The calculated distance (air line) between Sant Boi de Llobregat and Castelldefels is approximately 6 miles respectively 9 kilometers.
By car or train, the actual journey to Castelldefels is certainly longer, as only the direct route (as the crow flies) between Sant Boi de Llobregat and Castelldefels has been calculated here.
Air line (approximately)
9 kilometers
5 nautical miles