How this calculator works
Speed is distance divided by time. Converting between units therefore means converting both the length unit and the time unit. The metre per second is the SI unit. Kilometres per hour divide by 3.6 because one hour has 3,600 seconds and one kilometre has 1,000 metres. Miles per hour use the international mile of 1,609.344 m, and knots use the international nautical mile of exactly 1,852 m per hour. The speed of light is exactly 299,792,458 m/s. Mach is not a fixed speed; the value here uses the speed of sound in dry air at 15 C, about 340.3 m/s.
Formula
metres per second = entered value x F(unit). 1 km/h = 1/3.6 m/s. 1 mph = 0.44704 m/s. 1 ft/s = 0.3048 m/s. 1 knot = 1,852/3,600 m/s. 1 km/s = 1,000 m/s. 1 cm/s = 0.01 m/s. 1 ft/min = 0.00508 m/s. 1 mi/s = 1,609.344 m/s. Speed of light = 299,792,458 m/s. Beaufort force = (v / 0.836)^(2/3). Pace in minutes per kilometre = 1,000 / (v x 60).
Worked example
100 km/h is 27.778 m/s, 62.137 mph, 91.134 ft/s, 53.996 knots and 0.082 Mach. A 10 km/h run is a pace of 6 minutes per kilometre. The speed of light is 1,079,252,849 km/h.
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Frequently asked questions
Why does 100 km/h give 62.137 mph rather than 60?
A mile is longer than a kilometre, so a mile per hour is a faster speed than a kilometre per hour. Multiplying km/h by 0.621371 gives mph, and 100 x 0.621371 is 62.137.
Is a knot a speed over land?
A knot is one nautical mile per hour, used in navigation at sea and in the air. It is exactly 1.852 km/h, which is about 1.151 mph.
Why is Mach not included as a fixed unit everywhere?
The speed of sound depends on temperature, humidity and gas composition. On a hot day at altitude the same Mach number is a different ground speed. This page fixes Mach at 340.3 m/s for 15 C dry air.