SpeedConverter

Speed measures how quickly an object changes position over time: distance divided by time. The SI unit is meters per second (m/s), but km/h and mph are far more prevalent in everyday life. Knots — nautical miles per hour — are the international standard for aviation and maritime navigation, where one knot equals exactly 1.852 km/h. Mach numbers express speed as a multiple of the local speed of sound, which varies with altitude and temperature: at sea level and 20 °C, Mach 1 ≈ 343 m/s ≈ 1,235 km/h.

The history of speed measurement is intertwined with the history of navigation. Sailors in the 16th–17th centuries measured their ships' speed by throwing a 'chip log' — a wooden board attached to a knotted rope — overboard, then counting how many knots passed through the hand in a fixed interval (usually 30 seconds on a 28-second glass). This direct physical measurement gave us the unit we still use. Road speed measurement became important with the spread of railways in the 1830s, and the speedometer became a standard vehicle instrument by the early 20th century. Aviation introduced the Mach number in the 1930s as a way to relate aircraft speed to compressibility effects in air.

Speed conversion is important in several everyday contexts. A driver crossing from Canada into the US finds speed limits switching from km/h to mph and must mentally adjust — 100 km/h is about 62 mph. A cyclist checking fitness data from a European app may need to convert km/h to mph for a US audience. Pilots worldwide report altitude in feet but speed in knots, regardless of the country they are flying over. Weather forecasters report wind speeds in both km/h and knots, and a listener must know that a 30-knot gale equals 55.6 km/h (about 34.5 mph).