This study presents a mathematical framework for implementing process costing systems in leather tanneries with an application to Curtiduría Serrano in Ambato, Ecuador. The leather industry in Ecuador contributes significantly to the national GDP, yet many tanneries operate without standardized cost accounting methods, resulting in suboptimal pricing strategies and profitability challenges. Through a comprehensive analysis of production processes, we developed and tested a process costing system that mathematically models the relationship between direct inputs (raw materials, labor) and indirect manufacturing costs across four sequential production stages: ribera (preparation), tanning, post-tanning, and finishing. Data was collected through structured interviews, direct observation, and financial record analysis. The mathematical model incorporates process-specific cost functions that quantify the relationship between batch size, material consumption ratios, and cost distributions. Results demonstrate that implementing the proposed system enabled precise calculation of unit costs for five product lines, revealing profitability variations ranging from $669.23 to $2,719.23 per batch of 250 units. The framework quantified previously untracked indirect costs, amounting to $1,978.17 monthly. Statistical analysis confirmed a significant correlation ( \(p<0.05\) ) between systematic cost tracking and profitability margins. This research contributes to both theoretical literature on process costing and practical applications for small to medium-sized manufacturers in developing economies, providing a replicable mathematical framework for optimizing resource allocation and enhancing profitability in the leather processing industry.