The calculated distance (air line) between Dordrecht and Alphen aan den Rijn is approximately 22 miles respectively 36 kilometers.
By car or train, the actual journey to Alphen aan den Rijn is certainly longer, as only the direct route (as the crow flies) between Dordrecht and Alphen aan den Rijn has been calculated here.
Air line (approximately)
36 kilometers
19 nautical miles