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The Stadion (Ancient Greek: στάδιον, plural: στάδια, stadia) was a prominent ancient Greek unit of length. Its primary basis was the standard length of the track in a Greek stadion (stadium) structure, the venue for footraces and other athletic events.

Like many ancient units, its precise length varied depending on the specific location (polis) and time period, as different stadiums had slightly different track lengths. However, it was generally understood to be 600 Greek feet (podes). Since the Greek foot itself varied, so did the stadion.

Commonly cited values include:

• Olympic Stadion: Approximately 192.27 meters (m) or about 630.8 feet (ft). Based on the stadium at Olympia.
• Attic (Athenian) Stadion: Often estimated around 185 meters (m) or about 607 feet (ft). This is a frequently used general approximation.
• Ptolemaic (Egyptian) Stadion: Used later, sometimes estimated around 157.5 meters (m).

The stadion was a fundamental unit in the Hellenic world, dating back to at least the Archaic period (c. 800–480 BCE).

• Origin: Directly tied to the length of the stadion footrace, which was the premier event at the ancient Olympic Games and other Panhellenic games. The track length became a standard measure.
• Variability: Different major sites (Olympia, Delphi, Athens, Epidaurus) had stadiums of slightly different lengths, leading to regional variations of the stadion unit. The 600-foot standard was common, but the length of the 'foot' differed.
• Geographical Measurement: It became the standard unit for measuring larger distances, such as travel routes between cities or the dimensions of territories. Historians like Herodotus and geographers like Strabo used it extensively.
• Eratosthenes' Calculation: Most famously, Eratosthenes of Cyrene (c. 276–194 BCE) used the stadion to estimate the Earth's circumference with remarkable accuracy for his time. His result depended crucially on the specific stadion length he assumed (likely the Attic or Egyptian stadion).
• Roman Influence: While the Romans adopted their own system (based on the passus and Roman mile), Greek units like the stadion continued to be understood and sometimes used in the eastern parts of the Roman Empire and referenced by Roman writers like Pliny the Elder when discussing Greek geography.
• Decline: With the decline of the Greek city-states and the dominance of Rome, followed by later measurement reforms, the stadion gradually fell out of practical use, eventually being entirely superseded by Roman, Byzantine, and later metric or imperial units.

The stadion is now obsolete but was historically essential for:

• Athletics: Defining the length of the primary footrace and the stadium itself.
• Geography & Cartography: Measuring distances between locations, the size of regions, and geographical features in the Greek world.
• Astronomy: Used by figures like Eratosthenes and Ptolemy for astronomical distances and calculations (e.g., Earth's circumference, distances to celestial bodies).
• Architecture & Urban Planning: Sometimes used for measuring large structures or city dimensions.
• Historical Texts: Crucial for understanding distances mentioned in ancient Greek literature, historical accounts (like Thucydides or Herodotus), and scientific treatises.