The calculated distance (air line) between Mäntsälä and Nurmijärvi is approximately 21 miles respectively 34 kilometers.
By car or train, the actual journey to Nurmijärvi is certainly longer, as only the direct route (as the crow flies) between Mäntsälä and Nurmijärvi has been calculated here.
Air line (approximately)
34 kilometers
18 nautical miles