed funds can be achieved, so when contrasting these with the profitability of the investment a higher economic value added can be obtained.