Objective: The aim of this study is to evaluate mastoid volume and dimensions in patients with unilateral microtia using High-Resolution Computed Tomography (HRCT) to enhance the precision of reconstructive surgical planning. Methods: A retrospective analysis of HRCT mastoid scans from patients with unilateral microtia was carried out at Cipto Mangunkusumo General Hospital between May 2020 and August 2022. Parameters such as mastoid volume, height, and surface area were measured at the ear canal, Superior Semicircular Canal (SCC), and lateral SCC levels. Results: The analysis revealed statistically significant decreases in median mastoid air cell volume and mastoid bone volume in the affected ears compared to contralateral ears (p = 0.0312 and p = 0.02, respectively). Additionally, decreased mastoid height and surface areas at the ear canal and superior SCC levels were identified in affected ears (p < 0.05). Conclusions: Patients with unilateral microtia have diminished mastoid bone volumetric parameters and dimensions on the affected side. These findings offer critical data for surgeons in preoperative planning, enabling the selection of appropriate reconstructive techniques and providing comprehensive patient counselling. Level of evidence: Level 4. © 2024 Associação Brasileira de Otorrinolaringologia e Cirurgia Cérvico-Facial