This prospective study aimed to establish reference values for thoracic inlet diameter-normalized radiographic indices to evaluate cardiac size in clinically healthy domestic shorthair cats. Standardized thoracic radiographs were obtained from 50 cats at defined respiratory phases. Two metrics were evaluated: the cardiac silhouette axis index, on the basis of the sum of orthogonal long and short axes, and the cardiac silhouette circumference index, derived from the total cardiac perimeter. The thoracic inlet diameter, readily measurable on right and left lateral thoracic radiographs, remained consistent regardless of laterality and respiratory phase and showed strong correlations with cardiac measurements (Pearson's r = 0.86-0.96), supporting its use as a reliable anatomic reference for standardizing cardiac dimensions. Mean cardiac silhouette axis index values at end-inspiration and end-expiration were 2.54 and 2.57 in the right lateral, 2.55 and 2.58 in the left lateral, 2.70 and 2.72 in the dorsoventral, and 2.68 and 2.71 in the ventrodorsal views. Corresponding cardiac silhouette circumference index values were 4.29 and 4.31 in the right lateral, 4.30 and 4.32 in the left lateral, 4.44 and 4.46 in the dorsoventral, and 4.42 and 4.43 in the ventrodorsal views. Unlike the axis index, which is restricted to orthogonal long and short cardiac axes, the circumference index integrates the entire cardiac silhouette. This yields a dimensionless, shape-sensitive parameter that potentially offers a more precise reflection of true cardiac size.