Convert speed

About the units

• Metres per second (m/s) – The metre per second is the SI coherent unit of speed, formed directly from the metre and the second, and the unit every other speed measure is converted through. It is a legal unit across the EU and the obvious one in physics and engineering. Ten metres per second is 36 kilometres per hour, and one metre per second is 2.24 miles per hour. The factor against kilometres per hour is 3.6 and is often dropped. At sea and in the air, speed is counted in knots.
• Kilometres per hour (km/h) – The kilometre per hour is built from two legal units and is itself legal throughout the EU. One kilometre per hour is one 3.6th of a metre per second. It is the unit on speedometers and speed limits across the union, and Swedish traffic rules set the limits in kilometres per hour, 50 in built-up areas and 70 outside them. A hundred kilometres per hour is just under 28 metres per second and 62 miles per hour. Write km/h, not “kmh” or “kph”.
• Miles per hour (mph) – A mile per hour is exactly 0.44704 metres per second, that is 1.609344 kilometres per hour. It is the unit on speedometers and road signs in the US and the UK, and EU law makes an explicit exception for British and Irish speed limits. American weather reports give wind in mph as well. Seventy miles per hour is 112.7 kilometres per hour. Two factors are easy to mix up: a mile per hour is 1.609 kilometres per hour, while a knot is 1.151 miles per hour.
• Knots (kn) – A knot is one nautical mile per hour, and the nautical mile has been fixed at 1,852 metres since 1929. That makes a knot exactly 1.852 kilometres per hour, or 0.514 metres per second. Knots are used in shipping and aviation, and European air traffic rules set a limit of 250 knots below 10,000 feet. The unit is absent from the EU units directive, and there is no internationally agreed symbol for it. “Knots per hour” is wrong: a knot is already a speed.
• Feet per second (ft/s) – A foot per second is exactly 0.3048 metres per second, since the foot has been exactly 0.3048 metres since 1959. It belongs to American engineering: the Federal Highway Administration’s drainage manual says storm drains are typically designed for full-flow velocities of 3 ft/s or more to stay self-cleaning, which is 0.91 metres per second. A handy anchor is that 60 miles per hour is exactly 88 feet per second. The trap is fps, which also means frames per second, so write ft/s.