What each Speed unit is
Metre per second m/s
Metres per second is the SI unit and the one physics is done in, which is why it turns up in wind readings and in almost nothing a driver would look at.
Kilometre per hour km/h
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.
Mile per hour mph
Miles per hour is the road speed of the United States and the United Kingdom, which is a rare pairing — the UK uses miles for road speed and metric for nearly everything else.
Knot kn
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.
Notes on the common Speed conversions
kmh to knots
Road intuition is the wrong instrument for water
A speed of 30 km/h is nothing on a road — it is a residential limit, slower than a competent cyclist. At sea it is 16.2 knots of wind, which is the upper end of a fresh breeze and enough that a cruising yacht is thinking about reducing sail. That mismatch is the real content of this conversion: the arithmetic is easy and the recalibration is not.
The same applies to vessel speeds from the other end. A boat doing 20 km/h is doing 10.8 knots, which is fast for a displacement hull and unremarkable for anything planing. A tidal stream of 5 km/h is 2.7 knots and will dominate the passage of a small boat entirely. Nothing about the road-speed number signals how significant these are, because roads are two orders of magnitude more forgiving of speed than water is.
kmh to meters per second
Nothing is rounded on the way to 3.6
The divisor comes from two definitions and nothing else. A kilometre is 1,000 metres by the definition of the prefix, and an hour is 3,600 seconds by the definition of the hour, so a speed in kilometres per hour is 1,000/3,600 of the same speed in metres per second. That fraction reduces to 1/3.6, and both of its inputs are stipulated rather than measured.
This is unusual and worth noticing. Most conversions on this site carry a factor with a long tail of digits, and the honest question is where to stop. Here there is no tail: the only decimals in your answer are the ones your own input put there. 90 km/h is 25 m/s exactly, 36 km/h is 10 m/s exactly, and anything not divisible by 3.6 gives a recurring decimal rather than an approximation.
kmh to mph
The sign values, and what they feel like in mph
Five numbers cover most of continental Europe. 50 km/h is the default in built-up areas and comes out at 31 mph; 80 or 90 km/h on an ordinary rural road is 50 or 56 mph; 110 or 120 km/h on a dual carriageway is 68 or 75 mph; and 130 km/h, the motorway figure in France, Italy and much of eastern Europe, is 81 mph. None of those is far from a limit a British or American driver already knows, which is why the unit change is a smaller adjustment than it looks.
The one that catches people is the low end. A 30 km/h zone — increasingly the default in city centres — is 19 mph, not 30, and reading it as a familiar number puts you at roughly half again the posted speed. The same trap sits at 20 km/h in a residential or shared-space zone, which is 12 mph and slower than most people cycle.
knots to kmh
The air runs on knots and feet, wherever it is flying
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.
knots to meters per second
A fraction that is exact and a decimal that never ends
The conversion is 1852 over 3600, and that fraction is exact — both numbers are definitions rather than measurements. Reduced it is 463/900, and 900 carries a factor of 9 that decimal notation cannot resolve, so the decimal form is 0.514444 with the 4 repeating forever. Every published value of this factor is therefore a rounding, including the one on this page.
That is a different situation from most conversions here, where the factor terminates and the only question is how many digits to copy. Here there is no last digit to copy. Carrying 0.5144 is accurate to under a part in ten thousand and is more than any real calculation needs; carrying the fraction itself through a spreadsheet is better still, because it removes the rounding from the chain entirely rather than making it small.
knots to mph
Fifteen per cent, and it is almost exactly fifteen
The factor is 1.150779, which means adding fifteen per cent is right to seven parts in ten thousand. Twenty knots becomes 23 mph, forty becomes 46, sixty becomes 69, a hundred becomes 115. At a hundred knots the shortcut is off by seven hundredths of a mile an hour, which is smaller than the rounding in any bulletin that will ever quote it.
That makes this one of the few conversions on the site where the mental version and the exact version are interchangeable for practical purposes. There is no reason to reach for a calculator to read a marine forecast, and no benefit in carrying more digits than the forecast itself does — wind speeds are issued to the nearest knot at best and usually in five-knot bands.