The calculated distance (air line) between Bonn and Hennef (Sieg) is approximately 9 miles respectively 14 kilometers.
By car or train, the actual journey to Hennef (Sieg) is certainly longer, as only the direct route (as the crow flies) between Bonn and Hennef (Sieg) has been calculated here.
Air line (approximately)
14 kilometers
8 nautical miles