The calculated distance (air line) between Herdecke and Oer-Erkenschwick is approximately 18 miles respectively 29 kilometers.
By car or train, the actual journey to Oer-Erkenschwick is certainly longer, as only the direct route (as the crow flies) between Herdecke and Oer-Erkenschwick has been calculated here.
Air line (approximately)
29 kilometers
16 nautical miles