The calculated distance (air line) between Dortmund and Mechelen-aan-de-Maas is approximately 86 miles respectively 138 kilometers.
By car or train, the actual journey to Mechelen-aan-de-Maas is certainly longer, as only the direct route (as the crow flies) between Dortmund and Mechelen-aan-de-Maas has been calculated here.
Air line (approximately)
138 kilometers
74 nautical miles