SI System (International System of Units)
A system of units internationally standardized for science and technology.
Meters/second (m/s)
SI base unit, standardly used in physics and engineering. Represents the distance traveled in meters in one second. Walking speed is about 1.4 m/s, running speed about 5-7 m/s.
Kilometers/hour (km/h)
The most familiar velocity unit in everyday life. Used on vehicle speedometers and road signs. Normal roads are 40-60 km/h, highways about 100 km/h.
Imperial System
A system of units primarily used in the USA and UK.
Miles/hour (mph)
Used on road signs and speedometers in the USA and UK. 1 mile = 1.60934 km. US highways are typically 55-75 mph.
Feet/second (ft/s)
Used in engineering, especially in fluid dynamics and aerodynamics. 1 foot = 0.3048 m. Used to measure flow velocity in pipes and wind speed.
Aviation & Nautical Units
Specialized units used internationally in aviation and shipping industries.
Knot (kt, knot)
The speed that travels 1 nautical mile (1,852 m) in one hour. Used to display aircraft and ship speed. Standard unit of the International Civil Aviation Organization (ICAO). Cruising speed of passenger aircraft is about 450-550 knots.
Mach (Mach)
Relative velocity with speed of sound = 1. Mach 1 at sea level at 15°C is about 340 m/s (1,225 km/h). Speed of sound varies with temperature and altitude. Supersonic aircraft are Mach 1 or more, hypersonic aircraft are Mach 5 or more.
Special Units
Special velocity units used in scientific research and specific fields.
Speed of light (c)
The speed of light in vacuum, a fundamental constant in physics. Exactly 299,792,458 m/s. In relativity, no speed exists that exceeds this. In particle accelerators, there are particles reaching over 99.9% of the speed of light.