The calculated distance (air line) between Saint-Quentin-en-Yvelines and Fontenay-sous-Bois is approximately 22 miles respectively 35 kilometers.
By car or train, the actual journey to Fontenay-sous-Bois is certainly longer, as only the direct route (as the crow flies) between Saint-Quentin-en-Yvelines and Fontenay-sous-Bois has been calculated here.
Air line (approximately)
35 kilometers
19 nautical miles