The calculated distance (air line) between Reykjanesbær and Mosfellsbær is approximately 28 miles respectively 46 kilometers.
By car or train, the actual journey to Mosfellsbær is certainly longer, as only the direct route (as the crow flies) between Reykjanesbær and Mosfellsbær has been calculated here.
Air line (approximately)
46 kilometers
25 nautical miles