Similar to protective headgear such as helmets, swim cap design and sizing for diverse hair types, particularly long and thick hair, remain a critical challenge in the sportswear and personal gear industries. Hair with structural characteristics such as coarseness and substantial volume often make conventional swim cap designs insufficient in accommodating hair dimensions. As a result, swim caps do not often provide functionality, such as keeping the hair dry or protecting it from chlorine or salt. This study explored the application of 3D scanning technology to develop methods for designing swim caps for individuals with different hair volumes. We analyzed key head- and swim cap-related measurements using the 3D anthropometric data of 31 female participants. Principal component and cluster analyses identified two distinct sizing categories. Furthermore, hair compression analysis revealed that hair type, length, and bun positioning influenced volume measurements in 3D head scans. Future research should expand participant diversity, integrate objective hair volume measurements, and conduct real-world fit testing to optimize swim cap functionality.