Convert km to nmi kilometres ↔ nautical miles Free Online Secure
Type a value in kilometres and read it in nautical miles at once. The factor is exact by definition, the arithmetic happens in your browser, and nothing is sent anywhere.
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Common values — Convert km to nmi (kilometres ↔ nautical miles)
| km | nmi |
|---|---|
| 1 | 0.539956803456 |
| 2 | 1.07991360691 |
| 3 | 1.61987041037 |
| 5 | 2.69978401728 |
| 10 | 5.39956803456 |
| 20 | 10.7991360691 |
| 50 | 26.9978401728 |
| 100 | 53.9956803456 |
Where these units come from
The metre was first defined in 1793 as a ten-millionth of the distance from the equator to the North Pole — a definition nobody could check without surveying a quarter of the planet. It has been redefined four times since, each time to something steadier: a platinum bar, then a prototype kept in Paris, then a wavelength of krypton light, and since 1983 the distance light travels in vacuum in 1/299,792,458 of a second. The inch stopped being a separate thing in 1959, when six countries agreed to define it as exactly 25.4 mm. That agreement is why every conversion on this page between a metric and an imperial length is exact rather than measured: both sides now descend from the same definition.
Where you meet them
Millimetres are the working unit of engineering drawings and of anything made in a workshop; centimetres belong to tailoring, furniture and school rulers; metres to rooms, fabric and running tracks. Feet and inches survive in aviation altitudes worldwide, in American and British construction, and in the diagonal of every screen you have ever bought. Nautical miles are not a quaint survival — one is a minute of latitude, which is why ships and aircraft still navigate in them.
Questions people ask
Which countries still measure road distances in miles?
The United States and the United Kingdom, where road signs show miles, together with a handful of smaller countries such as Liberia and Myanmar. Almost everywhere else road distances are in kilometres. One kilometre is 1000 metres and about 0.621 of a mile, so a distance in miles is about five-eighths of the same distance in kilometres.
Why is a nautical mile 1852 metres?
Because it was meant to equal one minute of latitude, which averages about 1852 metres along a meridian. An international conference in 1929 fixed the figure at exactly 1852 metres, and one knot is one nautical mile per hour. That link to the globe is why ships and aircraft still navigate in them.
Is the answer exact?
Where both units are defined in terms of one another by whole numbers, yes — the page says so above the figure, and the only rounding is in how many digits are shown. Where the ratio has no finite decimal, the page shows twelve significant digits and says the relation is approximate rather than pretending otherwise.
Does anything I type leave my device?
No. The arithmetic runs in your browser; nothing is uploaded and there is no server to upload it to. Load the page once and it keeps working with the network switched off.
From kilometres on the road to nautical miles at sea: how to convert km to nmi
On land everything is measured in kilometres; on the water and in the air the nautical mile rules. A harbour master gives the distance to the next port in nautical miles, a pilot files a flight plan in nautical miles, and a ferry timetable may show kilometres on one page and nautical miles on the next. Anyone who moves between the two worlds needs to convert km to nmi, and this page does it the moment you type the number.
One nautical mile is exactly 1.852 kilometres, a figure fixed by international agreement in 1929. To go from kilometres to nautical miles you therefore divide by 1.852, or multiply by about 0.54. Below you will find the rule, worked examples, an explanation of why the nautical mile exists at all, a comparison with the kilometre, the situations in which the conversion helps and the errors to avoid.
How to convert km to nmi: divide by 1.852
The rule
Since 1 nmi equals 1.852 km, the number of nautical miles is the number of kilometres divided by 1.852. 1.852 km is 1 nmi, 10 km is 5.4 nmi, 50 km is 27 nmi, 100 km is 54 nmi and 370.4 km is 200 nmi. Multiplying by 0.539957 gives the same answer. To go in the opposite direction, multiply the nautical miles by 1.852.
Worked examples
A harbour 25 km away is 13.5 nmi. A coastline stretch of 140 km is 75.6 nmi. A flight leg of 900 km is 486 nmi. A fishing ground 40 km offshore is 21.6 nmi. In every case the result is a little more than half of the kilometre figure.
The mental shortcut
Take a little more than half: multiply by 0.5, then add 8 per cent of the result. For 100 km: 50 plus 4 is 54 nmi, exactly right. For 30 km: 15 plus 1.2 is 16.2 nmi against the exact 16.2.
Precision
Give the nautical-mile figure to the same number of significant digits as the kilometre figure. A distance of 20 km is 10.8 nmi, not 10.799568. Navigators usually round to the nearest tenth of a mile.
The kilometre
Land distance
The kilometre, 1,000 metres, is the standard unit for road distances, running events and mapping in most countries. It is a decimal unit that fits neatly into the metric system, which is why scientists and engineers prefer it.
Why it does not fit the sea
At sea you cannot read a distance from a milestone. A navigator relies on the latitude scale of the chart, and the kilometre has no connection to that scale. That is what the nautical mile provides.
The nautical mile
One minute of latitude
To understand the “convert km to nmi” calculation step, note that the nautical mile was originally the length of one minute of arc along a meridian. Because there are 60 minutes in a degree and 360 degrees around the Earth, the circumference is 21,600 nautical miles, and each mile corresponds to a fixed step on the chart's latitude scale.
The exact value
Because the Earth is not a perfect sphere, the length of a minute of latitude varies slightly, from 1,843 m at the equator to 1,862 m near the poles. In 1929 the International Hydrographic Conference fixed the nautical mile at exactly 1,852 m, and the figure has been in use ever since. The symbol is nmi, M or NM.
Kilometre and nautical mile compared
A nautical mile is 1.852 kilometres, so the number of nautical miles is about 54 per cent of the number of kilometres. A distance of 1,000 km is 540 nmi. The nautical mile is longer than the statute mile, 1.609 km, by about 15 per cent.
Use kilometres for roads and nautical miles for sea and air. In texts with both audiences, give the primary unit first and the other in brackets, for example 100 nmi (185 km). Never use the abbreviation mi for a nautical mile, as it normally means a statute mile.
Where you will need it
Planning a sea passage
A sailing yacht averages 5 knots, which is 5 nautical miles per hour or 9.26 km/h. A passage of 150 km is 81 nmi and takes 16 hours. A skipper with a map in kilometres has to convert km to nmi before computing the time, because speeds on board are always in knots.
Coastal waters and borders
Territorial waters extend 12 nautical miles from the coast, which is 22.2 km. The contiguous zone reaches 24 nmi, 44.4 km, and the exclusive economic zone 200 nmi, 370.4 km. News about fishing rights or maritime boundaries quotes these figures in nautical miles, while maps may show kilometres.
Ferries and cruises
A ferry crossing of 240 km is 129.6 nmi. At 20 knots, the voyage lasts 6.5 hours. Timetables sometimes show the distance in kilometres for passengers and in nautical miles for the crew.
Offshore industry
Wind farms and oil platforms are located by distance from shore. A wind farm 45 km offshore is 24.3 nmi out, beyond the territorial sea. Cable lengths for the connection are counted in kilometres, but permits refer to nautical miles.
Latitude and the length of a mile
A degree of latitude
One degree of latitude is 60 nautical miles, 111.1 km. A navigator who moves from 44 degrees north to 45 degrees north has travelled 60 nmi due north. This simple rule makes a “convert km to nmi” calculation estimate on a chart easy: measure the distance against the latitude scale at the side of the chart and read off the miles.
From kilometres to latitude
To find how many degrees of latitude correspond to 250 km, convert to nautical miles, 135 nmi, then divide by 60 to get 2.25 degrees. This is a typical way to convert km to nmi before working with a chart.
Weather and sea state
Distance to a storm
A storm 55 km away is 29.7 nmi. At 25 knots it reaches a boat in about 1.2 hours. Meteorological bulletins give storm positions in nautical miles from a coast station, so a skipper who counts in kilometres should convert.
Waves and currents
Currents are measured in knots. A current of 2 knots carries a boat 2 nmi, 3.7 km, in an hour. Over a 12-hour tide the drift is 24 nmi, 44.4 km.
History in brief
Navigators and mapmakers
The idea of linking distance to latitude is old, but it was Mercator's 1569 map and later the work of English and French navigators that made nautical miles standard. Before 1929, countries used slightly different nautical miles. The British Admiralty mile was 1,853.18 m, the American one 1,853.25 m.
The 1929 agreement
The conference in Monaco settled on 1,852 m, the average length of a minute of latitude at 45 degrees. The United States adopted it in 1954 and the United Kingdom in 1970.
Common mistakes
Using the statute mile
Dividing kilometres by 1.609 gives statute miles, not nautical miles. The difference is 15 per cent: 100 km is 62.1 mi but 54.0 nmi.
Mixing units
Do not add nautical miles to kilometres in a route plan. Convert all to one unit first. A route with legs of 12 nmi and 30 km is 12 plus 16.2, which is 28.2 nmi.
A worked voyage from start to finish
The plan
A small cruising boat plans a passage of 185 km along a coast, with a stop halfway. The skipper begins with a “convert km to nmi” calculation step: 185 divided by 1.852 is 99.9, so 100 nmi, and the halfway stop is at 50 nmi. At a planned cruising speed of 6 knots the whole trip needs 16.7 hours, and each half 8.3 hours. With a 20 per cent allowance for weather, the plan becomes 20 hours, so a dawn start and an evening arrival.
Fuel and reserves
The engine burns 3 litres per hour at 6 knots, so the 20-hour plan needs 60 litres. The skipper carries 100 litres. The reserve of 40 litres is 67 per cent of the planned requirement, a typical margin for coastal work. The same sums in kilometres would use the speed 11.1 km/h, an awkward number that nobody uses on board.
Reading a chart with two scales
The latitude scale and the distance scale
A chart has a latitude scale in its margin and, often, a separate distance scale in kilometres or miles. The latitude scale is the one used for measuring, because a minute of latitude is a nautical mile. A kilometre scale on the same chart is a convenience for visitors. If you must convert km to nmi on a chart, use the latitude scale as your authority and keep the kilometre figure for communicating with people ashore.
Why the scale changes
On a Mercator chart the scale grows toward the poles, so a nautical mile occupies more paper at 60 degrees north than at the equator. That is why you measure on the scale next to your position, not at the bottom of the chart. A kilometre scale printed at one latitude would be wrong elsewhere.
Plotting a course
Dividers and scales
On a paper chart the navigator sets a pair of dividers to the latitude scale, marks off a step of 1 nmi, and walks it along the course line. If the course is 26 km long, it takes 14 steps. The classic method has largely been replaced by electronics but is still taught, because it works without power.
Electronic charts
Plotters show distance in the unit you select. Setting it to kilometres on a boat is possible, and then no convert km to nmi is needed on screen, but all tidal and current information is in nautical miles and knots, so most skippers keep the display in nautical miles.
Speeds at sea
The knot
A knot is one nautical mile per hour, 1.852 km/h. 20 knots is 37 km/h. A fast ferry at 35 knots makes 64.8 km/h. Container ships cruise at about 20 knots, sailboats at 4 to 8, and a rowing boat at 2.
Not knots per hour
Saying knots per hour is a common mistake, because the knot already contains the hour. A ship that speeds up by 2 knots in an hour has an acceleration of 2 knots per hour, which is a different quantity.
Rescue and safety
Distances in emergencies
Maritime radio calls give positions as bearing and distance in nautical miles from a known point. A vessel reporting a fire 12 miles south-west of a headland is 22.2 km away. Rescue helicopters have a range of about 150 nmi, 278 km, so a call at 200 km is within reach.
Visibility
Weather bulletins for the sea give visibility in nautical miles. Good visibility is more than 5 nmi, 9.3 km; moderate is 2 to 5 nmi, 3.7 to 9.3 km; poor is below 2 nmi.
Fishing and nets
Fishing grounds
A fishing boat that travels 60 km to the grounds goes 32.4 nmi. A trawler sets nets for 6 hours at 3 knots, covering 18 nmi, 33.3 km. Quotas and zones are defined in nautical miles from shore, so fishermen convert land-based maps to the units of their licences.
Fuel
Fuel consumption at sea is given per nautical mile or per hour. A boat that burns 40 litres per hour at 10 knots uses 4 litres per nautical mile, 2.16 litres per kilometre. The conversion shows how the cost compares with a car.
Spreadsheet formulas
Formula
To convert km to nmi for a column, put kilometres in column A and enter =A2/1.852 in column B. Format with one decimal. To convert a column of speeds in km/h to knots, use the same formula. For the reverse, multiply by 1.852.
Checking
Test with 1.852: you should get exactly 1. With 185.2 you should get 100. If your sheet shows 54 for 100 km, it is applying 0.54 and rounding, which is fine for estimates but not for charts.
Rounding rules
Navigation precision
Position fixes at sea are accurate to some tens of metres with GPS. Distances to the next waypoint are shown to a tenth of a nautical mile, 185 m. That is the precision to use. A harbour approach may need hundredths, 18.5 m.
Safety margins
When estimating fuel, round distances up. If the journey is 143 km, 77.2 nmi, plan for 80 nmi. If the margin of safety is important, add 10 per cent.
Earth geometry
The Earth in nautical miles
The circumference of the Earth is 21,600 nmi, 40,003 km. The distance from the equator to a pole is 5,400 nmi. A great circle route between two points is the shortest path, and a minute of arc along it is exactly a nautical mile by definition.
Great-circle distances
An air route from Bucharest to New York is about 4,300 nmi or 7,900 km. A pilot reading the distance in kilometres from a timetable converts it to nautical miles to compute the fuel.
Teaching the nautical mile
A globe in the classroom
Take a globe and a piece of string. Measure along a meridian between 40 and 50 degrees north. The string is 10 degrees long, which is 600 nmi, 1,111 km. Students see the link between angle and distance directly.
Estimation
Ask pupils to guess the nautical-mile distance between two cities and then check with the page. The usual error is to forget the 8 per cent correction.
More values for convert km to nmi
A longer list of conversions
If the value you need is not in the first list, these further conversions may be close enough to read off directly:
- 0.1 km = 0.0539957 nmi
- 0.5 km = 0.269978 nmi
- 3 km = 1.61987 nmi
- 4 km = 2.15983 nmi
- 7 km = 3.7797 nmi
- 15 km = 8.09935 nmi
- 20 km = 10.7991 nmi
- 75 km = 40.4968 nmi
- 200 km = 107.991 nmi
- 500 km = 269.978 nmi
- 2,000 km = 1,079.91 nmi
- 10,000 km = 5,399.57 nmi
Mistakes when you convert km to nmi
Multiplying instead of dividing
A nautical mile is longer than a kilometre, so the number of nautical miles is smaller. If 100 km becomes 185, you multiplied.
Forgetting that nautical is not statute
Use nmi, not mi. The symbols carry different values.
Rounding the factor
Using 0.5 instead of 0.54 gives an error of 8 per cent. Keep four digits at least.
Quick answers
How many kilometres in a nautical mile?
Exactly 1.852.
How do I convert km to nmi?
Divide the kilometres by 1.852.
What is 100 km in nautical miles?
About 54 nmi.
Is the conversion exact?
Yes, the nautical mile is defined as exactly 1,852 m.
Does my number leave my device?
No. The calculation runs in your browser.
Related pages and a last word
If you work with lengths, these conversions are close at hand: Convert km to mi, Convert km to ly, Convert mi to nmi, Convert nmi to mi, Convert °C to °F, Convert °F to °C, Convert L to gal, Convert gal to L, Convert tbsp to tsp. The reverse of this page is Convert nmi to km.
In short, to convert km to nmi divide by 1.852, remember that a nautical mile is longer than a kilometre, and keep only the digits your measurement supports. Your number stays on your device: nothing is uploaded and no account is needed.