The calculated distance (air line) between Llantwit Major and Dinas Powys is approximately 12 miles respectively 19 kilometers.
By car or train, the actual journey to Dinas Powys is certainly longer, as only the direct route (as the crow flies) between Llantwit Major and Dinas Powys has been calculated here.
Air line (approximately)
19 kilometers
10 nautical miles