SeaMemo
Nautical mile to km and miles: how long is a nautical mile, and what counts?
A nautical mile is exactly 1,852 metres, which is 1.852 km or about 1.151 statute miles (6,076 feet). To convert nautical miles to miles, multiply by 1.151; to get kilometres, multiply by 1.852. A quick mental shortcut: add 15 percent for miles. 10 nautical miles is roughly 11.5 miles.
The definition: 1 nautical mile = 1,852 metres
A nautical mile (NM, nmi, also called a sea mile) measures exactly 1,852 metres, which is 1.852 km or about 6,076 feet. The First International Extraordinary Hydrographic Conference in Monaco set this value as the international nautical mile in 1929, and it is the figure in the ISO 80000-3 standard and in the conversion tables of the US National Institute of Standards and Technology (NIST). The nautical mile is not an SI unit, but it is the unit of distance at sea and in the air worldwide.
It is not an arbitrary number: one nautical mile equals one minute of latitude, an angular minute along a meridian. That is the clever trick of the chart, more on that below.
Nautical mile converter
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Conversion chart: nautical miles to miles and km
| Nautical miles | Statute miles | Kilometres |
|---|---|---|
| 1 nm | ≈ 1.15 mi | 1.852 km |
| 2 nm | ≈ 2.30 mi | 3.704 km |
| 5 nm | ≈ 5.75 mi | 9.26 km |
| 10 nm | ≈ 11.51 mi | 18.52 km |
| 20 nm | ≈ 23.02 mi | 37.04 km |
| 50 nm | ≈ 57.54 mi | 92.6 km |
| 100 nm | ≈ 115.08 mi | 185.2 km |
| 300 nm (SKS) | ≈ 345.23 mi | 555.6 km |
| 1,000 nm (SSS, Switzerland) | ≈ 1,150.78 mi | 1,852 km |
Miles and km to nautical miles
| Distance | Statute miles to nm | Kilometres to nm |
|---|---|---|
| 1 | ≈ 0.87 nm | ≈ 0.54 nm |
| 5 | ≈ 4.34 nm | ≈ 2.70 nm |
| 10 | ≈ 8.69 nm | ≈ 5.40 nm |
| 20 | ≈ 17.38 nm | ≈ 10.80 nm |
| 50 | ≈ 43.45 nm | ≈ 27.00 nm |
| 100 | ≈ 86.90 nm | ≈ 54.00 nm |
Mental shortcuts: nautical miles to statute miles, add 15 percent (20 nm is about 23 mi). Nautical miles to km, double it and take off a little under a tenth (30 nm × 2 = 60, minus 6 gives 54 km; exactly it is 55.6 km).
Nautical mile vs mile vs kilometre
- Nautical mile (nm): 1,852 m, about 6,076 ft. Used for distances at sea and in aviation.
- Statute mile (mi): 1,609.344 m, exactly 5,280 ft. The mile on road signs in the US and UK.
- Kilometre (km): 1,000 m. Used on land almost everywhere else.
So a nautical mile is about 15 percent longer than a statute mile. When you take distances from a road map or a travel site, check which mile is meant. For licence and exam miles, it is always the nautical mile.
Nautical mile symbol: NM, nmi, nm or M?
There is no single internationally agreed symbol for the nautical mile. You will come across four:
- NM in aviation (ICAO) and much everyday English
- nmi in technical standards and US government publications
- M on many nautical charts, as used by the International Hydrographic Organization
- nm in plenty of apps, plotters and pilot books, even though it is really the symbol for nanometre
In German speaking countries you will also see sm for Seemeile, for example on sea miles certificates.
Why 1,852 metres? The nautical mile on the chart
Originally the nautical mile was the length of one minute of arc on a great circle of the Earth, a sixtieth of a degree of latitude. 360 degrees times 60 minutes gives 21,600 minutes, and the Earth's polar circumference is indeed very close to 21,600 nautical miles. Because the Earth is not a perfect sphere, a minute of latitude is not exactly the same length everywhere, so several values were in use before agreement, such as the British Admiralty mile of 6,080 feet. The United States adopted the international nautical mile in 1954, Britain in 1970.
Measuring distance with dividers
For navigation the link to the minute of latitude is still gold: on the side scale of the chart (the latitude scale), 1 minute equals exactly 1 nautical mile. Open the dividers across the distance and read it off the latitude scale at the same height. Never use the top or bottom edge, because minutes of longitude get shorter towards the poles.
From nautical miles to knots
Speed follows directly: 1 knot (kn) = 1 nautical mile per hour = 1.151 mph = 1.852 km/h. Sailing at 5 knots, you cover 5 nautical miles an hour, so 30 nautical miles to the next anchorage take about 6 hours. To convert knots to mph, km/h or m/s, use the converter and chart in knots to mph and km/h.
Over ground or through the water? GPS versus log
- Log (through the water): a small paddle wheel on the hull counts distance relative to the water. With current and tide this differs from the distance actually covered.
- GPS (over ground): satellites measure the real distance over the seabed, regardless of current. That is the distance that truly describes your trip.
For proof of sea miles, the distance actually covered counts, and GPS over ground shows it most honestly. That is why SeaMemo records your route with GPS tracking: no guessing and no estimating from the chart, but the distance sailed leg by leg. What else belongs in the log is covered in how to keep a sailing logbook.
Tacking: beating upwind you sail far more miles than the straight line to your destination. These real wake miles count, and a GPS record captures them while a straight line estimate leaves them out.
Which miles count for which licence?
- Germany (SKS, SSS): coastal or offshore waters. The IJsselmeer does not count for the SKS.
- Austria (FB2 to FB4): depending on the area, with extra conditions for sailing miles and night passages.
- Switzerland (offshore licence): everything outside inland waters, even the IJsselmeer and Markermeer.
- United Kingdom (RYA Yachtmaster): sea time within the last 10 years, at least half in tidal waters.
As a rule, lakes, rivers and canals are excluded almost everywhere: it has to be real sea miles at sea.
How many nautical miles do you sail in a day?
As a rough guide, a normal day's leg is 40 to 60 nautical miles (about 8 to 12 hours at 5 to 6 knots), roughly 45 to 70 statute miles. With a night passage or on fast yachts it is considerably more, 100 nm and upwards. For licence planning: the 300 SKS miles are doable in one intensive week, while 1,000 miles take several trips or a dedicated mile building trip.
Sources
- NIST Guide to the SI, Appendix B.8: 1 nautical mile = 1,852 m, 1 mile = 1,609.344 m
- Wikipedia: Nautical mile: 1929 Monaco definition, adoption by the US and UK, symbols NM, nmi and M
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Frequently asked questions
How long is a nautical mile in km?
A nautical mile is exactly 1.852 kilometres, or 1,852 metres. This value has applied internationally since 1929 and is also set out in the ISO 80000-3 standard.
How many miles is a nautical mile?
One nautical mile is about 1.151 statute miles, or 6,076 feet. One statute mile is about 0.869 nautical miles.
What is the difference between a nautical mile and a mile?
A nautical mile (1,852 m) is based on one minute of latitude and is used at sea and in aviation. A statute mile (1,609.344 m) is the mile on road signs in the US and UK. The nautical mile is about 15 percent longer.
What is the symbol for nautical mile?
There is no single internationally agreed symbol. NM is used in aviation, nmi in technical standards and US government texts, and M on many nautical charts. The abbreviation nm is common too, although it is really the symbol for nanometre.
Why is a nautical mile 1,852 metres?
It goes back to one minute of latitude, a sixtieth of a degree. Because the Earth is not a perfect sphere, that length varies slightly, so a fixed value of 1,852 metres was agreed in 1929.
Does distance over ground or through the water count?
What counts is the distance actually covered. GPS measures distance over ground regardless of current, so a GPS record is the most reliable basis.