Polyhydroxyalkanoates (PHAs) are biodegradable polyesters synthesized by microorganisms and have attracted significant attention as sustainable alternatives to petroleum-based plastics. Accurate quantification of intracellularly accumulated PHAs often requires complex pretreatment procedures, including polymer extraction and purification. In this study, we propose a simplified and reliable method for the quantification of PHAs based on alkaline decomposition followed by high-performance liquid chromatography (HPLC). We focused on the copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)], which consists of two monomer units: 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV). Under alkaline conditions, these monomers are decomposed into 2-butenoic acid (2BE) and 2-pentenoic acid (2PE), respectively. The molar conversion factors from PHB to 2BE (α) and PHV to 2PE (β) were experimentally determined through calibration curves constructed using both commercially available standards and alkaline-treated PHA samples. These constants were then used to calculate the concentrations of PHB and PHV. The conversion factors were calculated to be α = 3.26 and β = 3.30. Standard P(3HB-co-3HV) samples were analyzed using this method, and the measured values closely matched the theoretical concentrations, confirming the method's accuracy. This approach simplifies PHA quantification without compromising reliability and eliminates the need for solvent-based extraction. Future work will explore the applicability of this method for analyzing intracellular PHAs in bacterial cells, potentially streamlining PHA analysis in various environmental and industrial settings.