The calculated distance (air line) between Formello and Guidonia Montecelio is approximately 18 miles respectively 28 kilometers.
By car or train, the actual journey to Guidonia Montecelio is certainly longer, as only the direct route (as the crow flies) between Formello and Guidonia Montecelio has been calculated here.
Air line (approximately)
28 kilometers
15 nautical miles