The calculated distance (air line) between Hanoi and Dien Bien Phu is approximately 183 miles respectively 295 kilometers.
By car or train, the actual journey to Dien Bien Phu is certainly longer, as only the direct route (as the crow flies) between Hanoi and Dien Bien Phu has been calculated here.
Air line (approximately)
295 kilometers
159 nautical miles