The answer to how many km in a nautical mile is exactly 1.852, so one nautical mile is 1,852 meters. That number is a definition, not a measurement. It was agreed at an international hydrographic conference in Monaco in 1929. A statute mile, the everyday mile on land, is 1,609.344 meters, which makes a nautical mile about 15 percent longer: 1 nautical mile is 1.15078 statute miles.
The unit started as the length of one minute of latitude along a meridian, roughly 1/21,600 of the Earth's circumference. Because the Earth is not a sphere, that length changes with latitude, so the modern unit is a fixed value. The sections below cover where 1,852 comes from, how much the real minute varies, when the US and UK adopted it, and what a knot is.
How many km in a nautical mile?
How long is a nautical mile? One nautical mile is 1.852 km, 1,852 m and about 6,076.115 feet. That is also the answer to how many feet in a nautical mile: 1,852 ÷ 0.3048 = 6,076.115. For readers who ask how many nautical miles to miles, the answer is 1 nautical mile = 1.15078 statute miles, and 1 statute mile = 0.86898 nautical mile. Two formulas cover both directions:
- Nautical miles to km: km = nautical miles × 1.852
- Km to nautical miles: nautical miles = km ÷ 1.852
For 50 nautical miles the step is 50 × 1.852 = 92.6 km. In the other direction, 100 km ÷ 1.852 = 53.9957 nautical miles, or about 54. There is no internationally agreed unit symbol for the nautical mile, as the BIPM SI Brochure states. Aviation documents write NM, and "nm" is also the symbol for nanometer, so spelling out "nautical miles" next to a number avoids confusion.
Nautical mile vs statute mile: why they differ
Why is a nautical mile different than a regular mile? The two were defined for different jobs. The statute mile is a fixed land distance, while the nautical mile was tied to the size of the Earth. Today both are exact in meters, and they differ by 242.656 meters (1,852 − 1,609.344).
The statute mile is 5,280 feet
A statute mile is 5,280 feet. A foot is exactly 0.3048 meter, so 5,280 × 0.3048 = 1,609.344 m. NIST Handbook 44 lists 1 mile as exactly 1.609344 km. The 1959 agreement that fixed the foot and the yard is covered in our post on why a pound is exactly 0.45359237 kg.
One minute of latitude
A circle around the Earth is divided into 360 degrees, and each degree into 60 minutes. One minute is therefore 1/(360 × 60) = 1/21,600 of the circumference. Walk north or south along a meridian and one minute of latitude covers about 1.85 km of ground. The nautical mile was built on that length.
The practical benefit is that distance and position share a scale. Because one minute of latitude is about one nautical mile, the latitude scale on a chart doubles as a distance scale. Marine Insight gives the link to degrees of latitude as the reason sailors use nautical miles and knots.
Why a nautical mile is exactly 1,852 meters
Because the 1929 conference fixed it as a convention. The First International Extraordinary Hydrographic Conference in Monaco adopted the "international nautical mile" of 1,852 meters. NIST Handbook 44 credits that conference, and the BIPM brochure lists 1 nautical mile = 1852 m and says its size was defined by the 1929 conference.
Neither source explains how the number was chosen, but you can see why it works. On the GRS 80 reference ellipsoid, the full meridian circumference is 40,007.863 km. Divided by 21,600 that gives 1,852.216 m. We calculated this ourselves from the ellipsoid values in the EPSG registry. The 1929 figure is only about 22 centimeters short of the average minute of latitude.
A minute of latitude is not the same everywhere
The Earth is slightly flattened at the poles, so a minute of latitude gets longer toward the poles. The table below shows the effect on the GRS 80 ellipsoid with an equatorial radius of 6,378,137 m and an inverse flattening of 298.257222101. Each row is the length of a one-minute meridian arc centered on the stated latitude, found by numerical integration. The last row uses the final minute at the pole.
| Latitude | One minute of latitude (m) | Difference from 1,852 m (m) |
|---|---|---|
| 0° (equator) | 1,842.90 | −9.10 |
| 30° | 1,847.54 | −4.46 |
| 45° | 1,852.20 | +0.20 |
| 60° | 1,856.87 | +4.87 |
| 90° (pole) | 1,861.57 | +9.57 |
From equator to pole the minute grows by 18.67 meters, about 1 percent. The fixed 1,852 m is never more than about 0.5 percent away from the real minute, so counting one minute on a chart as one nautical mile is a close approximation, not an identity. The table reflects the ellipsoid model only. It leaves out local gravity variations and map projection effects.
A minute of longitude is not a nautical mile
The same rule does not apply to longitude. At the equator one minute of longitude is 1,855.3 m, and it shrinks as meridians converge toward the poles. The same calculation gives 1,421.3 m at 40° latitude and 927.7 m at 60°. That is why distances on a chart are read from the latitude scale, not from the longitude scale.
When the US and UK switched to 1,852 meters
The United States switched on July 1, 1954. According to NIST Handbook 44, the 1,852 m value was adopted in the US by identical directives of the Departments of Commerce and Defense, effective July 1, 1954. Before that the US used 6,080.20 feet for the nautical mile.
The United Kingdom used the Admiralty mile of 6,080 feet. The UK Metric Association timeline says the UK Hydrographic Office adopted the 1,852 m international nautical mile in September 1970. That date comes from a secondary source and was not confirmed in an official document. British law still knows the old unit: the Units of Measurement Regulations 1995 list a "nautical mile (UK)" with a metric equivalent of 1,853 meters.
The table compares the three definitions. Feet were converted to meters with the international foot (0.3048 m). The historical difference between the US foot and the international foot amounts to a few millimeters here and does not change the result.
| Definition | Feet | Meters | Difference from 1,852 m |
|---|---|---|---|
| International nautical mile (1929) | 6,076.115 | 1,852.000 | 0 |
| US nautical mile (before 1954) | 6,080.20 | 1,853.245 | +1.245 m (0.067%) |
| British Admiralty mile | 6,080 | 1,853.184 | +1.184 m (0.064%) |
The old values were about 1.2 meters longer. Over a 1,000 nautical mile voyage that adds up to a little more than 1.2 km.
What is a knot, and how fast is it in km/h and mph?
A knot is a speed of one nautical mile per hour, which is exactly 1.852 km/h. The BIPM brochure writes it as 1 nautical mile per hour = (1852/3600) m/s, about 0.5144 m/s. In US customary units 1 knot is 1.15078 mph, so knots to km/h is one multiplication by 1.852 and knots to mph one by 1.15078. "Knots per hour" is a common slip, because the hour is already part of the definition. The name comes from the old knotted line used to measure a ship's speed (Webtekno, in Turkish).
The table converts common speeds using 1 knot = 1.852 km/h. The mph column uses the 1,609.344 m statute mile.
| Speed (knots) | km/h | mph | m/s |
|---|---|---|---|
| 5 | 9.26 | 5.75 | 2.57 |
| 10 | 18.52 | 11.51 | 5.14 |
| 15 | 27.78 | 17.26 | 7.72 |
| 20 | 37.04 | 23.02 | 10.29 |
| 30 | 55.56 | 34.52 | 15.43 |
| 40 | 74.08 | 46.03 | 20.58 |
So 10 knots is 18.52 km/h and 30 knots is 55.56 km/h, or 34.52 mph. These are exact conversions. Some aviation documents use a shortcut for wind: ICAO annexes use 1 kt = 0.5 m/s when representing wind speed, as the Philippine CAAP adoption of Annex 5 notes. That is about 2.8 percent below the true 0.5144 m/s.
Why ships and aircraft use nautical miles and knots
Ships use knots instead of mph, and aircraft navigation uses nautical miles for distance, because the units keep distance, speed and position on one scale. The BIPM brochure says the nautical mile and the knot are used in marine and aerial navigation for distance and speed, and it lists both among the non-SI units accepted for use.
First, positions are given in latitude and longitude, and a nautical mile stays close to one minute of latitude, so a distance can be read against the coordinates. Second, a knot is tied directly to the nautical mile, so the formula distance = speed × time needs no conversion factor: 120 nautical miles, 20 knots and 6 hours fit together as they are. Third, regulations fix them. The International Maritime Organization's Resolution A.824(19) sets performance standards for ship speed and distance indicators in knots and nautical miles. The Philippine adoption of ICAO Annex 5 states that no termination date has been set for the nautical mile and the knot in aviation.
Nautical mile, mile and knot: names and symbols
A nautical mile and a "regular" mile are different lengths, and a knot is a speed, not a distance. When a source just says "mile" without more context, it can mean either the 1,609.344 m statute mile or the nautical mile, and aviation writes it SM against NM for the nautical mile (Fly Away Simulation explains the split). Legal texts avoid the ambiguity. UN treaty and US proclamation language below both say "nautical miles" in full.
Nautical miles to km and miles conversion chart
Converting takes one multiplication. The first table converts nautical miles to km by multiplying by 1.852 and to statute miles by multiplying by 1.852 ÷ 1.609344.
| Nautical miles | Kilometers | Statute miles |
|---|---|---|
| 1 | 1.852 | 1.15 |
| 5 | 9.26 | 5.75 |
| 10 | 18.52 | 11.51 |
| 12 | 22.224 | 13.81 |
| 20 | 37.04 | 23.02 |
| 50 | 92.6 | 57.54 |
| 100 | 185.2 | 115.08 |
| 120 | 222.24 | 138.09 |
| 200 | 370.4 | 230.16 |
| 500 | 926 | 575.39 |
Going the other way, divide kilometers by 1.852. The second table shows the result to two decimals.
| Kilometers | Nautical miles |
|---|---|
| 1 | 0.54 |
| 10 | 5.40 |
| 20 | 10.80 |
| 50 | 27.00 |
| 100 | 54.00 |
| 200 | 107.99 |
| 500 | 269.98 |
| 1,000 | 539.96 |
To convert between kilometers and statute miles, the length converter works. Its unit list has no nautical mile, so apply the 1.852 factor above first.
Worked example: a 120 nautical mile trip at 20 knots
Take a hypothetical ferry route of 120 nautical miles at a steady 20 knots. There are two ways to find the time:
- Convert the distance: 120 × 1.852 = 222.24 km.
- Convert the speed: 20 × 1.852 = 37.04 km/h.
- Divide: time = distance ÷ speed = 222.24 ÷ 37.04 = 6 hours.
Because both quantities use the same unit, 120 ÷ 20 = 6 hours works directly with no conversion. At 25 knots the trip takes 120 ÷ 25 = 4.8 hours, or 4 hours 48 minutes, and at 15 knots it takes 8 hours.
Aviation uses the same logic. As a hypothetical, a 540 NM flight at a ground speed of 450 knots takes 540 ÷ 450 = 1.2 hours, or 72 minutes, ignoring wind, takeoff and landing. To start from a map, take the straight-line distance in km from the distance calculator and divide by 1.852. Real routes are longer than a straight line, so the result is a lower bound.