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km/h
100 km/h
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Instantly convert between m/s, km/h, mph, knots, Mach, speed of light and more.
The SI unit of speed is metres per second (m/s). Kilometres per hour (km/h) is used for road speeds worldwide. Miles per hour (mph) is used in the US and UK. Knots (kn) are used in aviation and maritime navigation — 1 knot = 1 nautical mile per hour.
Mach 1 is the speed of sound in air at sea level (~340.29 m/s or ~1,235 km/h at 15°C). Mach numbers vary with altitude and temperature. Supersonic means Mach 1–5, hypersonic means Mach 5+. The SR-71 Blackbird flew at Mach 3.3.
The speed of light in a vacuum (c) is exactly 299,792,458 m/s (~300,000 km/s). Nothing with mass can reach this speed. Light takes ~8.3 minutes to travel from the Sun to Earth. 1% of the speed of light is still ~10,790 km/s — 31× faster than Earth's orbital speed.
One knot equals one nautical mile (1,852 m) per hour, or approximately 1.852 km/h. Commercial aircraft cruise at 450–500 knots (~830–930 km/h). The term "knot" comes from the historic practice of measuring ship speed using a knotted rope thrown overboard.