The calculated distance (air line) between Rennes and Montfort-sur-Meu is approximately 13 miles respectively 21 kilometers.
By car or train, the actual journey to Montfort-sur-Meu is certainly longer, as only the direct route (as the crow flies) between Rennes and Montfort-sur-Meu has been calculated here.
Air line (approximately)
21 kilometers
11 nautical miles