A league is an old unit of distance, not specifically a unit of depth, and it is usually taken as about 3 miles or roughly 4.8 kilometers in modern terms.

What a league means

  • A league historically described how far a person could walk in about an hour, so it was always a measure of horizontal distance along the ground or sea, not vertical depth.
  • In English-speaking contexts on land, a league is generally accepted as 3 statute miles, which is about 4.83 kilometers.

Variations in leagues

  • Different countries used slightly different leagues, ranging from about 2.4 to 4.6 miles (around 3.9 to 7.4 kilometers), depending on time and place.
  • At sea, a league is often described as 3 nautical miles, which is about 5.56 kilometers.

So, “how deep is a league”?

  • If someone casually asks “how deep is a league,” the usual modern answer is “about 3 miles (around 4.8 km),” even though that is technically a distance along the surface, not depth.
  • When people talk about things like “20,000 leagues under the sea,” the phrase refers to the distance traveled through the water, not a literal vertical depth straight down from the surface.

Information gathered from public forums or data available on the internet and portrayed here.