The calculated distance (air line) between Boulogne-Billancourt and Montrouge is approximately 4 miles respectively 6 kilometers.
By car or train, the actual journey to Montrouge is certainly longer, as only the direct route (as the crow flies) between Boulogne-Billancourt and Montrouge has been calculated here.
Air line (approximately)
6 kilometers
3 nautical miles