The calculated distance (air line) between Hafnarfjörður and Mosfellsbær is approximately 10 miles respectively 16 kilometers.
By car or train, the actual journey to Mosfellsbær is certainly longer, as only the direct route (as the crow flies) between Hafnarfjörður and Mosfellsbær has been calculated here.
Air line (approximately)
16 kilometers
9 nautical miles