Nautical Mile Converter (NM, km, mi)

Understanding the nautical mile converter — concept and principle

The nautical mile (NM) is the distance unit of the sea and the air. Ship and aircraft routes, chart distances, territorial waters and visibility reports are given in nautical miles, and speeds in knots (nautical miles per hour). This converter changes nautical miles into kilometres, statute (land) miles, metres, feet and cables, and back.

How it is calculated

Values are converted to nautical miles and then to the target: result = value × (NM per "from" unit) ÷ (NM per "to" unit). The definitions behind the factors are exact:

• 1 NM = 1,852 m = 1.852 km
• 1 statute mile = 1,609.344 m, so 1 NM ≈ 1.150779 mi
• 1 ft = 0.3048 m, so 1 NM ≈ 6,076.115 ft
• 1 cable = 0.1 NM = 185.2 m (the international cable used here)

The principle

A nautical mile was originally one minute of arc of latitude measured along a meridian. Because the Earth is slightly flattened, that minute varies a little between the equator and the poles, so in 1929 the International Hydrographic Conference fixed the international nautical mile at exactly 1,852 m, close to the average. The link to latitude is what makes it practical: on a nautical chart, one minute on the latitude scale at the side of the chart is about one nautical mile, so navigators measure distance with dividers straight from the chart. One degree of latitude is therefore about 60 NM.

Worked example

1. Inputs: value = 12, from nautical miles, to kilometres.
2. 12 × 1.852 = 22.224 km.
3. In metres: 12 × 1,852 = 22,224 m.
4. In statute miles: 22,224 ÷ 1,609.344 = 13.80935338 mi.
5. In feet: 22,224 ÷ 0.3048 = 72,913.38583 ft; in cables: 12 ÷ 0.1 = 120.

How to read the result

Twelve nautical miles is a familiar figure: under the UN Convention on the Law of the Sea, a coastal state may claim a territorial sea of up to 12 NM from its baselines, about 22.2 km. A speed of 20 knots covers 20 NM, or 37.04 km, in an hour. Because 1 NM is longer than both a kilometre and a statute mile, a distance in NM is always the smallest number of the three.

Common mistakes

• Using the statute mile (1,609.344 m) when a chart or flight plan means nautical miles; the error is about 15%.
• Measuring distance on the longitude scale (top or bottom of a chart). Minutes of longitude shrink toward the poles; use the latitude scale.
• Writing "knots per hour". A knot already means nautical miles per hour.

Frequently asked questions

How many kilometres is one nautical mile?

Exactly 1.852 km.

Why do ships and aircraft use nautical miles?

Because one nautical mile is about one minute of latitude, distances can be read directly from chart latitude scales and match the geographic coordinate grid.

What is the difference between a nautical mile and a mile?

A nautical mile is 1,852 m; a statute mile is 1,609.344 m. One NM is about 1.15 statute miles.

Sources: NIST Special Publication 811 (2008), Appendix B · BIPM, The International System of Units (SI Brochure), 9th edition, 2019 · United Nations Convention on the Law of the Sea (1982), Article 3