Universal Speed Converter

Convert across many related units. Enter your value and which unit it uses.

One column on this page is a fixed constant of nature and another only pretends to be. The speed of light is 299,792,458 metres per second exactly, since 1983 that value has defined the metre itself rather than being measured against it. Mach, by contrast, is pinned here at 343 m/s, the speed of sound through dry air at roughly 20 °C at sea level — and the speed of sound is a function of air temperature, falling to about 295 m/s in the cold of the stratosphere. An airliner cruising at Mach 0.85 is therefore travelling far slower than 0.85 times this column suggests. The everyday units are on firmer ground: metres per second as the base, km/h at 0.277778, mph at 0.44704, feet per second at 0.3048, and the knot at 0.514444, which is one nautical mile of 1852 metres in an hour. Two limits to keep in view. Speed here is a bare scalar with no direction attached, so it cannot separate airspeed from ground speed or account for wind and current. And because results are rounded to nine decimals, an ordinary walking or driving speed expressed as a fraction of light retains only one significant figure.