The nautical league is a unit of length equal to three nautical miles, or exactly 5556 metres. It is longer than the land league by about 728 metres, since it is built from the nautical mile of 1852 metres rather than the statute mile.
The unit belonged to an era when it was the practical measure of coastal jurisdiction. The cannon shot rule, articulated in the seventeenth and eighteenth centuries, held that a state's authority extended as far as it could defend from shore, conventionally taken as three nautical miles. That distance became the standard territorial sea limit and was expressed as one marine league. Many older treaties and statutes define maritime boundaries in leagues for this reason.
The three-mile limit held for centuries before eroding through the twentieth. States progressively claimed wider waters, and the United Nations Convention on the Law of the Sea settled the matter in 1982 by establishing a twelve nautical mile territorial sea and a two hundred mile exclusive economic zone. Both figures are stated in nautical miles rather than leagues, and the older unit fell out of legal use.
Charts and pilot books from the age of sail used the league routinely for describing offshore distances and the visibility of landmarks. Reading those sources accurately requires knowing which league is meant, since the same word could refer to a land league, a marine league or a Spanish or Portuguese variant, and the differences are large enough to matter when reconstructing a historical position.
Today the unit has no navigational role. Charts are marked in nautical miles, and distances at sea are quoted the same way. The nautical league appears only in historical and legal texts.
The unit also underlies a familiar figure in visibility calculations. From a height of about three metres above sea level, the horizon lies roughly one nautical league away, which is why the three-mile limit corresponded loosely to what a lookout could see as well as to what a gun could reach.
One nautical league equals 3 nautical miles, 5556 metres, 5.556 kilometres, or about 3.452 statute miles.
| Unit | Symbol | 1 Nautical league equals | 1 of these equals | Converter |
|---|---|---|---|---|
| Ångström | Å | 55560000000000 Å | 1.79985601152 × 10-14 nlea | Nautical leagues to Ångströms |
| Nanometer | nm | 5556000000000 nm | 1.79985601152 × 10-13 nlea | Nautical leagues to Nanometers |
| Micrometer | μm | 5556000000 μm | 1.79985601152 × 10-10 nlea | Nautical leagues to Micrometers |
| Millimeter | mm | 5556000 mm | 0.000000179985601152 nlea | Nautical leagues to Millimeters |
| Centimeter | cm | 555600 cm | 0.00000179985601152 nlea | Nautical leagues to Centimeters |
| Decimeter | dm | 55560 dm | 0.0000179985601152 nlea | Nautical leagues to Decimeters |
| Meter | m | 5556 m | 0.000179985601152 nlea | Nautical leagues to Meters |
| Decameter | dam | 555.6 dam | 0.00179985601152 nlea | Nautical leagues to Decameters |
| Hectometer | hm | 55.56 hm | 0.0179985601152 nlea | Nautical leagues to Hectometers |
| Kilometer | km | 5.556 km | 0.179985601152 nlea | Nautical leagues to Kilometers |
| Inch | in | 218740.15748 in | 0.00000457163426926 nlea | Nautical leagues to Inches |
| Hand | hh | 54685.0393701 hh | 0.000018286537077 nlea | Nautical leagues to Hands |
| Foot | ft | 18228.3464567 ft | 0.0000548596112311 nlea | Nautical leagues to Feet |
| Yard | yd | 6076.11548556 yd | 0.000164578833693 nlea | Nautical leagues to Yards |
| Chain | ch | 276.187067526 ch | 0.00362073434125 nlea | Nautical leagues to Chains |
| Furlong | fur | 27.6187067526 fur | 0.0362073434125 nlea | Nautical leagues to Furlongs |
| Mile | mi | 3.45233834407 mi | 0.2896587473 nlea | Nautical leagues to Miles |
| Nautical mile | nmi | 3 nmi | 0.333333333333 nlea | Nautical leagues to Nautical miles |
| League | lea | 1.15077944802 lea | 0.868976241901 nlea | Nautical leagues to Leagues |
| Astronomical unit | au | 0.0000000371395660513 au | 26925462.689 nlea | Nautical leagues to Astronomical units |
| Light year | ly | 5.87269663384 × 10-13 ly | 1702795261444 nlea | Nautical leagues to Light years |
| Parsec | pc | 1.80057697322 × 10-13 pc | 5553775344657 nlea | Nautical leagues to Parsecs |