The calculated distance (air line) between Katwijk aan den Rijn and Zaanstad is approximately 24 miles respectively 39 kilometers.
By car or train, the actual journey to Zaanstad is certainly longer, as only the direct route (as the crow flies) between Katwijk aan den Rijn and Zaanstad has been calculated here.
Air line (approximately)
39 kilometers
21 nautical miles