Nautical Miles, Fathoms and Leagues: Old Distance Units
A mile on a road sign and a mile on a ship's chart are different lengths. Navigation uses the nautical mile, while road distances use the statute mile. Older units such as the fathom and the league also survive in sea charts, novels and idioms, and they are easy to misread without knowing their size.
This guide defines each unit, explains why the nautical mile is tied to the shape of the Earth, and gives worked conversions and shortcuts you can do in your head.
Statute miles versus nautical miles
The statute mile is 1,609.344 meters (5,280 feet). The nautical mile is defined as exactly 1,852 meters, about 6,076 feet, so it is roughly 15% longer. The two are related by a fixed ratio.
1 nautical mile = 1,852 ÷ 1,609.344 = 1.1508 statute milesGoing the other way, 1 statute mile = 0.8690 nautical miles.
The nautical mile exists because it matches the globe. One minute of arc of latitude, a sixtieth of a degree, is very close to one nautical mile; the true length varies slightly from the equator to the poles, but stays in the neighborhood of 1,840 to 1,860 meters. That makes distances easy to read directly off a chart's latitude scale. A change of one degree of latitude is 60 nautical miles, or 60 × 1.852 = 111.12 km.
Speed follows the same logic. A knot is one nautical mile per hour, so a vessel traveling at 15 knots covers 15 nautical miles each hour. Aviation also commonly uses nautical miles for distance and knots for speed.
Converting nautical miles
| Nautical miles | Kilometers | Statute miles |
|---|---|---|
| 1 | 1.852 | 1.151 |
| 5 | 9.26 | 5.754 |
| 10 | 18.52 | 11.508 |
| 50 | 92.60 | 57.539 |
| 100 | 185.20 | 115.078 |
Worked example, nautical miles to kilometers and miles: a passage of 250 nautical miles is 250 × 1.852 = 463 km, or 250 × 1.1508 = 287.7 statute miles.
Worked example, kilometers to nautical miles: a 450 km flight is 450 ÷ 1.852 = 243 nautical miles.
- Nautical miles to kilometers: multiply by 1.85. 250 × 1.85 = 462.5, against an exact 463.
- Kilometers to nautical miles: multiply by 0.54. 450 × 0.54 = 243, which matches the exact value to the nearest mile.
- Nautical miles to statute miles: add 15%. 250 + 37.5 = 287.5, against an exact 287.7.
For exact answers use the nautical miles to kilometers converter, or the miles to nautical miles converter if the starting figure is a road mile.
Fathoms and depth
A fathom is exactly 6 feet, which is 1.8288 meters. It was a natural length for a sailor measuring depth with a weighted line, roughly the span of outstretched arms. Many modern charts show depth in meters, but some still use fathoms or feet, so check the legend before reading a number.
meters = fathoms × 1.8288
Worked example: a chart marks a depth of 20 fathoms. That is 20 × 6 = 120 feet, or 20 × 1.8288 = 36.6 meters. In the other direction, 12 meters of water is 12 ÷ 1.8288 = 6.56 fathoms. The fathoms to feet converter handles the arithmetic.
Leagues
A league is commonly described as the distance a person could walk in about an hour. Its length varied between countries and eras, and it is most often taken as three miles, which is 4.83 km. At sea the 'marine league' is traditionally three nautical miles rather than three statute miles, so a text that uses the unit should be read with care.
The book title 'Twenty Thousand Leagues Under the Sea' is a good example. It refers to the distance traveled, not depth. At three statute miles per league, 20,000 leagues is 60,000 miles, or about 96,560 km. That is roughly 2.4 times the Earth's circumference at the equator (about 40,075 km), which fits a long voyage around the world. A depth of that size would be physically impossible.
Avoiding mistakes
- Check whether 'mile' means statute or nautical. On a chart, a flight plan or a ship's log it normally means nautical.
- Never say 'knots per hour'. A knot is already a speed. For speed conversions see the article on km/h, mph, m/s and knots.
- For depth, look for the chart's unit. A reading of 20 could be meters, feet or fathoms, and the differences are large.
- Treat a league as approximate unless the source defines it.
- Nautical Miles to Kilometers Converter: Enter a distance in nautical miles and the converter returns kilometers, rounded to six decimals. One nautical mile is exactly 1.852 kilometers.
- Miles to Nautical Miles Converter: Enter a distance in statute miles and the converter returns nautical miles, rounded to six decimal places. One nautical mile equals 1.150779 land miles, so one land mile is about 0.868976 nautical miles.
- Fathoms to Feet Converter: Type a depth or length in fathoms and the converter shows feet immediately. One fathom is 6 feet, and the guide below covers the rule, the history and worked examples.
This article is general information, not professional advice. See our Disclaimer and Editorial Policy.
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