The calculated distance (air line) between Boulogne-Billancourt and Châtillon is approximately 3 miles respectively 5 kilometers.
By car or train, the actual journey to Châtillon is certainly longer, as only the direct route (as the crow flies) between Boulogne-Billancourt and Châtillon has been calculated here.
Air line (approximately)
5 kilometers
3 nautical miles