Speed converter
Convert between metres per second, kilometres per hour, miles per hour, feet per second and knots. Exact factors, and why aviation uses knots.
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Every speed unit, one page
Pick any two units above. The conversions people look up most often have dedicated pages with reference tables, linked below.
The factors are exact
| Unit | Metres per second | Note |
|---|---|---|
| Metre per second | 1 | SI |
| Kilometre per hour | 0.277778 | 1000 ÷ 3600 |
| Mile per hour | 0.44704 | exact — 1609.344 ÷ 3600 |
| Foot per second | 0.3048 | exact |
| Knot | 0.514444 | exact — 1852 ÷ 3600 |
Every speed factor is a length factor divided by 3600, so the exactness of the length conversions carries straight through.
Four systems, four audiences
| Unit | Used for |
|---|---|
| km/h | road speed almost everywhere |
| mph | road speed in the US, UK and a handful of others |
| knots | aviation, shipping, and wind in forecasts |
| m/s | physics, engineering, and wind in East Asian forecasts |
Wind is the one measured differently by different people looking at the same weather. A Taiwanese or Japanese forecast gives wind in m/s; a Hong Kong one gives km/h; an aviation forecast gives knots; a US one gives mph. All four describe the same air.
Why aviation and shipping use knots
A knot is one nautical mile per hour, and a nautical mile is one minute of latitude. That relationship is the whole point: on a chart, an hour at ten knots is ten minutes of latitude, measurable directly off the side of the map with dividers. No conversion, no scale factor.
The convenience is real enough that it survived the arrival of GPS. Aviation reports airspeed and wind in knots and altitude in feet, in countries that measure everything else in metric.
Reference speeds
| m/s | km/h | mph | knots | |
|---|---|---|---|---|
| Brisk walk | 1.4 | 5.0 | 3.1 | 2.7 |
| Cyclist | 5.6 | 20 | 12.4 | 10.8 |
| Usain Bolt’s peak | 12.4 | 44.7 | 27.8 | 24.1 |
| Urban speed limit | 13.9 | 50 | 31.1 | 27.0 |
| Motorway limit | 27.8 | 100 | 62.1 | 54.0 |
| Airliner cruise | 250 | 900 | 559 | 486 |
| Speed of sound at sea level | 343 | 1235 | 767 | 667 |
Where people go wrong
A speed conversion is not a distance conversion done twice. It is the same factor applied once: 100 km/h is 62.1 mph for exactly the reason 100 km is 62.1 miles.
Mach numbers are not a speed. Mach 1 is the local speed of sound, which falls with temperature and therefore with altitude. Mach 1 at sea level is 1235 km/h; at cruising altitude it is closer to 1062 km/h. A “Mach 0.85” airliner is not doing a fixed speed.