The insider's guide, from travelers who've actually been through.
Fiumicino splits international arrivals mainly across Terminal 1 and Terminal 3. Most long-haul flights use T3. The Leonardo Express to Roma Termini runs every 30 minutes, takes about 32 minutes, and costs around 14 euros one way; buy tickets from machines in the arrivals hall to skip the desk queue. With 6 hours you have enough time to reach central Rome, walk the Forum area or Trastevere, and get back with buffer. Under 5 hours, stay airside: T3 has a decent food court and free WiFi throughout. Verify the fare before you travel, as prices change periodically.
