This study focuses on the accurate evaluation of carbon dioxide capture in cementitious materials, a critical factor in reducing the carbon footprint of the concrete industry. Currently, there is no standardized method accurately quantifying fixed CO2 in carbonated cementitious materials. Additionally, since cementitious materials may contain organic carbon (TOC), which can lead to an overestimation of total inorganic carbon (TIC), separating and quantifying TIC and TOC is essential but remains challenging. We introduce a novel, straightforward approach for separately quantifying TIC and TOC based on total carbon measurement. Experiments were conducted using carbonated cement paste and a mixture containing polypropylene powder at various mass ratios. The optimal dissolution method for measuring TIC via acid solution and subsequent pretreatment was validated using X-ray diffraction analysis. This study proposes a subtraction method for determining TIC and TOC content in cementitious materials, providing a practical solution for environmental assessments in the concrete industry.