Speed converter

The meter per second (m/s) is this converter's reference unit, and every other speed unit is defined as a multiple of it. A kilometer per hour equals 0.2777778 m/s, a mile per hour equals 0.44704 m/s exactly, a knot (used at sea and in the air) equals 0.5144444 m/s, and a foot per second equals 0.3048 m/s. At the extreme ends of the list, Mach 1 at sea level sits at roughly 340.29 m/s, while the speed of light is fixed at exactly 299,792,458 m/s — a value so precise because the meter itself is now defined from it.

The most requested pair in this category is kilometer per hour to mile per hour. Dividing the km/h factor (0.2777778) by the mph factor (0.44704) gives the multiplier: mph = km/h × 0.6213711922. Reverse the direction by multiplying by 1.609344 instead, which is the same number turned upside down.

A highway speed limit of 100 km/h works out to 100 × 0.6213711922 = 62.14 mph. Going the other way, a 60 mph speed limit converts to 60 × 1.609344 = 96.56 km/h — useful to know before driving a rental car across a border where the signs suddenly change units.

Which unit shows up depends on the field. Meteorologists and sailors report wind and vessel speed in knots, because the knot is tied to the nautical mile and makes dead-reckoning navigation arithmetic simpler. Pilots and aerospace engineers switch to Mach number near the speed of sound, since aerodynamic behavior changes sharply around Mach 1 regardless of the aircraft's actual m/s or mph reading. Scientific papers, ballistics and track-and-field timing almost always use meters per second, while road signage splits the world between km/h (most countries) and mph (the United States, the UK and a handful of others).

The easiest mistake in this category is treating a knot as if it were a mph. They're close — 1 knot is about 1.15078 mph — but not identical, and the gap compounds over long distances in aviation and marine navigation. A second pitfall is assuming Mach 1 is a fixed speed: because it is defined relative to the local speed of sound, which depends on air temperature and altitude, Mach 1 at sea level (about 340.29 m/s) is noticeably faster than Mach 1 at cruising altitude, where the air is much colder.

Several of the smaller units in this list are built for specific industries. Conveyor belts and 3D printers are often rated in feet per minute, a slow-moving number that would look awkward in km/h. Mile per minute and mile per second appear in rocketry and ballistics, where mph alone would require unwieldy six- and seven-digit figures. Centimeter and millimeter per second, meanwhile, are standard in laboratory and robotics work, where motion happens over millimeters rather than miles.