The calculated distance (air line) between Dien Bien Phu and Sơn La is approximately 58 miles respectively 93 kilometers.
By car or train, the actual journey to Sơn La is certainly longer, as only the direct route (as the crow flies) between Dien Bien Phu and Sơn La has been calculated here.
Air line (approximately)
93 kilometers
50 nautical miles