The calculated distance (air line) between Merthyr Tydfil and Llantrisant is approximately 14 miles respectively 23 kilometers.
By car or train, the actual journey to Llantrisant is certainly longer, as only the direct route (as the crow flies) between Merthyr Tydfil and Llantrisant has been calculated here.
Air line (approximately)
23 kilometers
12 nautical miles