Speed Converter

About speed units

Speed is the rate at which distance is covered over time — a derived quantity combining length and duration. Kilometers per hour (km/h) is the everyday unit used on road signs in most countries. Miles per hour (mph) is standard in the United States, United Kingdom, and a handful of other countries. Aviation and maritime navigation use knots: 1 knot equals exactly 1 nautical mile per hour, or about 1.852 km/h. Meters per second (m/s) is the SI unit and appears in physics and engineering. Mach, the ratio of an object's speed to the local speed of sound, is used for aircraft and is not a fixed unit — it varies with altitude and air temperature.

History & origin

The nautical mile and the knot have roots in celestial navigation: a nautical mile was defined as one arcminute of latitude, making chart calculations straightforward at sea. The unit 'knot' itself comes from the 17th-century practice of throwing a knotted rope overboard and counting knots paid out in a fixed time. Miles per hour became practical with railways in the 19th century. The Mach number was named after physicist Ernst Mach in the 20th century to describe ratios that matter in supersonic aerodynamics.

Tips

Mach is the one speed unit that cannot be pinned to a single conversion factor — Mach 1 at sea level is about 340 m/s (1,225 km/h), but at cruising altitude where air is colder, it drops to roughly 295 m/s. Converters that use a fixed Mach factor are giving you an approximation. For road use, the key to remember is that 100 km/h ≈ 62 mph, and 60 mph ≈ 96.6 km/h — neither is a round number in the other system.

Common questions

How many km/h is one knot?

Exactly 1.852 km/h by international definition, because 1 nautical mile = 1,852 meters. One knot is also approximately 1.151 mph or 0.5144 m/s.

Why is Mach not a fixed speed?

Mach is a ratio relative to the local speed of sound, which depends on air temperature (and to a lesser extent pressure). At sea level on a standard day, Mach 1 ≈ 343 m/s. At 35,000 ft (cruise altitude), the colder air puts Mach 1 at about 295 m/s. Most converters use the sea-level standard value.