Cardiovascular diseases in the common mynah (Acridotheres tristis) often go undiagnosed due to limitations with the physical examination and common concurrent illnesses. The aim of this study was to establish accurate reference intervals for cardiac size based on radiographs for this species. All birds were considered healthy based on physical examination. High-quality ventrodorsal radiographic images were obtained from 34 healthy common mynah birds. The cardiac width (CW), thoracic width, distance between third and fourth ribs, synsacral width, coracoid width, and distance between clavicular bones were evaluated on the ventrodorsal radiographic images. Additionally, the ratio between the CW and these anatomical indices were measured. In order to evaluate the correlation between CW and the radiographic indices, a linear regression model was used. The mean ± SD for CW in the 34 birds was 16.1 ± 0.9 mm (95% confidence interval: 14.1–18 mm). Significant moderate correlation was found for CW with thoracic width (R² = 0.37; P < 0.001), coracoid width (R² = 0.25; P = 0.002), and the distance between the third and fourth ribs (R² = 0.34; P < 0.001). The radiographic reference intervals calculated in this study can be used to evaluate cardiac size in the common mynah bird.