The calculated distance (air line) between Garðabær and Reykjanesbær is approximately 20 miles respectively 33 kilometers.
By car or train, the actual journey to Reykjanesbær is certainly longer, as only the direct route (as the crow flies) between Garðabær and Reykjanesbær has been calculated here.
Air line (approximately)
33 kilometers
18 nautical miles