The calculated distance (air line) between Karlovac and Kostanjevica na Krki is approximately 25 miles respectively 41 kilometers.
By car or train, the actual journey to Kostanjevica na Krki is certainly longer, as only the direct route (as the crow flies) between Karlovac and Kostanjevica na Krki has been calculated here.
Air line (approximately)
41 kilometers
22 nautical miles