The ability to determine urinary-stone composition is valuable for urologists, allowing them to identify the possible causes of stone formation and to provide appropriate drug treatment and diet-control advice to patients, which can lower recurrence rates. Calcium-based stones account for the majority of urinary stones seen in patients, and calcium oxalate comprises most of these calcium-based stones. This study accomplishes a quantitative analysis of two forms of calcium oxalate stone, those comprised of calcium oxalate monohydrate (COM) or calcium oxalate dihydrate (COD), using Fourier transform infrared spectroscopy (FTIR). The use of a newly validated absorption peak ratio, 950/912 cm−1 (corresponding to COM and COD, respectively), is demonstrated to achieve quantitative estimation of the COM and COD composition. The correlation coefficient between the COM/(COM + COD) composition ratio and the selected peak ratio is 0.99 for standard compound mixtures. This peak-ratio analysis method could aid clinicians in quantitatively determining the COM and COD compositions of calcium oxalate stones, facilitating the design of appropriate treatment plans and preventive measures for calcium-oxalate-stone patients.