The calculated distance (air line) between Mandalay and Nay Pyi Taw is approximately 154 miles respectively 248 kilometers.
By car or train, the actual journey to Nay Pyi Taw is certainly longer, as only the direct route (as the crow flies) between Mandalay and Nay Pyi Taw has been calculated here.
Air line (approximately)
248 kilometers
134 nautical miles