Background <p>Proper coronal alignment plays a critical role in the effectiveness of medial unicompartmental knee arthroplasty (UKA). This research seeks to explore the connection between the arithmetic hip-knee-ankle angle (aHKA) and the actual postoperative (postop) HKA angle after undergoing medial UKA.</p> Methods <p>A retrospective analysis was conducted on individuals who received medial UKA at a specialized orthopedic hospital between January 1, 2024, and July 31, 2024. The aHKA was determined using the formula: medial proximal tibial angle (MPTA) minus lateral distal femoral angle (LDFA), plus 180°. The relationships between the postop HKA angle and the aHKA, MPTA, and LDFA were analyzed. Patients were further divided into three categories based on their postop HKA angle: greater than 180°, between 175° and 180°, and less than or equal to 175°. These groups were then compared in terms of aHKA, LDFA, MPTA, and preoperative HKA angle.</p> Results <p>A total of 242 patients (254 knees) were included in this study. The postop HKA was nearly equal to the preoperative aHKA (176.09° ± 2.86° vs. 176.23° ± 3.15°). Statistical analysis revealed a positive association between aHKA and postop HKA angle (R² = 0.4595, <i>P</i> &lt; 0.05), as well as between MPTA and postop HKA angle (R² = 0.2072, <i>P</i> &lt; 0.05). Conversely, a negative correlation was identified between LDFA and postop HKA angle (R² = 0.2448, <i>P</i> &lt; 0.05). These patterns held true for both fixed-bearing and mobile-bearing UKA prostheses. Notable differences among the three HKA groups were found regarding aHKA, MPTA, LDFA, and preoperative HKA angle (<i>P</i> &lt; 0.05).</p> Conclusion <p>The findings indicate that aHKA has a strong relationship with the postop HKA angle, suggesting its potential as an effective predictor for postop coronal alignment after medial UKA, irrespective of the type of prosthesis used.</p>