The calculated distance (air line) between Merthyr Tydfil and Caerphilly is approximately 14 miles respectively 22 kilometers.
By car or train, the actual journey to Caerphilly is certainly longer, as only the direct route (as the crow flies) between Merthyr Tydfil and Caerphilly has been calculated here.
Air line (approximately)
22 kilometers
12 nautical miles