The calculated distance (air line) between Ouderkerk aan de Amstel and Haarlem is approximately 13 miles respectively 21 kilometers.
By car or train, the actual journey to Haarlem is certainly longer, as only the direct route (as the crow flies) between Ouderkerk aan de Amstel and Haarlem has been calculated here.
Air line (approximately)
21 kilometers
11 nautical miles