T The construction industry necessitates effective production cost control to maintain product quality and competitiveness. pt bba, a producer of k-350 ready- mix concrete, implemented a substitution of crushed stone (batu split) with bombana sand to optimize production cost efficiency. this research aims to analyze the variance between standard and actual costs, evaluate the k-350 laboratory quality, and assess the impact of the substitution on the efficiency of the cost of production (cop).the research methodology employs a pre-post study design, comparing the period before the substitution (january–june 2025) with the period after the substitution (july–december 2025). the data analyzed includes raw material costs, labor costs, manufacturing overhead, and laboratory quality test results, including the slump test and compressive strength.the findings indicate a significant change in standard cost variance, with the k-350 quality consistently meeting the technical specification standards. the substitution of bombana sand is proven to increase cop efficiency through the optimization of material composition without compromising the quality of the final product. these findings provide strategic recommendations for pt bba in cost control and the enhancement of batching plant operational efficiency.