Convert kn to km/h

A knot is one nautical mile per hour, and a nautical mile is exactly 1,852 metres — originally one minute of latitude, so a knot relates speed directly to position on a chart. Its name comes from counting knots in a rope paid out behind a ship.

Kilometres per hour is the speed limit almost everywhere in the world. Dividing by 3.6 gives metres per second, because there are 3,600 seconds in an hour and 1,000 metres in a kilometre.

The factor is 1.852, and almost nobody carries that around. Rounded to 1.85 it is off by 0.11 % — which stays invisible on small numbers and turns into a whole unit somewhere around 1,000 kn.

That is the number worth knowing before you round: not the error itself, but where it stops being ignorable. Below that point the shorter factor is the sensible one; above it, use the field above, which never rounds until it prints.

A knot is one nautical mile per hour, and the nautical mile is exactly 1,852 metres — fixed by definition in 1929, not measured. It was chosen to be one minute of latitude, which is what makes it useful at sea: sixty nautical miles is one degree up the chart, on any chart.

So the conversion to kilometre per hour is exact rather than approximate, but the round number is on the nautical side. Expect the metric figure to be the untidy one here, and note that the knot is already a speed — "knots per hour" is a quantity nobody means.

Aviation is the one large field where a single pair of units is used almost everywhere on earth regardless of what the country underneath does. Airspeeds and wind are in knots; altitudes and flight levels are in feet. A German, Brazilian or Japanese airliner uses both, in a country where every road sign, fuel pump and weather forecast is metric.

The reason is standardisation rather than tradition alone: an international air traffic system cannot have aircraft at the same place using different vertical references, and the world converged on feet. The notable exceptions are on the altitude side rather than the speed side, with China and North Korea using metres for flight levels — which is why an aircraft crossing into that airspace performs a level change to a metric equivalent and back again on the way out. Russia was the third until November 2011, when it moved to feet.

A flight tracker shows groundspeed: how fast the aircraft's position over the earth is changing, usually converted to km/h for a general audience. The crew is looking at indicated airspeed, which is essentially a measurement of the air pressure the aircraft is pushing into and is what the aeroplane's handling and structural limits are stated against. Between them sits true airspeed, the actual speed through the air mass.

The gap is enormous at altitude. Air at 11,000 metres is about three tenths of sea-level density, so an aircraft indicating around 280 knots is doing roughly 500 knots true — and if it has 100 knots of jet stream behind it, the tracker shows 600 knots, or 1,111 km/h. Converting the tracker figure gives you a real number about the ground and tells you almost nothing about what the aircraft is doing, which is worth knowing before comparing two flights on the same route in opposite directions.

The knot figures for shipping are small and the metric translations make that concrete. A large container ship cruises at 16 to 22 knots, which is 30 to 41 km/h. A ferry at 25 knots is doing 46. A bulk carrier at 12 knots is doing 22, slower than a cyclist on a good day.

At 20 knots a vessel covers about 890 kilometres in a day, which is the number that explains sea freight schedules better than any speed figure does. Slow steaming — deliberately reducing speed to cut fuel consumption, because resistance rises steeply with speed — takes a ship down to around 16 knots, or 30 km/h, and adds days to a crossing in exchange for a large fuel saving.

European marine bulletins are issued in knots and the news coverage of the same weather is in kilometres per hour, which is why a storm can appear to have two wind speeds in the same day's reporting. The thresholds are the giveaway: a gale warning at 34 knots is 63 km/h, a storm warning at 48 knots is 89 km/h, and hurricane-force at 64 knots is 118.5 km/h.

None of the metric numbers is round, and all of the knot numbers are, which settles the question of which set came first. It also gives a metric reader a useful rule for reading marine coverage: a wind described in a round number of kilometres per hour has been converted and rounded by a journalist, while one described in a round number of knots came off the bulletin.

Doubling is the reflex and it is 8 per cent high — 400 knots becomes 800 against a true 741, which is a 59 km/h error and far too much. Taking a twelfth off the doubled figure gets to 1.833, which is 1 per cent low; taking off eight per cent gets to 1.84. The cleanest version to remember is to multiply by 1.85, which is 0.1 per cent low and can be done as double minus 7.5 per cent.

For anything you are only trying to picture, doubling and remembering that the answer is high is enough: 20 knots is under 40 km/h, 500 knots is under 1,000. The precision only starts to matter when the figure is being compared against a threshold, and marine thresholds sit at 34, 48 and 64 knots, where an eight per cent error is five kilometres an hour and can move a forecast across a line.

Because a knot is one nautical mile per hour, a speed in knots and a distance in nautical miles divide into hours with no factor at all — and both convert to metric with the same 1.852. A three-hour ferry crossing at 25 knots covers 75 nautical miles, which is 139 kilometres.

That is the practical reason the unit survives in an otherwise metric world, and it is why converting only the speed leaves the arithmetic half done. If you are checking a published journey time against a distance you looked up in kilometres, convert the speed to km/h and use the metric distance throughout, or stay in knots and nautical miles throughout. Mixing the two produces an answer that is out by exactly 1.852 and looks plausible.

It is easy to assume that a fully metric country converts aviation to its own units internally and translates only for foreign traffic. It does not. A pilot in Germany, Brazil or Japan files a flight plan in knots, reads an airspeed indicator in knots, is assigned levels in feet and reports wind in knots, in an aircraft fuelled in litres or kilograms and maintained in millimetres.

The reason is that air traffic is a single system with aircraft from everywhere in the same airspace, and interoperability outranks national convention. Metric alternatives are permitted in international standards and are used in the few places noted above, but the practical default is knots and feet. So a kilometres-per-hour figure in a news report about a flight has almost always been converted by the journalist rather than taken from any source the crew or the controller saw.

The kilometres-per-hour figure is always larger, by a bit under double. An answer smaller than the knot value you typed means the conversion ran backwards, and the result understates the speed by nearly half.

The other check is on what the source meant. A speed in knots from an aviation source may be indicated, true or over the ground, and those three can differ by a factor of two on the same aircraft at the same moment. The conversion is exact and the ambiguity is entirely in the number you were given, so it is worth knowing which one a tracker, a report or a press release is quoting before the metric figure gets repeated.