The calculated distance (air line) between El Prat de Llobregat and Castelldefels is approximately 7 miles respectively 12 kilometers.
By car or train, the actual journey to Castelldefels is certainly longer, as only the direct route (as the crow flies) between El Prat de Llobregat and Castelldefels has been calculated here.
Air line (approximately)
12 kilometers
6 nautical miles