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