How long (in kilometers) was the Achaemenid Royal Road from Susa to Sardis?

The Achaemenid Royal Road from Susa to Sardis was about 2,700 kilometers long.

The route connected Susa in southwestern Iran with Sardis in western Anatolia, crossing much of the Achaemenid Empire. It followed or reorganized earlier roads and was probably brought to a high level of efficiency under Darius I in the fifth century BCE. Its purpose was administrative as much as commercial: royal messengers, officials, soldiers, and traders could move across the empire.

Herodotus described the road as 450 parasangs, or 13,500 stades, and mentioned 111 royal post stations. Modern conversions vary because ancient distance units are not perfectly fixed; estimates commonly fall around 2,500–2,700 kilometers. Thus 2,700 kilometers is the standard rounded quiz answer, not a survey-grade measurement of every branch or extension.

The route was later extended toward Ephesus in some descriptions. It should not be confused with every road called “Royal Road” in other ancient civilizations.